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To:     Jason Wang <jasowang@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>
References: <df278db6-1fc0-3d42-9c0e-f5a085c6351e@redhat.com>
 <8dfc0ee9-b97a-8ca8-d057-31c8cad3f5b6@redhat.com>
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 <475f84e2-78ee-1a24-ef57-b16c1f2651ed@redhat.com>
 <4b53e2be-c38e-7509-dfcf-94f5bf5dcc10@redhat.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: 5.13-rt1 + KVM = WARNING: at fs/eventfd.c:74 eventfd_signal()
Message-ID: <52348289-5d4d-f4a4-6fe3-f0c24cc6d9f9@redhat.com>
Date:   Thu, 15 Jul 2021 07:58:28 +0200
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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

> 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;
>>  }
>>
>