Overview ======== When a CPU chooses to call push_rt_task and picks a task to push to another CPU's runqueue then it will call find_lock_lowest_rq method which would take a double lock on both CPUs' runqueues. If one of the locks aren't readily available, it may lead to dropping the current runqueue lock and reacquiring both the locks at once. During this window it is possible that the task is already migrated and is running on some other CPU. These cases are already handled. However, if the task is migrated and has already been executed and another CPU is now trying to wake it up (ttwu) such that it is queued again on the runqeue (on_rq is 1) and also if the task was run by the same CPU, then the current checks will pass even though the task was migrated out and is no longer in the pushable tasks list.
Crashes ======= This bug resulted in quite a few flavors of crashes triggering kernel panics with various crash signatures such as assert failures, page faults, null pointer dereferences, and queue corruption errors all coming from scheduler itself.
Some of the crashes: -> kernel BUG at kernel/sched/rt.c:1616! BUG_ON(idx >= MAX_RT_PRIO) Call Trace: ? __die_body+0x1a/0x60 ? die+0x2a/0x50 ? do_trap+0x85/0x100 ? pick_next_task_rt+0x6e/0x1d0 ? do_error_trap+0x64/0xa0 ? pick_next_task_rt+0x6e/0x1d0 ? exc_invalid_op+0x4c/0x60 ? pick_next_task_rt+0x6e/0x1d0 ? asm_exc_invalid_op+0x12/0x20 ? pick_next_task_rt+0x6e/0x1d0 __schedule+0x5cb/0x790 ? update_ts_time_stats+0x55/0x70 schedule_idle+0x1e/0x40 do_idle+0x15e/0x200 cpu_startup_entry+0x19/0x20 start_secondary+0x117/0x160 secondary_startup_64_no_verify+0xb0/0xbb
-> BUG: kernel NULL pointer dereference, address: 00000000000000c0 Call Trace: ? __die_body+0x1a/0x60 ? no_context+0x183/0x350 ? __warn+0x8a/0xe0 ? exc_page_fault+0x3d6/0x520 ? asm_exc_page_fault+0x1e/0x30 ? pick_next_task_rt+0xb5/0x1d0 ? pick_next_task_rt+0x8c/0x1d0 __schedule+0x583/0x7e0 ? update_ts_time_stats+0x55/0x70 schedule_idle+0x1e/0x40 do_idle+0x15e/0x200 cpu_startup_entry+0x19/0x20 start_secondary+0x117/0x160 secondary_startup_64_no_verify+0xb0/0xbb
-> BUG: unable to handle page fault for address: ffff9464daea5900 kernel BUG at kernel/sched/rt.c:1861! BUG_ON(rq->cpu != task_cpu(p))
-> kernel BUG at kernel/sched/rt.c:1055! BUG_ON(!rq->nr_running) Call Trace: ? __die_body+0x1a/0x60 ? die+0x2a/0x50 ? do_trap+0x85/0x100 ? dequeue_top_rt_rq+0xa2/0xb0 ? do_error_trap+0x64/0xa0 ? dequeue_top_rt_rq+0xa2/0xb0 ? exc_invalid_op+0x4c/0x60 ? dequeue_top_rt_rq+0xa2/0xb0 ? asm_exc_invalid_op+0x12/0x20 ? dequeue_top_rt_rq+0xa2/0xb0 dequeue_rt_entity+0x1f/0x70 dequeue_task_rt+0x2d/0x70 __schedule+0x1a8/0x7e0 ? blk_finish_plug+0x25/0x40 schedule+0x3c/0xb0 futex_wait_queue_me+0xb6/0x120 futex_wait+0xd9/0x240 do_futex+0x344/0xa90 ? get_mm_exe_file+0x30/0x60 ? audit_exe_compare+0x58/0x70 ? audit_filter_rules.constprop.26+0x65e/0x1220 __x64_sys_futex+0x148/0x1f0 do_syscall_64+0x30/0x80 entry_SYSCALL_64_after_hwframe+0x62/0xc7
-> BUG: unable to handle page fault for address: ffff8cf3608bc2c0 Call Trace: ? __die_body+0x1a/0x60 ? no_context+0x183/0x350 ? spurious_kernel_fault+0x171/0x1c0 ? exc_page_fault+0x3b6/0x520 ? plist_check_list+0x15/0x40 ? plist_check_list+0x2e/0x40 ? asm_exc_page_fault+0x1e/0x30 ? _cond_resched+0x15/0x30 ? futex_wait_queue_me+0xc8/0x120 ? futex_wait+0xd9/0x240 ? try_to_wake_up+0x1b8/0x490 ? futex_wake+0x78/0x160 ? do_futex+0xcd/0xa90 ? plist_check_list+0x15/0x40 ? plist_check_list+0x2e/0x40 ? plist_del+0x6a/0xd0 ? plist_check_list+0x15/0x40 ? plist_check_list+0x2e/0x40 ? dequeue_pushable_task+0x20/0x70 ? __schedule+0x382/0x7e0 ? asm_sysvec_reschedule_ipi+0xa/0x20 ? schedule+0x3c/0xb0 ? exit_to_user_mode_prepare+0x9e/0x150 ? irqentry_exit_to_user_mode+0x5/0x30 ? asm_sysvec_reschedule_ipi+0x12/0x20
Above are some of the common examples of the crashes that were observed due to this issue.
Details ======= Let's look at the following scenario to understand this race.
1) CPU A enters push_rt_task a) CPU A has chosen next_task = task p. b) CPU A calls find_lock_lowest_rq(Task p, CPU Z’s rq). c) CPU A identifies CPU X as a destination CPU (X < Z). d) CPU A enters double_lock_balance(CPU Z’s rq, CPU X’s rq). e) Since X is lower than Z, CPU A unlocks CPU Z’s rq. Someone else has locked CPU X’s rq, and thus, CPU A must wait.
2) At CPU Z a) Previous task has completed execution and thus, CPU Z enters schedule, locks its own rq after CPU A releases it. b) CPU Z dequeues previous task and begins executing task p. c) CPU Z unlocks its rq. d) Task p yields the CPU (ex. by doing IO or waiting to acquire a lock) which triggers the schedule function on CPU Z. e) CPU Z enters schedule again, locks its own rq, and dequeues task p. f) As part of dequeue, it sets p.on_rq = 0 and unlocks its rq.
3) At CPU B a) CPU B enters try_to_wake_up with input task p. b) Since CPU Z dequeued task p, p.on_rq = 0, and CPU B updates B.state = WAKING. c) CPU B via select_task_rq determines CPU Y as the target CPU.
4) The race a) CPU A acquires CPU X’s lock and relocks CPU Z. b) CPU A reads task p.cpu = Z and incorrectly concludes task p is still on CPU Z. c) CPU A failed to notice task p had been dequeued from CPU Z while CPU A was waiting for locks in double_lock_balance. If CPU A knew that task p had been dequeued, it would return NULL forcing push_rt_task to give up the task p's migration. d) CPU B updates task p.cpu = Y and calls ttwu_queue. e) CPU B locks Ys rq. CPU B enqueues task p onto Y and sets task p.on_rq = 1. f) CPU B unlocks CPU Y, triggering memory synchronization. g) CPU A reads task p.on_rq = 1, cementing its assumption that task p has not migrated. h) CPU A decides to migrate p to CPU X.
This leads to A dequeuing p from Y's queue and various crashes down the line.
Solution ======== The solution here is fairly simple. After obtaining the lock (at 4a), the check is enhanced to make sure that the task is still at the head of the pushable tasks list. If not, then it is anyway not suitable for being pushed out.
Testing ======= The fix is tested on a cluster of 3 nodes, where the panics due to this are hit every couple of days. A fix similar to this was deployed on such cluster and was stable for more than 30 days.
Co-developed-by: Jon Kohler jon@nutanix.com Signed-off-by: Jon Kohler jon@nutanix.com Co-developed-by: Gauri Patwardhan gauri.patwardhan@nutanix.com Signed-off-by: Gauri Patwardhan gauri.patwardhan@nutanix.com Co-developed-by: Rahul Chunduru rahul.chunduru@nutanix.com Signed-off-by: Rahul Chunduru rahul.chunduru@nutanix.com Signed-off-by: Harshit Agarwal harshit@nutanix.com Tested-by: Will Ton william.ton@nutanix.com Reviewed-by: Steven Rostedt (Google) rostedt@goodmis.org Cc: stable@vger.kernel.org --- Changes in v2: - As per Steve's suggestion, removed some checks that are done after obtaining the lock that are no longer needed with the addition of new check. - Moved up is_migration_disabled check. - Link to v1: https://lore.kernel.org/lkml/20250211054646.23987-1-harshit@nutanix.com/
Changes in v3: - Updated commit message to add stable maintainers and reviewed-by tag. - Link to v2: https://lore.kernel.org/lkml/20250214170844.201692-1-harshit@nutanix.com/ --- kernel/sched/rt.c | 54 +++++++++++++++++++++++------------------------ 1 file changed, 26 insertions(+), 28 deletions(-)
diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c index 4b8e33c615b1..4762dd3f50c5 100644 --- a/kernel/sched/rt.c +++ b/kernel/sched/rt.c @@ -1885,6 +1885,27 @@ static int find_lowest_rq(struct task_struct *task) return -1; }
+static struct task_struct *pick_next_pushable_task(struct rq *rq) +{ + struct task_struct *p; + + if (!has_pushable_tasks(rq)) + return NULL; + + p = plist_first_entry(&rq->rt.pushable_tasks, + struct task_struct, pushable_tasks); + + BUG_ON(rq->cpu != task_cpu(p)); + BUG_ON(task_current(rq, p)); + BUG_ON(task_current_donor(rq, p)); + BUG_ON(p->nr_cpus_allowed <= 1); + + BUG_ON(!task_on_rq_queued(p)); + BUG_ON(!rt_task(p)); + + return p; +} + /* Will lock the rq it finds */ static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq) { @@ -1915,18 +1936,16 @@ static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq) /* * We had to unlock the run queue. In * the mean time, task could have - * migrated already or had its affinity changed. - * Also make sure that it wasn't scheduled on its rq. + * migrated already or had its affinity changed, + * therefore check if the task is still at the + * head of the pushable tasks list. * It is possible the task was scheduled, set * "migrate_disabled" and then got preempted, so we must * check the task migration disable flag here too. */ - if (unlikely(task_rq(task) != rq || + if (unlikely(is_migration_disabled(task) || !cpumask_test_cpu(lowest_rq->cpu, &task->cpus_mask) || - task_on_cpu(rq, task) || - !rt_task(task) || - is_migration_disabled(task) || - !task_on_rq_queued(task))) { + task != pick_next_pushable_task(rq))) {
double_unlock_balance(rq, lowest_rq); lowest_rq = NULL; @@ -1946,27 +1965,6 @@ static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq) return lowest_rq; }
-static struct task_struct *pick_next_pushable_task(struct rq *rq) -{ - struct task_struct *p; - - if (!has_pushable_tasks(rq)) - return NULL; - - p = plist_first_entry(&rq->rt.pushable_tasks, - struct task_struct, pushable_tasks); - - BUG_ON(rq->cpu != task_cpu(p)); - BUG_ON(task_current(rq, p)); - BUG_ON(task_current_donor(rq, p)); - BUG_ON(p->nr_cpus_allowed <= 1); - - BUG_ON(!task_on_rq_queued(p)); - BUG_ON(!rt_task(p)); - - return p; -} - /* * If the current CPU has more than one RT task, see if the non * running task can migrate over to a CPU that is running a task
Hi Harshit,
On 25/02/25 18:05, Harshit Agarwal wrote:
Overview
When a CPU chooses to call push_rt_task and picks a task to push to another CPU's runqueue then it will call find_lock_lowest_rq method which would take a double lock on both CPUs' runqueues. If one of the locks aren't readily available, it may lead to dropping the current runqueue lock and reacquiring both the locks at once. During this window it is possible that the task is already migrated and is running on some other CPU. These cases are already handled. However, if the task is migrated and has already been executed and another CPU is now trying to wake it up (ttwu) such that it is queued again on the runqeue (on_rq is 1) and also if the task was run by the same CPU, then the current checks will pass even though the task was migrated out and is no longer in the pushable tasks list.
...
kernel/sched/rt.c | 54 +++++++++++++++++++++++------------------------ 1 file changed, 26 insertions(+), 28 deletions(-)
diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c index 4b8e33c615b1..4762dd3f50c5 100644 --- a/kernel/sched/rt.c +++ b/kernel/sched/rt.c @@ -1885,6 +1885,27 @@ static int find_lowest_rq(struct task_struct *task) return -1; } +static struct task_struct *pick_next_pushable_task(struct rq *rq) +{
- struct task_struct *p;
- if (!has_pushable_tasks(rq))
return NULL;- p = plist_first_entry(&rq->rt.pushable_tasks,
struct task_struct, pushable_tasks);- BUG_ON(rq->cpu != task_cpu(p));
- BUG_ON(task_current(rq, p));
- BUG_ON(task_current_donor(rq, p));
- BUG_ON(p->nr_cpus_allowed <= 1);
- BUG_ON(!task_on_rq_queued(p));
- BUG_ON(!rt_task(p));
- return p;
+}
/* Will lock the rq it finds */ static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq) { @@ -1915,18 +1936,16 @@ static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq) /* * We had to unlock the run queue. In * the mean time, task could have
* migrated already or had its affinity changed.* Also make sure that it wasn't scheduled on its rq.
* migrated already or had its affinity changed,* therefore check if the task is still at the* head of the pushable tasks list. * It is possible the task was scheduled, set * "migrate_disabled" and then got preempted, so we must * check the task migration disable flag here too. */
if (unlikely(task_rq(task) != rq ||
if (unlikely(is_migration_disabled(task) || !cpumask_test_cpu(lowest_rq->cpu, &task->cpus_mask) ||
task_on_cpu(rq, task) ||!rt_task(task) ||is_migration_disabled(task) ||!task_on_rq_queued(task))) {
task != pick_next_pushable_task(rq))) {double_unlock_balance(rq, lowest_rq); lowest_rq = NULL; @@ -1946,27 +1965,6 @@ static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq) return lowest_rq; } -static struct task_struct *pick_next_pushable_task(struct rq *rq) -{
- struct task_struct *p;
- if (!has_pushable_tasks(rq))
return NULL;- p = plist_first_entry(&rq->rt.pushable_tasks,
struct task_struct, pushable_tasks);- BUG_ON(rq->cpu != task_cpu(p));
- BUG_ON(task_current(rq, p));
- BUG_ON(task_current_donor(rq, p));
- BUG_ON(p->nr_cpus_allowed <= 1);
- BUG_ON(!task_on_rq_queued(p));
- BUG_ON(!rt_task(p));
- return p;
-}
As usual, we have essentially the same in deadline.c, do you think we should/could implement the same fix proactively in there as well? Steve?
Thanks, Juri
On Tue, 4 Mar 2025 09:15:55 +0000 Juri Lelli juri.lelli@redhat.com wrote:
As usual, we have essentially the same in deadline.c, do you think we should/could implement the same fix proactively in there as well? Steve?
Probably. It would be better if we could find a way to consolidate the functionality so that when we fix a bug in one, the other gets fixed too.
-- Steve
On 04/03/25 10:30, Steven Rostedt wrote:
On Tue, 4 Mar 2025 09:15:55 +0000 Juri Lelli juri.lelli@redhat.com wrote:
As usual, we have essentially the same in deadline.c, do you think we should/could implement the same fix proactively in there as well? Steve?
Probably. It would be better if we could find a way to consolidate the functionality so that when we fix a bug in one, the other gets fixed too.
That would be nice indeed.
Thanks, Juri
Thanks Juri for pointing this out. I can send the fix for deadline as well. Is it okay if I do it in a separate patch?
From taking a quick look at the code, I can see that the same fix won’t apply as is in case of deadline since it has two different callers for find_lock_later_rq. One is push_dl_task for which we can call pick_next_pushable_dl_task and make sure it is at the head. This is where we have the bug. Another one is dl_task_offline_migration which gets the task from dl_task_timer which in turn gets it from sched_dl_entity. I haven’t gone through the deadline code thoroughly but I think this race shouldn’t exist for the offline task (2nd) case. If that is true then the fix could be to check in push_dl_task if the task returned by find_lock_later_rq is still at the head of the queue or not. Let me know Steve and Juri if this plan sounds good. I will send the fix accordingly.
Regards, Harshit
On Mar 4, 2025, at 8:18 AM, Juri Lelli juri.lelli@redhat.com wrote:
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On 04/03/25 10:30, Steven Rostedt wrote:
On Tue, 4 Mar 2025 09:15:55 +0000 Juri Lelli juri.lelli@redhat.com wrote:
As usual, we have essentially the same in deadline.c, do you think we should/could implement the same fix proactively in there as well? Steve?
Probably. It would be better if we could find a way to consolidate the functionality so that when we fix a bug in one, the other gets fixed too.
That would be nice indeed.
Thanks, Juri
Thanks Juri for pointing this out. I can send the fix for deadline as well. Is it okay if I do it in a separate patch?
From taking a quick look at the code, I can see that the same fix won’t apply as is in case of deadline since it has two different callers for find_lock_later_rq. One is push_dl_task for which we can call pick_next_pushable_dl_task and make sure it is at the head. This is where we have the bug. Another one is dl_task_offline_migration which gets the task from dl_task_timer which in turn gets it from sched_dl_entity. I haven’t gone through the deadline code thoroughly but I think this race shouldn’t exist for the offline task (2nd) case. If that is true then the fix could be to check in push_dl_task if the task returned by find_lock_later_rq is still at the head of the queue or not. Steve and Juri, let me know if this plan sounds good. I will send the fix accordingly.
Regards, Harshit
On Mar 4, 2025, at 8:18 AM, Juri Lelli juri.lelli@redhat.com wrote:
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On 04/03/25 10:30, Steven Rostedt wrote:
On Tue, 4 Mar 2025 09:15:55 +0000 Juri Lelli juri.lelli@redhat.com wrote:
As usual, we have essentially the same in deadline.c, do you think we should/could implement the same fix proactively in there as well? Steve?
Probably. It would be better if we could find a way to consolidate the functionality so that when we fix a bug in one, the other gets fixed too.
That would be nice indeed.
Thanks, Juri
On 04/03/25 18:37, Harshit Agarwal wrote:
Thanks Juri for pointing this out. I can send the fix for deadline as well. Is it okay if I do it in a separate patch?
Yes, we would need a separate patch.
From taking a quick look at the code, I can see that the same fix won’t apply as is in case of deadline since it has two different callers for find_lock_later_rq.
Right, indeed.
One is push_dl_task for which we can call pick_next_pushable_dl_task and make sure it is at the head. This is where we have the bug.
OK.
Another one is dl_task_offline_migration which gets the task from dl_task_timer which in turn gets it from sched_dl_entity. I haven’t gone through the deadline code thoroughly but I think this race shouldn’t exist for the offline task (2nd) case. If that is true then the fix could be to check in push_dl_task if the task returned by find_lock_later_rq is still at the head of the queue or not.
I believe that won't work as dl_task_offline_migration() gets called in case the replenishment timer for a task fires (to unthrottle it) and it finds the old rq the task was running on has been offlined in the meantime. The task is still throttled at this point and so it is not enqueued in the dl_rq nor in the pushable task list/tree, so the check you are adding won't work I am afraid. Maybe we can use dl_se->dl_throttled to differentiate this different case.
Thanks, Juri
On Mar 5, 2025, at 2:43 AM, Juri Lelli juri.lelli@redhat.com wrote:
Another one is dl_task_offline_migration which gets the task from dl_task_timer which in turn gets it from sched_dl_entity. I haven’t gone through the deadline code thoroughly but I think this race shouldn’t exist for the offline task (2nd) case. If that is true then the fix could be to check in push_dl_task if the task returned by find_lock_later_rq is still at the head of the queue or not.
I believe that won't work as dl_task_offline_migration() gets called in case the replenishment timer for a task fires (to unthrottle it) and it finds the old rq the task was running on has been offlined in the meantime. The task is still throttled at this point and so it is not enqueued in the dl_rq nor in the pushable task list/tree, so the check you are adding won't work I am afraid. Maybe we can use dl_se->dl_throttled to differentiate this different case.
Thanks Juri. I sent the fix please take a look: https://lore.kernel.org/lkml/20250307204255.60640-1-harshit@nutanix.com/ Instead of changing find_lock_later_rq, I added the handling in the caller i.e. push_dl_task since that’s the one affected by the race. I think we don’t need to handle the other case at all as the race is not applicable for offline migration case. Let me know if this sounds fine or if I am missing something.
Regards, Harshit
Sorry for the repeat messages, my email client’s acting up. :)
On Mar 4, 2025, at 8:18 AM, Juri Lelli juri.lelli@redhat.com wrote:
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On 04/03/25 10:30, Steven Rostedt wrote:
On Tue, 4 Mar 2025 09:15:55 +0000 Juri Lelli juri.lelli@redhat.com wrote:
As usual, we have essentially the same in deadline.c, do you think we should/could implement the same fix proactively in there as well? Steve?
Probably. It would be better if we could find a way to consolidate the functionality so that when we fix a bug in one, the other gets fixed too.
That would be nice indeed.
Thanks, Juri
On Tue, Feb 25, 2025 at 06:05:53PM +0000 Harshit Agarwal wrote:
Overview
When a CPU chooses to call push_rt_task and picks a task to push to another CPU's runqueue then it will call find_lock_lowest_rq method which would take a double lock on both CPUs' runqueues. If one of the locks aren't readily available, it may lead to dropping the current runqueue lock and reacquiring both the locks at once. During this window it is possible that the task is already migrated and is running on some other CPU. These cases are already handled. However, if the task is migrated and has already been executed and another CPU is now trying to wake it up (ttwu) such that it is queued again on the runqeue (on_rq is 1) and also if the task was run by the same CPU, then the current checks will pass even though the task was migrated out and is no longer in the pushable tasks list.
Crashes
This bug resulted in quite a few flavors of crashes triggering kernel panics with various crash signatures such as assert failures, page faults, null pointer dereferences, and queue corruption errors all coming from scheduler itself.
Some of the crashes: -> kernel BUG at kernel/sched/rt.c:1616! BUG_ON(idx >= MAX_RT_PRIO) Call Trace: ? __die_body+0x1a/0x60 ? die+0x2a/0x50 ? do_trap+0x85/0x100 ? pick_next_task_rt+0x6e/0x1d0 ? do_error_trap+0x64/0xa0 ? pick_next_task_rt+0x6e/0x1d0 ? exc_invalid_op+0x4c/0x60 ? pick_next_task_rt+0x6e/0x1d0 ? asm_exc_invalid_op+0x12/0x20 ? pick_next_task_rt+0x6e/0x1d0 __schedule+0x5cb/0x790 ? update_ts_time_stats+0x55/0x70 schedule_idle+0x1e/0x40 do_idle+0x15e/0x200 cpu_startup_entry+0x19/0x20 start_secondary+0x117/0x160 secondary_startup_64_no_verify+0xb0/0xbb
-> BUG: kernel NULL pointer dereference, address: 00000000000000c0 Call Trace: ? __die_body+0x1a/0x60 ? no_context+0x183/0x350 ? __warn+0x8a/0xe0 ? exc_page_fault+0x3d6/0x520 ? asm_exc_page_fault+0x1e/0x30 ? pick_next_task_rt+0xb5/0x1d0 ? pick_next_task_rt+0x8c/0x1d0 __schedule+0x583/0x7e0 ? update_ts_time_stats+0x55/0x70 schedule_idle+0x1e/0x40 do_idle+0x15e/0x200 cpu_startup_entry+0x19/0x20 start_secondary+0x117/0x160 secondary_startup_64_no_verify+0xb0/0xbb
-> BUG: unable to handle page fault for address: ffff9464daea5900 kernel BUG at kernel/sched/rt.c:1861! BUG_ON(rq->cpu != task_cpu(p))
-> kernel BUG at kernel/sched/rt.c:1055! BUG_ON(!rq->nr_running) Call Trace: ? __die_body+0x1a/0x60 ? die+0x2a/0x50 ? do_trap+0x85/0x100 ? dequeue_top_rt_rq+0xa2/0xb0 ? do_error_trap+0x64/0xa0 ? dequeue_top_rt_rq+0xa2/0xb0 ? exc_invalid_op+0x4c/0x60 ? dequeue_top_rt_rq+0xa2/0xb0 ? asm_exc_invalid_op+0x12/0x20 ? dequeue_top_rt_rq+0xa2/0xb0 dequeue_rt_entity+0x1f/0x70 dequeue_task_rt+0x2d/0x70 __schedule+0x1a8/0x7e0 ? blk_finish_plug+0x25/0x40 schedule+0x3c/0xb0 futex_wait_queue_me+0xb6/0x120 futex_wait+0xd9/0x240 do_futex+0x344/0xa90 ? get_mm_exe_file+0x30/0x60 ? audit_exe_compare+0x58/0x70 ? audit_filter_rules.constprop.26+0x65e/0x1220 __x64_sys_futex+0x148/0x1f0 do_syscall_64+0x30/0x80 entry_SYSCALL_64_after_hwframe+0x62/0xc7
-> BUG: unable to handle page fault for address: ffff8cf3608bc2c0 Call Trace: ? __die_body+0x1a/0x60 ? no_context+0x183/0x350 ? spurious_kernel_fault+0x171/0x1c0 ? exc_page_fault+0x3b6/0x520 ? plist_check_list+0x15/0x40 ? plist_check_list+0x2e/0x40 ? asm_exc_page_fault+0x1e/0x30 ? _cond_resched+0x15/0x30 ? futex_wait_queue_me+0xc8/0x120 ? futex_wait+0xd9/0x240 ? try_to_wake_up+0x1b8/0x490 ? futex_wake+0x78/0x160 ? do_futex+0xcd/0xa90 ? plist_check_list+0x15/0x40 ? plist_check_list+0x2e/0x40 ? plist_del+0x6a/0xd0 ? plist_check_list+0x15/0x40 ? plist_check_list+0x2e/0x40 ? dequeue_pushable_task+0x20/0x70 ? __schedule+0x382/0x7e0 ? asm_sysvec_reschedule_ipi+0xa/0x20 ? schedule+0x3c/0xb0 ? exit_to_user_mode_prepare+0x9e/0x150 ? irqentry_exit_to_user_mode+0x5/0x30 ? asm_sysvec_reschedule_ipi+0x12/0x20
Above are some of the common examples of the crashes that were observed due to this issue.
Details
Let's look at the following scenario to understand this race.
- CPU A enters push_rt_task
a) CPU A has chosen next_task = task p. b) CPU A calls find_lock_lowest_rq(Task p, CPU Z’s rq). c) CPU A identifies CPU X as a destination CPU (X < Z). d) CPU A enters double_lock_balance(CPU Z’s rq, CPU X’s rq). e) Since X is lower than Z, CPU A unlocks CPU Z’s rq. Someone else has locked CPU X’s rq, and thus, CPU A must wait.
- At CPU Z
a) Previous task has completed execution and thus, CPU Z enters schedule, locks its own rq after CPU A releases it. b) CPU Z dequeues previous task and begins executing task p. c) CPU Z unlocks its rq. d) Task p yields the CPU (ex. by doing IO or waiting to acquire a lock) which triggers the schedule function on CPU Z. e) CPU Z enters schedule again, locks its own rq, and dequeues task p. f) As part of dequeue, it sets p.on_rq = 0 and unlocks its rq.
- At CPU B
a) CPU B enters try_to_wake_up with input task p. b) Since CPU Z dequeued task p, p.on_rq = 0, and CPU B updates B.state = WAKING. c) CPU B via select_task_rq determines CPU Y as the target CPU.
- The race
a) CPU A acquires CPU X’s lock and relocks CPU Z. b) CPU A reads task p.cpu = Z and incorrectly concludes task p is still on CPU Z. c) CPU A failed to notice task p had been dequeued from CPU Z while CPU A was waiting for locks in double_lock_balance. If CPU A knew that task p had been dequeued, it would return NULL forcing push_rt_task to give up the task p's migration. d) CPU B updates task p.cpu = Y and calls ttwu_queue. e) CPU B locks Ys rq. CPU B enqueues task p onto Y and sets task p.on_rq = 1. f) CPU B unlocks CPU Y, triggering memory synchronization. g) CPU A reads task p.on_rq = 1, cementing its assumption that task p has not migrated. h) CPU A decides to migrate p to CPU X.
This leads to A dequeuing p from Y's queue and various crashes down the line.
Solution
The solution here is fairly simple. After obtaining the lock (at 4a), the check is enhanced to make sure that the task is still at the head of the pushable tasks list. If not, then it is anyway not suitable for being pushed out.
Testing
The fix is tested on a cluster of 3 nodes, where the panics due to this are hit every couple of days. A fix similar to this was deployed on such cluster and was stable for more than 30 days.
Co-developed-by: Jon Kohler jon@nutanix.com Signed-off-by: Jon Kohler jon@nutanix.com Co-developed-by: Gauri Patwardhan gauri.patwardhan@nutanix.com Signed-off-by: Gauri Patwardhan gauri.patwardhan@nutanix.com Co-developed-by: Rahul Chunduru rahul.chunduru@nutanix.com Signed-off-by: Rahul Chunduru rahul.chunduru@nutanix.com Signed-off-by: Harshit Agarwal harshit@nutanix.com Tested-by: Will Ton william.ton@nutanix.com Reviewed-by: Steven Rostedt (Google) rostedt@goodmis.org Cc: stable@vger.kernel.org
We've got some crashes that seem to be from the same race so this will be a nice fix to have. Thanks!
Reviewed-by: Phil Auld pauld@redhat.com
Cheers, Phil
Changes in v2:
- As per Steve's suggestion, removed some checks that are done after obtaining the lock that are no longer needed with the addition of new check.
- Moved up is_migration_disabled check.
- Link to v1: https://lore.kernel.org/lkml/20250211054646.23987-1-harshit@nutanix.com/
Changes in v3:
- Updated commit message to add stable maintainers and reviewed-by tag.
- Link to v2: https://lore.kernel.org/lkml/20250214170844.201692-1-harshit@nutanix.com/
kernel/sched/rt.c | 54 +++++++++++++++++++++++------------------------ 1 file changed, 26 insertions(+), 28 deletions(-)
diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c index 4b8e33c615b1..4762dd3f50c5 100644 --- a/kernel/sched/rt.c +++ b/kernel/sched/rt.c @@ -1885,6 +1885,27 @@ static int find_lowest_rq(struct task_struct *task) return -1; } +static struct task_struct *pick_next_pushable_task(struct rq *rq) +{
- struct task_struct *p;
- if (!has_pushable_tasks(rq))
return NULL;- p = plist_first_entry(&rq->rt.pushable_tasks,
struct task_struct, pushable_tasks);- BUG_ON(rq->cpu != task_cpu(p));
- BUG_ON(task_current(rq, p));
- BUG_ON(task_current_donor(rq, p));
- BUG_ON(p->nr_cpus_allowed <= 1);
- BUG_ON(!task_on_rq_queued(p));
- BUG_ON(!rt_task(p));
- return p;
+}
/* Will lock the rq it finds */ static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq) { @@ -1915,18 +1936,16 @@ static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq) /* * We had to unlock the run queue. In * the mean time, task could have
* migrated already or had its affinity changed.* Also make sure that it wasn't scheduled on its rq.
* migrated already or had its affinity changed,* therefore check if the task is still at the* head of the pushable tasks list. * It is possible the task was scheduled, set * "migrate_disabled" and then got preempted, so we must * check the task migration disable flag here too. */
if (unlikely(task_rq(task) != rq ||
if (unlikely(is_migration_disabled(task) || !cpumask_test_cpu(lowest_rq->cpu, &task->cpus_mask) ||
task_on_cpu(rq, task) ||!rt_task(task) ||is_migration_disabled(task) ||!task_on_rq_queued(task))) {
task != pick_next_pushable_task(rq))) {double_unlock_balance(rq, lowest_rq); lowest_rq = NULL; @@ -1946,27 +1965,6 @@ static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq) return lowest_rq; } -static struct task_struct *pick_next_pushable_task(struct rq *rq) -{
- struct task_struct *p;
- if (!has_pushable_tasks(rq))
return NULL;- p = plist_first_entry(&rq->rt.pushable_tasks,
struct task_struct, pushable_tasks);- BUG_ON(rq->cpu != task_cpu(p));
- BUG_ON(task_current(rq, p));
- BUG_ON(task_current_donor(rq, p));
- BUG_ON(p->nr_cpus_allowed <= 1);
- BUG_ON(!task_on_rq_queued(p));
- BUG_ON(!rt_task(p));
- return p;
-}
/*
- If the current CPU has more than one RT task, see if the non
- running task can migrate over to a CPU that is running a task
-- 2.22.3
On Tue, Feb 25, 2025 at 06:05:53PM +0000, Harshit Agarwal wrote:
Details
Let's look at the following scenario to understand this race.
- CPU A enters push_rt_task
a) CPU A has chosen next_task = task p. b) CPU A calls find_lock_lowest_rq(Task p, CPU Z’s rq). c) CPU A identifies CPU X as a destination CPU (X < Z). d) CPU A enters double_lock_balance(CPU Z’s rq, CPU X’s rq). e) Since X is lower than Z, CPU A unlocks CPU Z’s rq. Someone else has locked CPU X’s rq, and thus, CPU A must wait.
- At CPU Z
a) Previous task has completed execution and thus, CPU Z enters schedule, locks its own rq after CPU A releases it. b) CPU Z dequeues previous task and begins executing task p. c) CPU Z unlocks its rq. d) Task p yields the CPU (ex. by doing IO or waiting to acquire a lock) which triggers the schedule function on CPU Z. e) CPU Z enters schedule again, locks its own rq, and dequeues task p. f) As part of dequeue, it sets p.on_rq = 0 and unlocks its rq.
- At CPU B
a) CPU B enters try_to_wake_up with input task p. b) Since CPU Z dequeued task p, p.on_rq = 0, and CPU B updates B.state = WAKING. c) CPU B via select_task_rq determines CPU Y as the target CPU.
- The race
a) CPU A acquires CPU X’s lock and relocks CPU Z. b) CPU A reads task p.cpu = Z and incorrectly concludes task p is still on CPU Z. c) CPU A failed to notice task p had been dequeued from CPU Z while CPU A was waiting for locks in double_lock_balance. If CPU A knew that task p had been dequeued, it would return NULL forcing push_rt_task to give up the task p's migration. d) CPU B updates task p.cpu = Y and calls ttwu_queue. e) CPU B locks Ys rq. CPU B enqueues task p onto Y and sets task p.on_rq = 1. f) CPU B unlocks CPU Y, triggering memory synchronization. g) CPU A reads task p.on_rq = 1, cementing its assumption that task p has not migrated. h) CPU A decides to migrate p to CPU X.
This leads to A dequeuing p from Y's queue and various crashes down the line.
Solution
The solution here is fairly simple. After obtaining the lock (at 4a), the check is enhanced to make sure that the task is still at the head of the pushable tasks list. If not, then it is anyway not suitable for being pushed out.
Testing
The fix is tested on a cluster of 3 nodes, where the panics due to this are hit every couple of days. A fix similar to this was deployed on such cluster and was stable for more than 30 days.
Co-developed-by: Jon Kohler jon@nutanix.com Signed-off-by: Jon Kohler jon@nutanix.com Co-developed-by: Gauri Patwardhan gauri.patwardhan@nutanix.com Signed-off-by: Gauri Patwardhan gauri.patwardhan@nutanix.com Co-developed-by: Rahul Chunduru rahul.chunduru@nutanix.com Signed-off-by: Rahul Chunduru rahul.chunduru@nutanix.com Signed-off-by: Harshit Agarwal harshit@nutanix.com Tested-by: Will Ton william.ton@nutanix.com Reviewed-by: Steven Rostedt (Google) rostedt@goodmis.org Cc: stable@vger.kernel.org
Thanks, I've picked this up to land after -rc1.
linux-stable-mirror@lists.linaro.org
-
Harshit Agarwal -
Juri Lelli -
Peter Zijlstra -
Phil Auld -
Steven Rostedt