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SubjectRe: [PATCH/RFC] Futex mmap_sem deadlock
Linus Torvalds wrote:
> > queue_me(...) etc.
> > current->flags |= PF_MMAP_SEM; <- new
> > ret = get_user(...);
> > current->flags &= PF_MMAP_SEM; <- new
> > /* the rest */
>
> That is uglee.
>
> We really have this already, and it's called "current->preempt". It
> handles any lock at all, and doesn't add yet another special case to all
> the architectures.

Ooh, I didn't know current->preempt did that (been away).

> repeat:
> down_read(&current->mm->mmap_sem);
> get_futex_key(...) etc.
> queue_me(...) etc.
> inc_preempt_count();
> ret = get_user(...);
> dec_preempt_count();
> if (unlikely(ret)) {
> up_read(&current->mm->mmap_sem);
> /* Re-do the access outside the lock */
> ret = get_user(...);
> if (!ret)
> goto repeat;
> return ret;
> }

That would work. I like it. :)

Page faults will enter the fault handler twice (i.e. slower), but
that's not really a disadvantage, because a program always references
the memory just before calling futex_wait anyway. A fault is rare.

There is one small but important error: the "return ret" mustn't just
return. It must call unqueue_me(&q) just like the code at out_unqueue,
_including_ the conditional "ret = 0", but _excluding_ the up_read().

Alternatively, since it's a rare case, just shuffle the loop around:

down_read(&current->mm->mmap_sem);
repeat:
get_futex_key(...) etc.
queue_me(...) etc.
inc_preempt_count();
ret = get_user(...);
dec_preempt_count();
if (unlikely(ret)) {
up_read(&current->mm->mmap_sem);
/* Re-do the access outside the lock */
ret = get_user(...);
down_read(&current->mm->mmap_sem);
if (!ret)
goto repeat;
goto out_unqueue;
}
-- Jamie
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