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SubjectRe: 5.13-rt1 + KVM = WARNING: at fs/eventfd.c:74 eventfd_signal()
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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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