From: Andrea Parri <andrea.parri@amarulasolutions.com>
To: Prateek Sood <prsood@codeaurora.org>
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: Wed, 12 Dec 2018 16:28:52 +0100 [thread overview]
Message-ID: <20181212152852.GA11111@andrea> (raw)
In-Reply-To: <1543590656-7157-1-git-send-email-prsood@codeaurora.org>
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();
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,
> --
> 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:[~2018-12-12 15:29 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 [this message]
2018-12-21 6:35 ` Prateek Sood
2019-01-21 11:25 ` [tip:locking/core] sched/wait: Fix rcuwait_wake_up() ordering tip-bot for Prateek Sood
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