Re: [Patch] rwsem: fix rwsem_is_locked() bug

From: Andrew Morton
Date: Wed Sep 30 2009 - 19:09:47 EST


On Tue, 29 Sep 2009 23:19:02 -0400
Amerigo Wang <amwang@xxxxxxxxxx> wrote:

>
> rwsem_is_locked() tests ->activity without locks, so we should always
> keep ->activity consistent. However, the code in __rwsem_do_wake()
> breaks this rule, it updates ->activity after _all_ readers waken up,
> this may give some reader a wrong ->activity value, thus cause
> rwsem_is_locked() behaves wrong.
>
> Brian has a kernel module to reproduce this, I can include it
> if any of you need. Of course, with Brian's approval.
>
> With this patch applied, I can't trigger that bug any more.
>

Changelog doesn't describe the bug well.

>
> ---
> diff --git a/lib/rwsem-spinlock.c b/lib/rwsem-spinlock.c
> index 9df3ca5..44e4484 100644
> --- a/lib/rwsem-spinlock.c
> +++ b/lib/rwsem-spinlock.c
> @@ -49,7 +49,6 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wakewrite)
> {
> struct rwsem_waiter *waiter;
> struct task_struct *tsk;
> - int woken;
>
> waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
>
> @@ -78,24 +77,21 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wakewrite)
>
> /* grant an infinite number of read locks to the front of the queue */
> dont_wake_writers:
> - woken = 0;
> while (waiter->flags & RWSEM_WAITING_FOR_READ) {
> struct list_head *next = waiter->list.next;
>
> + sem->activity++;
> list_del(&waiter->list);
> tsk = waiter->task;
> smp_mb();
> waiter->task = NULL;
> wake_up_process(tsk);
> put_task_struct(tsk);
> - woken++;
> if (list_empty(&sem->wait_list))
> break;
> waiter = list_entry(next, struct rwsem_waiter, list);
> }
>
> - sem->activity += woken;
> -
> out:
> return sem;
> }

So if I understand this correctly

- we have one or more processes sleeping in down_read(), waiting for access.

- we wake one or more processes up without altering ->activity

- they start to run and they do rwsem_is_locked(). This incorrectly
returns "false", because the waker process is still crunching away in
__rwsem_do_wake().

- the waker now alters ->activity, but it was too late.

And the patch fixes this by updating ->activity prior to waking the
sleeping processes. So when they run, they'll see a non-zero value of
->activity.

Fair enough, I guess.

I don't know if we really need this in -stable. Do we expect that
there will be any real runtime bugs arising from this?
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