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

[Davidlohr Bueso <dbueso@suse.de>]

On 2018-11-30 07:10, 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
> 

Hmm yeah we don't want rcu_wake_up() to get hoisted over the 
__this_cpu_dec(read_count). The smp_rmb() does not make sense to me here 
in the first place. Did you run into this scenario by code inspection or 
you actually it the issue?

Thanks,
Davidlohr