From: Waiman Long longman@redhat.com
[ Upstream commit ad789f84c9a145f8a18744c0387cec22ec51651e ]
The handling of sysrq key can be activated by echoing the key to /proc/sysrq-trigger or via the magic key sequence typed into a terminal that is connected to the system in some way (serial, USB or other mean). In the former case, the handling is done in a user context. In the latter case, it is likely to be in an interrupt context.
Currently in print_cpu() of kernel/sched/debug.c, sched_debug_lock is taken with interrupt disabled for the whole duration of the calls to print_*_stats() and print_rq() which could last for the quite some time if the information dump happens on the serial console.
If the system has many cpus and the sched_debug_lock is somehow busy (e.g. parallel sysrq-t), the system may hit a hard lockup panic depending on the actually serial console implementation of the system.
The purpose of sched_debug_lock is to serialize the use of the global cgroup_path[] buffer in print_cpu(). The rests of the printk calls don't need serialization from sched_debug_lock.
Calling printk() with interrupt disabled can still be problematic if multiple instances are running. Allocating a stack buffer of PATH_MAX bytes is not feasible because of the limited size of the kernel stack.
The solution implemented in this patch is to allow only one caller at a time to use the full size group_path[], while other simultaneous callers will have to use shorter stack buffers with the possibility of path name truncation. A "..." suffix will be printed if truncation may have happened. The cgroup path name is provided for informational purpose only, so occasional path name truncation should not be a big problem.
Fixes: efe25c2c7b3a ("sched: Reinstate group names in /proc/sched_debug") Suggested-by: Peter Zijlstra peterz@infradead.org Signed-off-by: Waiman Long longman@redhat.com Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Link: https://lkml.kernel.org/r/20210415195426.6677-1-longman@redhat.com Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/sched/debug.c | 42 +++++++++++++++++++++++++++++------------- 1 file changed, 29 insertions(+), 13 deletions(-)
diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c index 486f403a778b..9c8b3ed2199a 100644 --- a/kernel/sched/debug.c +++ b/kernel/sched/debug.c @@ -8,8 +8,6 @@ */ #include "sched.h"
-static DEFINE_SPINLOCK(sched_debug_lock); - /* * This allows printing both to /proc/sched_debug and * to the console @@ -470,16 +468,37 @@ static void print_cfs_group_stats(struct seq_file *m, int cpu, struct task_group #endif
#ifdef CONFIG_CGROUP_SCHED +static DEFINE_SPINLOCK(sched_debug_lock); static char group_path[PATH_MAX];
-static char *task_group_path(struct task_group *tg) +static void task_group_path(struct task_group *tg, char *path, int plen) { - if (autogroup_path(tg, group_path, PATH_MAX)) - return group_path; + if (autogroup_path(tg, path, plen)) + return;
- cgroup_path(tg->css.cgroup, group_path, PATH_MAX); + cgroup_path(tg->css.cgroup, path, plen); +}
- return group_path; +/* + * Only 1 SEQ_printf_task_group_path() caller can use the full length + * group_path[] for cgroup path. Other simultaneous callers will have + * to use a shorter stack buffer. A "..." suffix is appended at the end + * of the stack buffer so that it will show up in case the output length + * matches the given buffer size to indicate possible path name truncation. + */ +#define SEQ_printf_task_group_path(m, tg, fmt...) \ +{ \ + if (spin_trylock(&sched_debug_lock)) { \ + task_group_path(tg, group_path, sizeof(group_path)); \ + SEQ_printf(m, fmt, group_path); \ + spin_unlock(&sched_debug_lock); \ + } else { \ + char buf[128]; \ + char *bufend = buf + sizeof(buf) - 3; \ + task_group_path(tg, buf, bufend - buf); \ + strcpy(bufend - 1, "..."); \ + SEQ_printf(m, fmt, buf); \ + } \ } #endif
@@ -506,7 +525,7 @@ print_task(struct seq_file *m, struct rq *rq, struct task_struct *p) SEQ_printf(m, " %d %d", task_node(p), task_numa_group_id(p)); #endif #ifdef CONFIG_CGROUP_SCHED - SEQ_printf(m, " %s", task_group_path(task_group(p))); + SEQ_printf_task_group_path(m, task_group(p), " %s") #endif
SEQ_printf(m, "\n"); @@ -543,7 +562,7 @@ void print_cfs_rq(struct seq_file *m, int cpu, struct cfs_rq *cfs_rq)
#ifdef CONFIG_FAIR_GROUP_SCHED SEQ_printf(m, "\n"); - SEQ_printf(m, "cfs_rq[%d]:%s\n", cpu, task_group_path(cfs_rq->tg)); + SEQ_printf_task_group_path(m, cfs_rq->tg, "cfs_rq[%d]:%s\n", cpu); #else SEQ_printf(m, "\n"); SEQ_printf(m, "cfs_rq[%d]:\n", cpu); @@ -614,7 +633,7 @@ void print_rt_rq(struct seq_file *m, int cpu, struct rt_rq *rt_rq) { #ifdef CONFIG_RT_GROUP_SCHED SEQ_printf(m, "\n"); - SEQ_printf(m, "rt_rq[%d]:%s\n", cpu, task_group_path(rt_rq->tg)); + SEQ_printf_task_group_path(m, rt_rq->tg, "rt_rq[%d]:%s\n", cpu); #else SEQ_printf(m, "\n"); SEQ_printf(m, "rt_rq[%d]:\n", cpu); @@ -666,7 +685,6 @@ void print_dl_rq(struct seq_file *m, int cpu, struct dl_rq *dl_rq) static void print_cpu(struct seq_file *m, int cpu) { struct rq *rq = cpu_rq(cpu); - unsigned long flags;
#ifdef CONFIG_X86 { @@ -717,13 +735,11 @@ do { \ } #undef P
- spin_lock_irqsave(&sched_debug_lock, flags); print_cfs_stats(m, cpu); print_rt_stats(m, cpu); print_dl_stats(m, cpu);
print_rq(m, rq, cpu); - spin_unlock_irqrestore(&sched_debug_lock, flags); SEQ_printf(m, "\n"); }