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SubjectRe: [Xen-devel] [PATCH 1/2] xen: fix race in xen_qlock_wait()
>>> On 01.10.18 at 09:16, <jgross@suse.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@vger.kernel.org
> Cc: Waiman.Long@hp.com
> Cc: peterz@infradead.org
> Signed-off-by: Juergen Gross <jgross@suse.com>

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

Jan


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