From: Tetsuo Handa penguin-kernel@I-love.SAKURA.ne.jp
[ Upstream commit c0feea594e058223973db94c1c32a830c9807c86 ]
Like Hillf Danton mentioned
syzbot should have been able to catch cancel_work_sync() in work context by checking lockdep_map in __flush_work() for both flush and cancel.
in [1], being unable to report an obvious deadlock scenario shown below is broken. From locking dependency perspective, sync version of cancel request should behave as if flush request, for it waits for completion of work if that work has already started execution.
---------- #include <linux/module.h> #include <linux/sched.h> static DEFINE_MUTEX(mutex); static void work_fn(struct work_struct *work) { schedule_timeout_uninterruptible(HZ / 5); mutex_lock(&mutex); mutex_unlock(&mutex); } static DECLARE_WORK(work, work_fn); static int __init test_init(void) { schedule_work(&work); schedule_timeout_uninterruptible(HZ / 10); mutex_lock(&mutex); cancel_work_sync(&work); mutex_unlock(&mutex); return -EINVAL; } module_init(test_init); MODULE_LICENSE("GPL"); ----------
The check this patch restores was added by commit 0976dfc1d0cd80a4 ("workqueue: Catch more locking problems with flush_work()").
Then, lockdep's crossrelease feature was added by commit b09be676e0ff25bd ("locking/lockdep: Implement the 'crossrelease' feature"). As a result, this check was once removed by commit fd1a5b04dfb899f8 ("workqueue: Remove now redundant lock acquisitions wrt. workqueue flushes").
But lockdep's crossrelease feature was removed by commit e966eaeeb623f099 ("locking/lockdep: Remove the cross-release locking checks"). At this point, this check should have been restored.
Then, commit d6e89786bed977f3 ("workqueue: skip lockdep wq dependency in cancel_work_sync()") introduced a boolean flag in order to distinguish flush_work() and cancel_work_sync(), for checking "struct workqueue_struct" dependency when called from cancel_work_sync() was causing false positives.
Then, commit 87915adc3f0acdf0 ("workqueue: re-add lockdep dependencies for flushing") tried to restore "struct work_struct" dependency check, but by error checked this boolean flag. Like an example shown above indicates, "struct work_struct" dependency needs to be checked for both flush_work() and cancel_work_sync().
Link: https://lkml.kernel.org/r/20220504044800.4966-1-hdanton@sina.com [1] Reported-by: Hillf Danton hdanton@sina.com Suggested-by: Lai Jiangshan jiangshanlai@gmail.com Fixes: 87915adc3f0acdf0 ("workqueue: re-add lockdep dependencies for flushing") Cc: Johannes Berg johannes.berg@intel.com Signed-off-by: Tetsuo Handa penguin-kernel@I-love.SAKURA.ne.jp Signed-off-by: Tejun Heo tj@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/workqueue.c | 6 ++---- 1 file changed, 2 insertions(+), 4 deletions(-)
diff --git a/kernel/workqueue.c b/kernel/workqueue.c index b1bb6cb5802e..4ea2f7fd20ce 100644 --- a/kernel/workqueue.c +++ b/kernel/workqueue.c @@ -2917,10 +2917,8 @@ static bool __flush_work(struct work_struct *work, bool from_cancel) if (WARN_ON(!work->func)) return false;
- if (!from_cancel) { - lock_map_acquire(&work->lockdep_map); - lock_map_release(&work->lockdep_map); - } + lock_map_acquire(&work->lockdep_map); + lock_map_release(&work->lockdep_map);
if (start_flush_work(work, &barr, from_cancel)) { wait_for_completion(&barr.done);