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

[Juergen Gross]

Correcting Waiman's mail address

On 01/10/2018 09:16, Juergen Gross 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()
> frees the lock
> -> xen_qlock_kick(cpu2)
> 
> 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: longman@xxxxxxxxxx
> Cc: peterz@xxxxxxxxxxxxx
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> ---
>  arch/x86/xen/spinlock.c | 15 +++++----------
>  1 file changed, 5 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/x86/xen/spinlock.c b/arch/x86/xen/spinlock.c
> index 973f10e05211..cd210a4ba7b1 100644
> --- a/arch/x86/xen/spinlock.c
> +++ b/arch/x86/xen/spinlock.c
> @@ -45,17 +45,12 @@ static void xen_qlock_wait(u8 *byte, u8 val)
>  	if (irq == -1)
>  		return;
>  
> -	/* clear pending */
> -	xen_clear_irq_pending(irq);
> -	barrier();
> +	/* If irq pending already clear it and return. */
> +	if (xen_test_irq_pending(irq)) {
> +		xen_clear_irq_pending(irq);
> +		return;
> +	}
>  
> -	/*
> -	 * We check the byte value after clearing pending IRQ to make sure
> -	 * that we won't miss a wakeup event because of the clearing.
> -	 *
> -	 * The sync_clear_bit() call in xen_clear_irq_pending() is atomic.
> -	 * So it is effectively a memory barrier for x86.
> -	 */
>  	if (READ_ONCE(*byte) != val)
>  		return;
>  
>