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From:   Nicolas Saenz Julienne <nsaenzju@redhat.com>
To:     bristot@kernel.org
Cc:     axboe@kernel.dk, bigeasy@linutronix.de,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        pbonzini@redhat.com, stable@vger.kernel.org, tglx@linutronix.de,
        viro@zeniv.linux.org.uk, zhe.he@windriver.com,
        Nicolas Saenz Julienne <nsaenzju@redhat.com>
Subject: Re: [PATCH] eventfd: protect eventfd_wake_count with a local_lock
Date:   Fri, 23 Jul 2021 16:53:04 +0200
Message-Id: <20210723145303.272822-1-nsaenzju@redhat.com>
In-Reply-To: <523c91c4a30f21295508004c81cd2e46ccc37dc2.1626680553.git.bristot@kernel.org>
References: <523c91c4a30f21295508004c81cd2e46ccc37dc2.1626680553.git.bristot@kernel.org>
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On Mon, 19 Jul 2021 09:54:52 +0200, Daniel Bristot de Oliveira wrote:
> eventfd_signal assumes that spin_lock_irqsave/spin_unlock_irqrestore is
> non-preemptable and therefore increments and decrements the percpu
> variable inside the critical section.
> 
> 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.
> 
> Reported-by: Daniel Bristot de Oliveira <bristot@kernel.org>
> Fixes: b5e683d5cab8 ("eventfd: track eventfd_signal() recursion depth")
> Cc: He Zhe <zhe.he@windriver.com>
> Cc: Jens Axboe <axboe@kernel.dk>
> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Cc: stable@vger.kernel.org
> Cc: linux-fsdevel@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Co-developed-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Daniel Bristot de Oliveira <bristot@kernel.org>
> ---

Tested-by: Nicolas Saenz Julienne <nsaenzju@redhat.com>

Thanks!

--
Nicolás Sáenz