linux-kernel - Re: [PATCH] zram: fix possible race when checking idle

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Date:	Fri, 7 Aug 2015 18:58:16 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:	Joonsoo Kim <js1304@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Minchan Kim <minchan@...nel.org>,
	Nitin Gupta <ngupta@...are.org>, linux-kernel@...r.kernel.org,
	Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCH] zram: fix possible race when checking idle_strm

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).
if wakeup arrives after prepare_to_wait_event(), then we are fine by
defintion.

so, I'm puzzled a bit. do we have a problem or we are ok.

	-ss
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