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

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