linux-kernel - Re: [PATCH] KVM: selftests: Always run vCPU thread with blocked SIG

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Message-ID: <84c52ebe-58a2-6188-270e-c86409e44fa3@redhat.com>
Date:   Tue, 20 Apr 2021 18:24:50 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH] KVM: selftests: Always run vCPU thread with blocked
 SIG_IPI

On 20/04/21 17:32, Peter Xu wrote:
> On Tue, Apr 20, 2021 at 10:37:39AM -0400, Peter Xu wrote:
>> On Tue, Apr 20, 2021 at 04:16:14AM -0400, Paolo Bonzini wrote:
>>> The main thread could start to send SIG_IPI at any time, even before signal
>>> blocked on vcpu thread.  Therefore, start the vcpu thread with the signal
>>> blocked.
>>>
>>> Without this patch, on very busy cores the dirty_log_test could fail directly
>>> on receiving a SIGUSR1 without a handler (when vcpu runs far slower than main).
>>>
>>> Reported-by: Peter Xu <peterx@...hat.com>
>>> Cc: stable@...r.kernel.org
>>> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
>>
>> Yes, indeed better! :)
>>
>> Reviewed-by: Peter Xu <peterx@...hat.com>
> 
> I just remembered one thing: this will avoid program quits, but still we'll get
> the signal missing.

In what sense the signal will be missing?  As long as the thread exists, 
the signal will be accepted (but not delivered because it is blocked); 
it will then be delivered on the first KVM_RUN.

Paolo

   From that pov I slightly prefer the old patch.  However
> not a big deal so far as only dirty ring uses SIG_IPI, so there's always ring
> full which will just delay the kick. It's just we need to remember this when we
> extend IPI to non-dirty-ring tests as the kick is prone to be lost then.
> 
> Thanks,
>