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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>
References: <df278db6-1fc0-3d42-9c0e-f5a085c6351e@redhat.com>
 <8dfc0ee9-b97a-8ca8-d057-31c8cad3f5b6@redhat.com>
 <f0254740-944d-201b-9a66-9db1fe480ca6@redhat.com>
 <475f84e2-78ee-1a24-ef57-b16c1f2651ed@redhat.com>
 <87pmv23lru.ffs@nanos.tec.linutronix.de>
From:   Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: 5.13-rt1 + KVM = WARNING: at fs/eventfd.c:74 eventfd_signal()
Message-ID: <810e01ef-9b71-5b44-8498-b8a377d4e51b@redhat.com>
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. */
>   
>