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

Sergey Senozhatsky
2015-08-07T09:57:43Z

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