pthread_mutex_unlock potentially cause invalid access

[KOSAKI Motohiro]

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),

> #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.