Re: pthread_mutex_unlock potentially cause invalid access

["Carlos O'Donell" <carlos at systemhalted dot org>]

On Mon, Feb 13, 2012 at 9:22 AM, KOSAKI Motohiro
<kosaki.motohiro@gmail.com> wrote:
> 2012/2/13 Atsushi Nemoto <anemo@mba.ocn.ne.jp>:
>> On Sun, 12 Feb 2012 17:38:33 -0500, "Carlos O'Donell" <carlos@systemhalted.org> wrote:
>>>> On point "A", the mutex is actually unlocked, so other threads can
>>>> lock the mutex, unlock, destroy and free. ?If the mutex was destroyed
>>>> and freed by other thread, reading '__kind' on point "B" is not valid.
>>>
>>> No valid conforming program has this behaviour.
>> ...
>>> If {X-X'} need to be prevented from using M then you need to use
>>> *another* synchronization primitive, we call it P, to prevent the use
>>> M during the destruction of M.
>>
>> How about following example in pthread_mutex_destroy manual?
>> It seems it can be possible without P.
>>
>> http://pubs.opengroup.org/onlinepubs/007904875/functions/pthread_mutex_destroy.html
>> ------------------------------------------------------------------------
>> ? ?Destroying Mutexes
>>
>> ? ?A mutex can be destroyed immediately after it is unlocked. For
>> ? ?example, consider the following code:
>>
>> ? ?struct obj {
>> ? ?pthread_mutex_t om;
>> ? ? ? ?int refcnt;
>> ? ? ? ?...
>> ? ?};
>>
>> ? ?obj_done(struct obj *op)
>> ? ?{
>> ? ? ? ?pthread_mutex_lock(&op->om);
>> ? ? ? ?if (--op->refcnt == 0) {
>> ? ? ? ? ? ?pthread_mutex_unlock(&op->om);
>> ? ?(A) ? ? pthread_mutex_destroy(&op->om);
>> ? ?(B) ? ? free(op);
>> ? ? ? ?} else
>> ? ?(C) ? ? pthread_mutex_unlock(&op->om);
>> ? ?}
>>
>> ? ?In this case obj is reference counted and obj_done() is called
>> ? ?whenever a reference to the object is dropped. Implementations are
>> ? ?required to allow an object to be destroyed and freed and potentially
>> ? ?unmapped (for example, lines A and B) immediately after the object is
>> ? ?unlocked (line C).
>> ------------------------------------------------------------------------
>>
>> In this example, (A) and (B) can be executed in middle of (C) execution.
>> Thus, problem can be happen even on a fully conforming program, no?
>
> Heh, Interesting. I think you are right.
>
> At first, I thought we don't need to fix this likes Carlos because the
> standard says
>
> http://pubs.opengroup.org/onlinepubs/007904875/functions/pthread_mutex_destroy.html
>
> pthread_mutex_destroy:
> ? It shall be safe to destroy an initialized mutex that is unlocked.
> Attempting to destroy a locked mutex results in undefined behavior.
>
> So, while thread T is under mutex_unlock, any other thread shouldn't
> call pthread_mutex_destroy(). Thus, current implementation fullfill a
> requirement of the standard. But, yes, it also violate an example code
> of the standard. iow, SUS is suck.
>
> And, now lll_unlock has costly atomic operation (A) and syscall (B),

The atomic operation and syscall are *required* to implement the
semantics of a futex.

>> #define lll_unlock(lock, private) \
>> ?((void) ({ ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?\
>> ? ?int *__futex = &(lock); ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? \
>> ? ?int __val = atomic_exchange_rel (__futex, 0); ? ? ? ? ? ? \ ?(A)
>> ? ?if (__builtin_expect (__val > 1, 0)) ? ? ? ? ? ? ? ? ? ? ?\
>> ? ? ?lll_futex_wake (__futex, 1, private); ? ? ? ? ? ? ? ? ? \ ? (B)
>> ?}))
>
> thus, a few register move are much less costly than that. Plus,
> userland application developers hope non-x86 code works the same as
> x86. so, I think your proposal has good benefit/cost ratio.

I don't understand your suggestion.

If you have a suggestion for an enhancement please file a bugzilla
with a patch and we can talk about the benefits.

Adding instructions to the fast path to reduce the possibility of a
segfault in a application with problems is not a good benefit/cost.

Cheers,
Carlos.