Re: futex+io_uring: futex_q::task can maybe be dangling (but is not actually accessed, so it's fine)

[Jens Axboe]

On 1/15/25 8:23 AM, Jens Axboe wrote:
> On 1/15/25 3:20 AM, Thomas Gleixner wrote:
>> On Mon, Jan 13 2025 at 15:38, Peter Zijlstra wrote:
>>> On Fri, Jan 10, 2025 at 08:33:34PM -0700, Jens Axboe wrote:
>>>
>>>> @@ -548,7 +549,7 @@ void __futex_queue(struct futex_q *q, struct futex_hash_bucket *hb)
>>>>  
>>>>  	plist_node_init(&q->list, prio);
>>>>  	plist_add(&q->list, &hb->chain);
>>>> -	q->task = current;
>>>> +	q->task = task;
>>>>  }
>>>>  
>>>>  /**
>>>
>>> The alternative is, I suppose, to move the q->task assignment out to
>>> these two callsites instead. Thomas, any opinions?
>>
>> That's fine as long as hb->lock is held, but the explicit argument makes
>> all of this simpler to understand.
>>
>> Though I'm not really a fan of this part:
>>
>>> +		__futex_queue(&ifd->q, hb, NULL);
>>> +		spin_unlock(&hb->lock);
>>
>> Can we please add that @task argument to futex_queue() and keep the
>> internals in the futex code instead of pulling more stuff into io_uring?
> 
> Sure, was trying to keep the change more minimal, but we can certainly
> add it to futex_queue() instead rather than needing to work around it on
> the io_uring side.
> 
> I'll be happy to send out a patch for that.

Here's the raw patch. Should've done this initially rather than just
tackle __futex_queue(), for some reason I thought/assumed that
futex_queue() was more widely used.

What do you think?


diff --git a/io_uring/futex.c b/io_uring/futex.c
index e29662f039e1..f108da4ff863 100644
--- a/io_uring/futex.c
+++ b/io_uring/futex.c
@@ -349,7 +349,7 @@ int io_futex_wait(struct io_kiocb *req, unsigned int issue_flags)
 		hlist_add_head(&req->hash_node, &ctx->futex_list);
 		io_ring_submit_unlock(ctx, issue_flags);
 
-		futex_queue(&ifd->q, hb);
+		futex_queue(&ifd->q, hb, NULL);
 		return IOU_ISSUE_SKIP_COMPLETE;
 	}
 
diff --git a/kernel/futex/core.c b/kernel/futex/core.c
index ebdd76b4ecbb..3db8567f5a44 100644
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -532,7 +532,8 @@ void futex_q_unlock(struct futex_hash_bucket *hb)
 	futex_hb_waiters_dec(hb);
 }
 
-void __futex_queue(struct futex_q *q, struct futex_hash_bucket *hb)
+void __futex_queue(struct futex_q *q, struct futex_hash_bucket *hb,
+		   struct task_struct *task)
 {
 	int prio;
 
@@ -548,7 +549,7 @@ void __futex_queue(struct futex_q *q, struct futex_hash_bucket *hb)
 
 	plist_node_init(&q->list, prio);
 	plist_add(&q->list, &hb->chain);
-	q->task = current;
+	q->task = task;
 }
 
 /**
diff --git a/kernel/futex/futex.h b/kernel/futex/futex.h
index 618ce1fe870e..11de6405c4e3 100644
--- a/kernel/futex/futex.h
+++ b/kernel/futex/futex.h
@@ -285,13 +285,15 @@ static inline int futex_get_value_locked(u32 *dest, u32 __user *from)
 }
 
 extern void __futex_unqueue(struct futex_q *q);
-extern void __futex_queue(struct futex_q *q, struct futex_hash_bucket *hb);
+extern void __futex_queue(struct futex_q *q, struct futex_hash_bucket *hb,
+				struct task_struct *task);
 extern int futex_unqueue(struct futex_q *q);
 
 /**
  * futex_queue() - Enqueue the futex_q on the futex_hash_bucket
  * @q:	The futex_q to enqueue
  * @hb:	The destination hash bucket
+ * @task: Task queueing this futex
  *
  * The hb->lock must be held by the caller, and is released here. A call to
  * futex_queue() is typically paired with exactly one call to futex_unqueue().  The
@@ -299,11 +301,14 @@ extern int futex_unqueue(struct futex_q *q);
  * or nothing if the unqueue is done as part of the wake process and the unqueue
  * state is implicit in the state of woken task (see futex_wait_requeue_pi() for
  * an example).
+ *
+ * Note that @task may be NULL, for async usage of futexes.
  */
-static inline void futex_queue(struct futex_q *q, struct futex_hash_bucket *hb)
+static inline void futex_queue(struct futex_q *q, struct futex_hash_bucket *hb,
+			       struct task_struct *task)
 	__releases(&hb->lock)
 {
-	__futex_queue(q, hb);
+	__futex_queue(q, hb, task);
 	spin_unlock(&hb->lock);
 }
 
diff --git a/kernel/futex/pi.c b/kernel/futex/pi.c
index d62cca5ed8f4..635c7d5d4222 100644
--- a/kernel/futex/pi.c
+++ b/kernel/futex/pi.c
@@ -982,7 +982,7 @@ int futex_lock_pi(u32 __user *uaddr, unsigned int flags, ktime_t *time, int tryl
 	/*
 	 * Only actually queue now that the atomic ops are done:
 	 */
-	__futex_queue(&q, hb);
+	__futex_queue(&q, hb, current);
 
 	if (trylock) {
 		ret = rt_mutex_futex_trylock(&q.pi_state->pi_mutex);
diff --git a/kernel/futex/waitwake.c b/kernel/futex/waitwake.c
index 3a10375d9521..a9056acb75ee 100644
--- a/kernel/futex/waitwake.c
+++ b/kernel/futex/waitwake.c
@@ -350,7 +350,7 @@ void futex_wait_queue(struct futex_hash_bucket *hb, struct futex_q *q,
 	 * access to the hash list and forcing another memory barrier.
 	 */
 	set_current_state(TASK_INTERRUPTIBLE|TASK_FREEZABLE);
-	futex_queue(q, hb);
+	futex_queue(q, hb, current);
 
 	/* Arm the timer */
 	if (timeout)
@@ -461,7 +461,7 @@ int futex_wait_multiple_setup(struct futex_vector *vs, int count, int *woken)
 			 * next futex. Queue each futex at this moment so hb can
 			 * be unlocked.
 			 */
-			futex_queue(q, hb);
+			futex_queue(q, hb, current);
 			continue;
 		}
 

-- 
Jens Axboe