Re: [PATCH] zram: fix possible race when checking idle_strm

From: Joonsoo Kim
Date: Sun Aug 09 2015 - 20:27:09 EST

On Fri, Aug 07, 2015 at 06:58:16PM +0900, Sergey Senozhatsky wrote:
> On (08/07/15 18:14), Sergey Senozhatsky wrote:
> > hm... I need to think about it more.
> >
> > we do wake_up every time we put stream back to the list
> >
> > zcomp_strm_multi_release():
> >
> > spin_lock(&zs->strm_lock);
> > if (zs->avail_strm <= zs->max_strm) {
> > list_add(&zstrm->list, &zs->idle_strm);
> > spin_unlock(&zs->strm_lock);
> > wake_up(&zs->strm_wait);
> > return;
> > }
> >
> >
> > but I can probably see what you mean... in some very extreme case,
> > though. I can't even formulate it... eh... we use a multi stream
> > backend with ->max_strm == 1 and there are two processes, one
> > just falsely passed the wait_event() `if (condition)' check, the
> > other one just put stream back to ->idle_strm and called wake_up(),
> > but the first process hasn't yet executed prepare_to_wait_event()
> > so it might miss a wakeup. and there should be no other process
> > doing read or write operation. otherwise, there will be wakeup
> > eventually.
> >
> > is this the case you were thinking of?... then yes, this spinlock
> > may help.
> >
> on the other hand... it's actually
> wait_event() is
> if (condition)
> break;
> prepare_to_wait_event(&wq, &__wait, state)
> if (condition)
> break;
> schedule();
> if first condition check was false and we missed a wakeup call between
> first condition and prepare_to_wait_event(), then second condition
> check should do the trick I think (or you expect that second condition
> check may be wrongly pre-fetched or something).

Hello, Sergey.

This is what I thought.
I expected that second condition can be false if compiler reuse result
of first check for optimization. I guess that there is no prevention
for this kind of optimization.

So, following is the problem sequence I thought.
T1 means thread 1, T2 means another thread, 2.

<T1-1> check if idle_strm is empty or not with holding the lock
<T1-2> It is empty so do spin_unlock and run wait_event macro
<T1-3> check if idle_strm is empty or not
<T1-4> It is still empty

<T2-1> do strm release
<T2-2> call wake_up

<T1-5> add T1 to wait queue
<T1-6> check if idle_strm is empty or not
<T1-7> compiler reuse <T1-4>'s result or CPU just fetch cached
result so T1 starts waiting

In this case, T1 can be sleep permanently. To prevent compiler
optimization or fetching cached value, we need a lock here.

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