From: Paolo Bonzini <pbonzini@redhat.com>
To: Thomas Gleixner <tglx@linutronix.de>,
Jason Wang <jasowang@redhat.com>,
Daniel Bristot de Oliveira <bristot@redhat.com>,
Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
"Michael S. Tsirkin" <mst@redhat.com>,
Juri Lelli <jlelli@redhat.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
Al Viro <viro@zeniv.linux.org.uk>, He Zhe <zhe.he@windriver.com>,
Jens Axboe <axboe@fb.com>
Subject: Re: 5.13-rt1 + KVM = WARNING: at fs/eventfd.c:74 eventfd_signal()
Date: Wed, 28 Jul 2021 12:21:20 +0200 [thread overview]
Message-ID: <810e01ef-9b71-5b44-8498-b8a377d4e51b@redhat.com> (raw)
In-Reply-To: <87pmv23lru.ffs@nanos.tec.linutronix.de>
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. */
>
>
next prev parent reply other threads:[~2021-07-28 10:21 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-07-14 8:01 5.13-rt1 + KVM = WARNING: at fs/eventfd.c:74 eventfd_signal() Daniel Bristot de Oliveira
2021-07-14 8:10 ` Paolo Bonzini
2021-07-14 9:23 ` Jason Wang
2021-07-14 10:35 ` Paolo Bonzini
2021-07-14 10:41 ` Michael S. Tsirkin
2021-07-14 10:44 ` Paolo Bonzini
2021-07-14 12:20 ` Daniel Bristot de Oliveira
2021-07-15 4:14 ` Jason Wang
2021-07-15 5:58 ` Paolo Bonzini
2021-07-15 6:45 ` Jason Wang
2021-07-15 8:22 ` Daniel Bristot de Oliveira
2021-07-15 8:44 ` He Zhe
2021-07-15 9:51 ` Paolo Bonzini
2021-07-15 10:10 ` He Zhe
2021-07-15 11:05 ` Paolo Bonzini
2021-07-16 2:26 ` Jason Wang
2021-07-16 2:43 ` He Zhe
2021-07-16 2:46 ` Jason Wang
2021-07-15 9:46 ` Paolo Bonzini
2021-07-15 12:34 ` Daniel Bristot de Oliveira
[not found] ` <20210715102249.2205-1-hdanton@sina.com>
2021-07-15 12:31 ` Daniel Bristot de Oliveira
[not found] ` <20210716020611.2288-1-hdanton@sina.com>
2021-07-16 6:54 ` Paolo Bonzini
[not found] ` <20210716075539.2376-1-hdanton@sina.com>
2021-07-16 7:59 ` Paolo Bonzini
[not found] ` <20210716093725.2438-1-hdanton@sina.com>
2021-07-16 11:55 ` Paolo Bonzini
[not found] ` <20210718124219.1521-1-hdanton@sina.com>
2021-07-19 15:38 ` Paolo Bonzini
[not found] ` <20210721070452.1008-1-hdanton@sina.com>
2021-07-21 7:25 ` Thomas Gleixner
[not found] ` <20210721101119.1103-1-hdanton@sina.com>
2021-07-21 10:59 ` Paolo Bonzini
[not found] ` <20210723022356.1301-1-hdanton@sina.com>
2021-07-23 7:59 ` Paolo Bonzini
[not found] ` <20210723094830.1375-1-hdanton@sina.com>
2021-07-23 10:56 ` Paolo Bonzini
[not found] ` <20210724043320.1654-1-hdanton@sina.com>
2021-07-26 11:03 ` Paolo Bonzini
2021-07-28 8:06 ` Thomas Gleixner
2021-07-28 10:21 ` Paolo Bonzini [this message]
2021-07-28 19:07 ` Thomas Gleixner
2021-07-29 11:01 ` [PATCH] eventfd: Make signal recursion protection a task bit Thomas Gleixner
2021-07-29 14:32 ` Daniel Bristot de Oliveira
2021-07-29 19:23 ` Daniel Bristot de Oliveira
2021-08-26 7:03 ` Jason Wang
2021-08-27 23:41 ` [tip: sched/core] " tip-bot2 for Thomas Gleixner
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