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Subject: Re: 5.13-rt1 + KVM = WARNING: at fs/eventfd.c:74 eventfd_signal()
To:     Paolo Bonzini <pbonzini@redhat.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        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>
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 <52348289-5d4d-f4a4-6fe3-f0c24cc6d9f9@redhat.com>
From:   Jason Wang <jasowang@redhat.com>
Message-ID: <987e69e1-21d7-625d-4e45-bc40857a526c@redhat.com>
Date:   Thu, 15 Jul 2021 14:45:14 +0800
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在 2021/7/15 下午1:58, Paolo Bonzini 写道:
> On 15/07/21 06:14, Jason Wang wrote:
>>> This obviously does not fly with PREEMPT_RT.  If eventfd_signal is
>>> preempted and an unrelated thread calls eventfd_signal, the result is
>>> a spurious WARN.  To avoid this, protect the percpu variable with a
>>> local_lock.
>>
>> But local_lock only disable migration not preemption.
>
> On mainline PREEMPT_RT, local_lock is an array of per-CPU spinlocks. 
> When two eventfd_signals run on the same CPU and one is preempted, the 
> spinlocks avoid that the second sees eventfd_wake_count > 0.
>
> Thanks,
>
> Paolo


Right, I see.

Thanks


>
>> Or anything I missed here?
>>
>> Thanks
>>
>>
>>>
>>> Reported-by: Daniel Bristot de Oliveira <bristot@redhat.com>
>>> Fixes: b5e683d5cab8 ("eventfd: track eventfd_signal() recursion depth")
>>> Cc: stable@vger.kernel.org
>>> Cc: He Zhe <zhe.he@windriver.com>
>>> Cc: Jens Axboe <axboe@kernel.dk>
>>> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>>>
>>> diff --git a/fs/eventfd.c b/fs/eventfd.c
>>> index e265b6dd4f34..7d27b6e080ea 100644
>>> --- a/fs/eventfd.c
>>> +++ b/fs/eventfd.c
>>> @@ -12,6 +12,7 @@
>>>  #include <linux/fs.h>
>>>  #include <linux/sched/signal.h>
>>>  #include <linux/kernel.h>
>>> +#include <linux/local_lock.h>
>>>  #include <linux/slab.h>
>>>  #include <linux/list.h>
>>>  #include <linux/spinlock.h>
>>> @@ -25,6 +26,7 @@
>>>  #include <linux/idr.h>
>>>  #include <linux/uio.h>
>>>
>>> +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);
>>>
>>>      return n;
>>>  }
>>>
>>
>