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Subject: Re: [PATCH] eventfd: Make signal recursion protection a task bit
To:     Thomas Gleixner <tglx@linutronix.de>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Jason Wang <jasowang@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>,
        Peter Zijlstra <peterz@infradead.org>
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From:   Daniel Bristot de Oliveira <bristot@redhat.com>
Message-ID: <d99d8f37-b217-266b-b7bb-6128dafb549b@redhat.com>
Date:   Thu, 29 Jul 2021 21:23:45 +0200
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On 7/29/21 1:01 PM, Thomas Gleixner wrote:
> The recursion protection for eventfd_signal() is based on a per CPU
> variable and relies on the !RT semantics of spin_lock_irqsave() for
> protecting this per CPU variable. On RT kernels spin_lock_irqsave() neither
> disables preemption nor interrupts which allows the spin lock held section
> to be preempted. If the preempting task invokes eventfd_signal() as well,
> then the recursion warning triggers.
> 
> Paolo suggested to protect the per CPU variable with a local lock, but
> that's heavyweight and actually not necessary. The goal of this protection
> is to prevent the task stack from overflowing, which can be achieved with a
> per task recursion protection as well.
> 
> Replace the per CPU variable with a per task bit similar to other recursion
> protection bits like task_struct::in_page_owner. This works on both !RT and
> RT kernels and removes as a side effect the extra per CPU storage.
> 
> No functional change for !RT kernels.
> 
> Reported-by: Daniel Bristot de Oliveira <bristot@redhat.com>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

Tested-by: Daniel Bristot de Oliveira <bristot@redhat.com>

Thanks!