Re: [PATCH] xen/pvcalls: fix potential endless loop in pvcalls-front.c

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

On 11/06/2017 05:17 PM, Stefano Stabellini wrote:
> mutex_trylock() returns 1 if you take the lock and 0 if not. Assume you
> take in_mutex on the first try, but you can't take out_mutex. Next times
> you call mutex_trylock() in_mutex is going to fail. It's an endless
> loop.
>
> Solve the problem by moving the two mutex_trylock calls to two separate
> loops.
>
> Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
> Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
> CC: boris.ostrovsky@oracle.com
> CC: jgross@suse.com

Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

BTW, I assume that when recvmsg or sendmsg is called no other locks are
held, right? (otherwise we'd get into a deadlock.)

-boris

> ---
>  drivers/xen/pvcalls-front.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/xen/pvcalls-front.c b/drivers/xen/pvcalls-front.c
> index 0c1ec68..047dce7 100644
> --- a/drivers/xen/pvcalls-front.c
> +++ b/drivers/xen/pvcalls-front.c
> @@ -1048,8 +1048,9 @@ int pvcalls_front_release(struct socket *sock)
>  		 * is set to NULL -- we only need to wait for the existing
>  		 * waiters to return.
>  		 */
> -		while (!mutex_trylock(&map->active.in_mutex) ||
> -			   !mutex_trylock(&map->active.out_mutex))
> +		while (!mutex_trylock(&map->active.in_mutex))
> +			cpu_relax();
> +		while (!mutex_trylock(&map->active.out_mutex))
>  			cpu_relax();
>  
>  		pvcalls_front_free_map(bedata, map);


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