From: Prateek Sood <prsood@codeaurora.org>
To: Andrea Parri <andrea.parri@amarulasolutions.com>
Cc: peterz@infradead.org, mingo@redhat.com, dbueso@suse.de,
linux-kernel@vger.kernel.org, sramana@codeaurora.org
Subject: Re: [PATCH] percpu_rwsem: fix missed wakeup due to reordering of load
Date: Fri, 21 Dec 2018 12:05:51 +0530 [thread overview]
Message-ID: <fa769c59-df1f-fab1-a574-594a0b1922d8@codeaurora.org> (raw)
In-Reply-To: <20181212152852.GA11111@andrea>
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@codeaurora.org>
>
> 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@amarulasolutions.com>
>
> 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
next prev parent reply other threads:[~2018-12-21 6:36 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-11-30 15:10 [PATCH] percpu_rwsem: fix missed wakeup due to reordering of load Prateek Sood
2018-12-03 6:38 ` Davidlohr Bueso
2018-12-03 19:36 ` Prateek Sood
2018-12-12 14:26 ` Prateek Sood
2018-12-12 15:31 ` Davidlohr Bueso
2018-12-21 7:29 ` Prateek Sood
2018-12-21 9:45 ` Andrea Parri
2018-12-12 15:28 ` Andrea Parri
2018-12-21 6:35 ` Prateek Sood [this message]
2019-01-21 11:25 ` [tip:locking/core] sched/wait: Fix rcuwait_wake_up() ordering tip-bot for Prateek Sood
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