Re: [PATCH/RFC] Futex mmap_sem deadlock

Linus Torvalds
2005-02-22T21:30:13Z

On Tue, 22 Feb 2005, Jamie Lokier wrote:
>
> A much simpler solution (and sorry for not offering it earlier,
> because Andrew Morton pointed out this bug long ago, but I was busy), is:
>
> In futex.c:
>
> down_read(&current->mm->mmap_sem);
> get_futex_key(...) etc.
> 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.

Just do

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;
}
...

and you should be ok.

No new special cases, no new abstractions. At most, we should probably
create a "get_user_inatomic()", to

- make it damn obvious what we're doing, and match the explicit
"inatomic" in the other place where we depend on this (fs/filemap.c)

- allow the regular "get_user()" to continue to do the normal
"might_sleep()" checks.

That's assuming we can't just make rwsem's nest nicely.

Linus