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

[Joonsoo Kim]

On Tue, Aug 11, 2015 at 08:26:33AM +0900, Minchan Kim wrote:
> Hi Joonsoo,
> 
> On Mon, Aug 10, 2015 at 09:32:30AM +0900, Joonsoo Kim wrote:
> > 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.
> 
> When I read Documentation/memory-barrier.txt, it shouldn't happen.
> 
> "All memory barriers except the data dependency barriers imply a compiler
> barrier. Data dependencies do not impose any additional compiler ordering."
> 
> "SLEEP AND WAKE-UP FUNCTIONS
> ---------------------------
> 
> Sleeping and waking on an event flagged in global data ...
> ...
> ...
> ...
> 
> A general memory barrier is interpolated automatically by set_current_state()
> after it has altered the task state:"
> 
> So I think your T1-7 assumption is not true.
> 
> As well, there are many examples under drivers/ to use the global data
> as event flag without locking or atomic.
> 

Okay. Now, I'm convinced that race is not possible. I will drop this
patch.

Thanks.
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