RE: [PATCH 02/12] evtchn: avoid race in get_xen_consumer()

[Paul Durrant <xadimgnik@xxxxxxxxx>]

> -----Original Message-----
> From: Xen-devel <xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx> On Behalf Of Jan 
> Beulich
> Sent: 28 September 2020 11:57
> To: xen-devel@xxxxxxxxxxxxxxxxxxxx
> Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>; George Dunlap 
> <George.Dunlap@xxxxxxxxxxxxx>; Ian
> Jackson <iwj@xxxxxxxxxxxxxx>; Julien Grall <julien@xxxxxxx>; Wei Liu 
> <wl@xxxxxxx>; Stefano Stabellini
> <sstabellini@xxxxxxxxxx>
> Subject: [PATCH 02/12] evtchn: avoid race in get_xen_consumer()
> 
> There's no global lock around the updating of this global piece of data.
> Make use of cmpxchg() to avoid two entities racing with their updates.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> TBD: Initially I used cmpxchgptr() here, until realizing Arm doesn't
>      have it. It's slightly more type-safe than cmpxchg() (requiring
>      all arguments to actually be pointers), so I now wonder whether Arm
>      should gain it (perhaps simply by moving the x86 implementation to
>      xen/lib.h), or whether we should purge it from x86 as being
>      pointless.
> 
> --- a/xen/common/event_channel.c
> +++ b/xen/common/event_channel.c
> @@ -57,7 +57,8 @@
>   * with a pointer, we stash them dynamically in a small lookup array which
>   * can be indexed by a small integer.
>   */
> -static xen_event_channel_notification_t xen_consumers[NR_XEN_CONSUMERS];
> +static xen_event_channel_notification_t __read_mostly
> +    xen_consumers[NR_XEN_CONSUMERS];
> 
>  /* Default notification action: wake up from wait_on_xen_event_channel(). */
>  static void default_xen_notification_fn(struct vcpu *v, unsigned int port)
> @@ -81,7 +82,7 @@ static uint8_t get_xen_consumer(xen_even
>      for ( i = 0; i < ARRAY_SIZE(xen_consumers); i++ )
>      {
>          if ( xen_consumers[i] == NULL )
> -            xen_consumers[i] = fn;
> +            (void)cmpxchg(&xen_consumers[i], NULL, fn);
>          if ( xen_consumers[i] == fn )

Why not use the return from cmpxchg() to determine success and break out of the 
loop rather than re-accessing the global array?

  Paul

>              break;
>      }
>