Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
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>
References: <df278db6-1fc0-3d42-9c0e-f5a085c6351e@redhat.com>
 <8dfc0ee9-b97a-8ca8-d057-31c8cad3f5b6@redhat.com>
 <f0254740-944d-201b-9a66-9db1fe480ca6@redhat.com>
 <475f84e2-78ee-1a24-ef57-b16c1f2651ed@redhat.com>
 <87pmv23lru.ffs@nanos.tec.linutronix.de>
 <810e01ef-9b71-5b44-8498-b8a377d4e51b@redhat.com> <875ywujlzx.ffs@tglx>
 <87wnp9idso.ffs@tglx>
From:   Daniel Bristot de Oliveira <bristot@redhat.com>
Message-ID: <4ebcc02b-4d42-ce69-c515-f27ce4db118c@redhat.com>
Date:   Thu, 29 Jul 2021 16:32:40 +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: <87wnp9idso.ffs@tglx>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Precedence: bulk

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>

Testing....

Thanks!
-- Daniel