Messages in this thread | | | From | "Eric W. Biederman" <> | Date | Mon, 27 Jun 2022 14:16:08 -0500 | Subject | Re: [PATCH v2] signal: break out of wait loops on kthread_stop() |
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"Jason A. Donenfeld" <Jason@zx2c4.com> writes:
> I was recently surprised to learn that msleep_interruptible(), > wait_for_completion_interruptible_timeout(), and related functions > simply hung when I called kthread_stop() on kthreads using them. The > solution to fixing the case with msleep_interruptible() was more simply > to move to schedule_timeout_interruptible(). Why? > > The reason is that msleep_interruptible(), and many functions just like > it, has a loop like this: > > while (timeout && !signal_pending(current)) > timeout = schedule_timeout_interruptible(timeout); > > The call to kthread_stop() woke up the thread, so schedule_timeout_ > interruptible() returned early, but because signal_pending() returned > true, it went back into another timeout, which was never woken up. > > This wait loop pattern is common to various pieces of code, and I > suspect that subtle misuse in a kthread that caused a deadlock in the > code I looked at last week is also found elsewhere. > > So this commit causes signal_pending() to return true when > kthread_stop() is called. This is already what's done for > TIF_NOTIFY_SIGNAL, for these same purposes of breaking out of wait > loops, so a similar KTHREAD_SHOULD_STOP check isn't too much > different.
Semantically this makes a lot of sense.
Bloating up signal_pending which is mainly called in non-kthread contexts is undesirable.
Instead could you modify kthread_stop to call set_notify_signal().
That is exactly what set_notify_signal is there for. When you don't actually have a signal but you want to break out of an interruptible loop. My last round of work in the area decoupled set_notify_signal from any other semantics.
It would be nice to get everything down so that we only need to test TIF_NOTIFY_SIGNAL in signal_pending. Unfortunately to do that I need to do something with task_sigpending, and it hasn't been important enough to weed through all of those details yet.
Eric
> Cc: Ingo Molnar <mingo@redhat.com> > Cc: Peter Zijlstra <peterz@infradead.org> > Cc: Eric W. Biederman <ebiederm@xmission.com> > Cc: Toke Høiland-Jørgensen <toke@redhat.com> > Cc: Kalle Valo <kvalo@kernel.org> > Cc: Johannes Berg <johannes@sipsolutions.net> > Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com> > --- > include/linux/kthread.h | 1 + > include/linux/sched/signal.h | 9 +++++++++ > kernel/kthread.c | 8 ++++++++ > 3 files changed, 18 insertions(+) > > diff --git a/include/linux/kthread.h b/include/linux/kthread.h > index 30e5bec81d2b..7061dde23237 100644 > --- a/include/linux/kthread.h > +++ b/include/linux/kthread.h > @@ -87,6 +87,7 @@ void kthread_bind(struct task_struct *k, unsigned int cpu); > void kthread_bind_mask(struct task_struct *k, const struct cpumask *mask); > int kthread_stop(struct task_struct *k); > bool kthread_should_stop(void); > +bool __kthread_should_stop(struct task_struct *k); > bool kthread_should_park(void); > bool __kthread_should_park(struct task_struct *k); > bool kthread_freezable_should_stop(bool *was_frozen); > diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h > index cafbe03eed01..08700c65b806 100644 > --- a/include/linux/sched/signal.h > +++ b/include/linux/sched/signal.h > @@ -11,6 +11,7 @@ > #include <linux/refcount.h> > #include <linux/posix-timers.h> > #include <linux/mm_types.h> > +#include <linux/kthread.h> > #include <asm/ptrace.h> > > /* > @@ -397,6 +398,14 @@ static inline int signal_pending(struct task_struct *p) > */ > if (unlikely(test_tsk_thread_flag(p, TIF_NOTIFY_SIGNAL))) > return 1; > + > + /* > + * Likewise, KTHREAD_SHOULD_STOP isn't really a signal, but it also > + * requires the same behavior, lest wait loops go forever. > + */ > + if (unlikely(__kthread_should_stop(p))) > + return 1; > + > return task_sigpending(p); > } > > diff --git a/kernel/kthread.c b/kernel/kthread.c > index 3c677918d8f2..80f6ba323060 100644 > --- a/kernel/kthread.c > +++ b/kernel/kthread.c > @@ -145,6 +145,14 @@ void free_kthread_struct(struct task_struct *k) > kfree(kthread); > } > > +bool __kthread_should_stop(struct task_struct *k) > +{ > + struct kthread *kthread = __to_kthread(k); > + > + return kthread && test_bit(KTHREAD_SHOULD_STOP, &kthread->flags); > +} > +EXPORT_SYMBOL_GPL(__kthread_should_stop); > + > /** > * kthread_should_stop - should this kthread return now? > *
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