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workqueue: Remove the warning in wq_worker_sleeping()
The kernel test robot triggered a warning with the following race:
task-ctx A interrupt-ctx B
worker
-> process_one_work()
-> work_item()
-> schedule();
-> sched_submit_work()
-> wq_worker_sleeping()
-> ->sleeping = 1
atomic_dec_and_test(nr_running)
__schedule(); *interrupt*
async_page_fault()
-> local_irq_enable();
-> schedule();
-> sched_submit_work()
-> wq_worker_sleeping()
-> if (WARN_ON(->sleeping)) return
-> __schedule()
-> sched_update_worker()
-> wq_worker_running()
-> atomic_inc(nr_running);
-> ->sleeping = 0;
-> sched_update_worker()
-> wq_worker_running()
if (!->sleeping) return
In this context the warning is pointless everything is fine.
An interrupt before wq_worker_sleeping() will perform the ->sleeping
assignment (0 -> 1 > 0) twice.
An interrupt after wq_worker_sleeping() will trigger the warning and
nr_running will be decremented (by A) and incremented once (only by B, A
will skip it). This is the case until the ->sleeping is zeroed again in
wq_worker_running().
Remove the WARN statement because this condition may happen. Document
that preemption around wq_worker_sleeping() needs to be disabled to
protect ->sleeping and not just as an optimisation.
Fixes: 6d25be5782
("sched/core, workqueues: Distangle worker accounting from rq lock")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Tejun Heo <tj@kernel.org>
Link: https://lkml.kernel.org/r/20200327074308.GY11705@shao2-debian
This commit is contained in:
parent
111688ca1c
commit
62849a9612
2 changed files with 6 additions and 3 deletions
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@ -4120,7 +4120,8 @@ static inline void sched_submit_work(struct task_struct *tsk)
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* it wants to wake up a task to maintain concurrency.
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* As this function is called inside the schedule() context,
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* we disable preemption to avoid it calling schedule() again
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* in the possible wakeup of a kworker.
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* in the possible wakeup of a kworker and because wq_worker_sleeping()
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* requires it.
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*/
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if (tsk->flags & (PF_WQ_WORKER | PF_IO_WORKER)) {
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preempt_disable();
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@ -858,7 +858,8 @@ void wq_worker_running(struct task_struct *task)
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* @task: task going to sleep
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*
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* This function is called from schedule() when a busy worker is
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* going to sleep.
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* going to sleep. Preemption needs to be disabled to protect ->sleeping
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* assignment.
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*/
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void wq_worker_sleeping(struct task_struct *task)
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{
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@ -875,7 +876,8 @@ void wq_worker_sleeping(struct task_struct *task)
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pool = worker->pool;
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if (WARN_ON_ONCE(worker->sleeping))
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/* Return if preempted before wq_worker_running() was reached */
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if (worker->sleeping)
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return;
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worker->sleeping = 1;
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