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([2001:b07:6468:f312:63a7:c72e:ea0e:6045]) by smtp.gmail.com with ESMTPSA id r67sm10558378wma.6.2021.07.16.00.59.15 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 16 Jul 2021 00:59:16 -0700 (PDT) Subject: Re: 5.13-rt1 + KVM = WARNING: at fs/eventfd.c:74 eventfd_signal() To: Hillf Danton Cc: Jason Wang , Thomas Gleixner , Sebastian Andrzej Siewior , "Michael S. Tsirkin" , Juri Lelli , LKML , Al Viro , He Zhe References: <8dfc0ee9-b97a-8ca8-d057-31c8cad3f5b6@redhat.com> <475f84e2-78ee-1a24-ef57-b16c1f2651ed@redhat.com> <20210715102249.2205-1-hdanton@sina.com> <20210716020611.2288-1-hdanton@sina.com> <20210716075539.2376-1-hdanton@sina.com> From: Paolo Bonzini Message-ID: <8759e324-418b-493a-adee-236738cc3a4f@redhat.com> Date: Fri, 16 Jul 2021 09:59:15 +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: <20210716075539.2376-1-hdanton@sina.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 16/07/21 09:55, Hillf Danton wrote: > On Fri, 16 Jul 2021 08:54:58 +0200 Paolo Bonzini wrote: >> On 16/07/21 04:06, Hillf Danton wrote: >>>> With the patch: >>>> - no warn >>>> - continue using the VM normally... >>> Well with the patch applied, the VM works fine without the stuff protected >>> by the spin_lock_irqsave(), then without the patch why simply printing a >>> warning makes the VM dumb, given the warning is there actually also >>> preventing you from touching the lock. >> >> If the warning is triggered, eventfd_signal will not do the wakeup. > > [I am assuming we are not talking about the deadlock in the comment.] > > No preemption occured without the warning printed. > Why will the wakeup behavior change without peemption? Sorry, I don't follow. What I'm saying is that without the patch: * the warning only occurs if preemption occurs during the spin_lock_irqsave critical section (and therefore it can only occur in PREEMPT_RT kernels) * the warning causes an early return 0 that messes up the VM's networking Paolo