linux-kernel - Re: [Xen-devel] [PATCH 1/2] xen: fix race in xen_qlock

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Date:   Mon, 1 Oct 2018 10:58:40 +0200
From:   Juergen Gross <jgross@...e.com>
To:     Jan Beulich <JBeulich@...e.com>
Cc:     Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        xen-devel <xen-devel@...ts.xenproject.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        longman@...hat.com, mingo@...hat.com,
        lkml <linux-kernel@...r.kernel.org>, stable@...r.kernel.org,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [Xen-devel] [PATCH 1/2] xen: fix race in xen_qlock_wait()

On 01/10/2018 10:54, Jan Beulich wrote:
>>>> On 01.10.18 at 09:16, <jgross@...e.com> wrote:
>> In the following situation a vcpu waiting for a lock might not be
>> woken up from xen_poll_irq():
>>
>> CPU 1:                CPU 2:                      CPU 3:
>> takes a spinlock
>>                       tries to get lock
>>                       -> xen_qlock_wait()
>>                         -> xen_clear_irq_pending()
> 
> Doesn't the last line above ...
> 
>> frees the lock
>> -> xen_qlock_kick(cpu2)
> 
> ... need to be below here?

You are right, of course!

Thanks for noticing.


Juergen