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

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Date:   Mon, 01 Oct 2018 02:54:13 -0600
From:   "Jan Beulich" <JBeulich@...e.com>
To:     "Juergen Gross" <jgross@...e.com>
Cc:     "Borislav Petkov" <bp@...en8.de>,
        "Peter Zijlstra" <peterz@...radead.org>,
        "the arch/x86 maintainers" <x86@...nel.org>, <tglx@...utronix.de>,
        "xen-devel" <xen-devel@...ts.xenproject.org>,
        "Boris Ostrovsky" <boris.ostrovsky@...cle.com>,
        <longman@...hat.com>, <mingo@...hat.com>,
        <linux-kernel@...r.kernel.org>, <stable@...r.kernel.org>,
        <hpa@...or.com>
Subject: Re: [Xen-devel] [PATCH 1/2] xen: fix race in xen_qlock_wait()

>>> 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?

> takes lock again
>                                                   tries to get lock
>                                                   -> *lock = _Q_SLOW_VAL
>                         -> *lock == _Q_SLOW_VAL ?
>                         -> xen_poll_irq()
> frees the lock
> -> xen_qlock_kick(cpu3)
> 
> And cpu 2 will sleep forever.
> 
> This can be avoided easily by modifying xen_qlock_wait() to call
> xen_poll_irq() only if the related irq was not pending and to call
> xen_clear_irq_pending() only if it was pending.
> 
> Cc: stable@...r.kernel.org 
> Cc: Waiman.Long@...com 
> Cc: peterz@...radead.org 
> Signed-off-by: Juergen Gross <jgross@...e.com>

Patch itself
Reviewed-by: Jan Beulich <jbeulich@...e.com>

Jan