Re: [PATCH] percpu_rwsem: fix missed wakeup due to reordering of load

[Andrea Parri]

On Fri, Nov 30, 2018 at 08:40:56PM +0530, Prateek Sood wrote:
> In a scenario where cpu_hotplug_lock percpu_rw_semaphore is already
> acquired for read operation by P1 using percpu_down_read().
> 
> Now we have P1 in the path of releaseing the cpu_hotplug_lock and P2
> is in the process of acquiring cpu_hotplug_lock.
> 
> P1                                               P2
> percpu_up_read() path                      percpu_down_write() path
> 
>                                           rcu_sync_enter() //gp_state=GP_PASSED
> 
> rcu_sync_is_idle() //returns false        down_write(rw_sem)
> 
> __percpu_up_read()
> 
> [L] task = rcu_dereference(w->task) //NULL
> 
> smp_rmb()                                  [S] w->task = current
> 
>                                             smp_mb()
> 
>                                            [L] readers_active_check() //fails
> 					     schedule()
> 
> [S] __this_cpu_dec(read_count)
> 
> Since load of task can result in NULL. This can lead to missed wakeup
> in rcuwait_wake_up(). Above sequence violated the following constraint
> in rcuwait_wake_up():
> 
> 	 WAIT                WAKE
> [S] tsk = current	  [S] cond = true
> MB (A)	                    MB (B)
> [L] cond		  [L] tsk
> 
> This can happen as smp_rmb() in rcuwait_wake_up() will provide ordering
> of load before barrier with load and store after barrier for arm64
> architecture. Here the requirement is to order store before smp_rmb()
> with load after the smp_rmb().
> 
> For the usage of rcuwait_wake_up() in __percpu_up_read() full barrier
> (smp_mb) is required to complete the constraint of rcuwait_wake_up().
> 
> Signed-off-by: Prateek Sood <prsood@xxxxxxxxxxxxxx>

I know this is going to sound ridiculous (coming from me or from
the Italian that I am), but it looks like we could both work on
our English. ;-)

But the fix seems correct to me:

Reviewed-by: Andrea Parri <andrea.parri@xxxxxxxxxxxxxxxxxxxx>

It might be a good idea to integrate this fix with fixes to the
inline comments/annotations: for example, I see that the comment
in rcuwait_wake_up() mentions a non-existing rcuwait_trywake();
moreover, the memory-barrier annotation "B" is used also for the
smp_mb() preceding the __this_cpu_dec() in __percpu_up_read().

  Andrea


> ---
>  kernel/exit.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/exit.c b/kernel/exit.c
> index f1d74f0..a10820d 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -306,7 +306,7 @@ void rcuwait_wake_up(struct rcuwait *w)
>  	 *        MB (A)	      MB (B)
>  	 *    [L] cond		  [L] tsk
>  	 */
> -	smp_rmb(); /* (B) */
> +	smp_mb(); /* (B) */
>  
>  	/*
>  	 * Avoid using task_rcu_dereference() magic as long as we are careful,
> -- 
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>