Messages in this thread |  | | From | Paolo Bonzini <> | Subject | Re: 5.13-rt1 + KVM = WARNING: at fs/eventfd.c:74 eventfd_signal() | Date | Wed, 28 Jul 2021 12:21:20 +0200 |
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On 28/07/21 10:06, Thomas Gleixner wrote: > On Wed, Jul 14 2021 at 12:35, Paolo Bonzini wrote: >> On 14/07/21 11:23, Jason Wang wrote: >> >> +static local_lock_t eventfd_wake_lock = INIT_LOCAL_LOCK(eventfd_wake_lock); >> DEFINE_PER_CPU(int, eventfd_wake_count); >> >> static DEFINE_IDA(eventfd_ida); >> @@ -71,8 +73,11 @@ __u64 eventfd_signal(struct eventfd_ctx *ctx, __u64 n) >> * it returns true, the eventfd_signal() call should be deferred to a >> * safe context. >> */ >> - if (WARN_ON_ONCE(this_cpu_read(eventfd_wake_count))) >> + local_lock(&eventfd_wake_lock); >> + if (WARN_ON_ONCE(this_cpu_read(eventfd_wake_count))) { >> + local_unlock(&eventfd_wake_lock); >> return 0; >> + } >> >> spin_lock_irqsave(&ctx->wqh.lock, flags); >> this_cpu_inc(eventfd_wake_count); >> @@ -83,6 +88,7 @@ __u64 eventfd_signal(struct eventfd_ctx *ctx, __u64 n) >> wake_up_locked_poll(&ctx->wqh, EPOLLIN); >> this_cpu_dec(eventfd_wake_count); >> spin_unlock_irqrestore(&ctx->wqh.lock, flags); >> + local_unlock(&eventfd_wake_lock); > > Yes, that cures it, but if you think about what this wants to prevent, > then having the recursion counter per CPU is at least suboptimal. > > Something like the untested below perhaps?
Yes, that works (it should just be #ifdef CONFIG_EVENTFD).
On !PREEMPT_RT the percpu variable consumes memory while your patch uses none (there are plenty of spare bits in current), but it is otherwise basically the same. On PREEMPT_RT the local_lock is certainly more expensive.
Thanks,
Paolo
> Thanks, > > tglx > --- > --- a/fs/aio.c > +++ b/fs/aio.c > @@ -1695,7 +1695,7 @@ static int aio_poll_wake(struct wait_que > list_del(&iocb->ki_list); > iocb->ki_res.res = mangle_poll(mask); > req->done = true; > - if (iocb->ki_eventfd && eventfd_signal_count()) { > + if (iocb->ki_eventfd && !eventfd_signal_allowed()) { > iocb = NULL; > INIT_WORK(&req->work, aio_poll_put_work); > schedule_work(&req->work); > --- a/fs/eventfd.c > +++ b/fs/eventfd.c > @@ -25,8 +25,6 @@ > #include <linux/idr.h> > #include <linux/uio.h> > > -DEFINE_PER_CPU(int, eventfd_wake_count); > - > static DEFINE_IDA(eventfd_ida); > > struct eventfd_ctx { > @@ -67,21 +65,21 @@ struct eventfd_ctx { > * Deadlock or stack overflow issues can happen if we recurse here > * through waitqueue wakeup handlers. If the caller users potentially > * nested waitqueues with custom wakeup handlers, then it should > - * check eventfd_signal_count() before calling this function. If > - * it returns true, the eventfd_signal() call should be deferred to a > + * check eventfd_signal_allowed() before calling this function. If > + * it returns false, the eventfd_signal() call should be deferred to a > * safe context. > */ > - if (WARN_ON_ONCE(this_cpu_read(eventfd_wake_count))) > + if (WARN_ON_ONCE(current->in_eventfd_signal)) > return 0; > > spin_lock_irqsave(&ctx->wqh.lock, flags); > - this_cpu_inc(eventfd_wake_count); > + current->in_eventfd_signal = 1; > if (ULLONG_MAX - ctx->count < n) > n = ULLONG_MAX - ctx->count; > ctx->count += n; > if (waitqueue_active(&ctx->wqh)) > wake_up_locked_poll(&ctx->wqh, EPOLLIN); > - this_cpu_dec(eventfd_wake_count); > + current->in_eventfd_signal = 0; > spin_unlock_irqrestore(&ctx->wqh.lock, flags); > > return n; > --- a/include/linux/eventfd.h > +++ b/include/linux/eventfd.h > @@ -43,11 +43,9 @@ int eventfd_ctx_remove_wait_queue(struct > __u64 *cnt); > void eventfd_ctx_do_read(struct eventfd_ctx *ctx, __u64 *cnt); > > -DECLARE_PER_CPU(int, eventfd_wake_count); > - > -static inline bool eventfd_signal_count(void) > +static inline bool eventfd_signal_allowed(void) > { > - return this_cpu_read(eventfd_wake_count); > + return !current->in_eventfd_signal; > } > > #else /* CONFIG_EVENTFD */ > @@ -78,9 +76,9 @@ static inline int eventfd_ctx_remove_wai > return -ENOSYS; > } > > -static inline bool eventfd_signal_count(void) > +static inline bool eventfd_signal_allowed(void) > { > - return false; > + return true; > } > > static inline void eventfd_ctx_do_read(struct eventfd_ctx *ctx, __u64 *cnt) > --- a/include/linux/sched.h > +++ b/include/linux/sched.h > @@ -863,6 +863,8 @@ struct task_struct { > /* Used by page_owner=on to detect recursion in page tracking. */ > unsigned in_page_owner:1; > #endif > + /* Recursion prevention for eventfd_signal() */ > + unsigned in_eventfd_signal:1; > > unsigned long atomic_flags; /* Flags requiring atomic access. */ > >
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