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

[Prateek Sood]

On 12/12/2018 08:58 PM, Andrea Parri wrote:
> 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();
Ok, I will update the comment in next version of the patch.

> moreover, the memory-barrier annotation "B" is used also for the
> smp_mb() preceding the __this_cpu_dec() in __percpu_up_read().
In this annotation "B" is corresponding to annotation "A" in
rcuwait_wait_event(). So this seems to be correct.

> 
>   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,
>> -- 
>> Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc., 
>> is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.
>>


-- 
Qualcomm India Private Limited, on behalf of Qualcomm Innovation
Center, Inc., is a member of Code Aurora Forum, a Linux Foundation
Collaborative Project