Re: io_uring: incorrect assumption about mutex behavior on unlock?

[Pavel Begunkov <asml.silence@gmail.com>]

On 12/1/23 16:41, Jann Horn wrote:
> mutex_unlock() has a different API contract compared to spin_unlock().
> spin_unlock() can be used to release ownership of an object, so that
> as soon as the spinlock is unlocked, another task is allowed to free
> the object containing the spinlock.
> mutex_unlock() does not support this kind of usage: The caller of
> mutex_unlock() must ensure that the mutex stays alive until
> mutex_unlock() has returned.
> (See the thread
> <https://lore.kernel.org/all/20231130204817.2031407-1-jannh@google.com/>
> which discusses adding documentation about this.)
> (POSIX userspace mutexes are different from kernel mutexes, in
> userspace this pattern is allowed.)
> 
> io_ring_exit_work() has a comment that seems to assume that the
> uring_lock (which is a mutex) can be used as if the spinlock-style API
> contract applied:
> 
>      /*
>      * Some may use context even when all refs and requests have been put,
>      * and they are free to do so while still holding uring_lock or
>      * completion_lock, see io_req_task_submit(). Apart from other work,
>      * this lock/unlock section also waits them to finish.
>      */
>      mutex_lock(&ctx->uring_lock);
> 

Oh crap. I'll check if there more suspects and patch it up, thanks

> I couldn't find any way in which io_req_task_submit() actually still
> relies on this. I think io_fallback_req_func() now relies on it,
> though I'm not sure whether that's intentional. ctx->fallback_work is
> flushed in io_ring_ctx_wait_and_kill(), but I think it can probably be
> restarted later on via:

Yes, io_fallback_req_func() relies on it, and it can be spinned up
asynchronously from different places, e.g. in-IRQ block request
completion.

> io_ring_exit_work -> io_move_task_work_from_local ->
> io_req_normal_work_add -> io_fallback_tw(sync=false) ->
> schedule_delayed_work
> 
> I think it is probably guaranteed that ctx->refs is non-zero when we
> enter io_fallback_req_func, since I think we can't enter
> io_fallback_req_func with an empty ctx->fallback_llist, and the
> requests queued up on ctx->fallback_llist have to hold refcounted
> references to the ctx. But by the time we reach the mutex_unlock(), I
> think we're not guaranteed to hold any references on the ctx anymore,
> and so the ctx could theoretically be freed in the middle of the
> mutex_unlock() call?

Right, it comes with refs but loses them in between lock()/unlock().

> I think that to make this code properly correct, it might be necessary
> to either add another flush_delayed_work() call after ctx->refs has
> dropped to zero and we know that the fallback work can't be restarted
> anymore, or create an extra ctx->refs reference that is dropped in
> io_fallback_req_func() after the mutex_unlock(). (Though I guess it's
> probably unlikely that this goes wrong in practice.)

-- 
Pavel Begunkov