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([2001:b07:6468:f312:5e2c:eb9a:a8b6:fd3e]) by smtp.gmail.com with ESMTPSA id y16sm5188687wrw.42.2021.07.14.22.58.29 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 14 Jul 2021 22:58:30 -0700 (PDT) To: Jason Wang , Daniel Bristot de Oliveira , Thomas Gleixner , Sebastian Andrzej Siewior , "Michael S. Tsirkin" , Juri Lelli Cc: LKML , Al Viro , He Zhe References: <8dfc0ee9-b97a-8ca8-d057-31c8cad3f5b6@redhat.com> <475f84e2-78ee-1a24-ef57-b16c1f2651ed@redhat.com> <4b53e2be-c38e-7509-dfcf-94f5bf5dcc10@redhat.com> From: Paolo Bonzini 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 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: <4b53e2be-c38e-7509-dfcf-94f5bf5dcc10@redhat.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 >> Fixes: b5e683d5cab8 ("eventfd: track eventfd_signal() recursion depth") >> Cc: stable@vger.kernel.org >> Cc: He Zhe >> Cc: Jens Axboe >> Signed-off-by: Paolo Bonzini >> >> 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 >>  #include >>  #include >> +#include >>  #include >>  #include >>  #include >> @@ -25,6 +26,7 @@ >>  #include >>  #include >> >> +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; >>  } >> >