Re: [Xen-devel] [PATCH 1/2] xen: fix race in xen_qlock_wait()

[Jan Beulich]

>>> On 01.10.18 at 09:16, <jgross@xxxxxxxx> 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@xxxxxxxxxxxxxxx 
> Cc: Waiman.Long@xxxxxx 
> Cc: peterz@xxxxxxxxxxxxx 
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>

Patch itself
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

Jan