[PATCH RFC v5] futex: Fix might_sleep() warning in futex_pivot_pending()

A recent change modified futex_pivot_pending() to acquire a mutex to fix a
race condition. However, futex_pivot_pending() is evaluated as a condition
inside wait_var_event() in futex_hash_allocate(). Since wait_var_event()
sets the task state to TASK_UNINTERRUPTIBLE before evaluating the
condition, calling a blocking operation like mutex_lock() is invalid and
triggers a might_sleep() warning:

do not call blocking ops when !TASK_RUNNING; state=2 set at
[<ffffffff819e8c8d>] prepare_to_wait_event+0x3dd/0x480
kernel/sched/wait.c:317
WARNING: kernel/sched/core.c:9124 at __might_sleep+0x92/0xf0
kernel/sched/core.c:9120
Call Trace:
 <TASK>
 __mutex_lock_common kernel/locking/mutex.c:623 [inline]
 __mutex_lock+0x118/0x1550 kernel/locking/mutex.c:821
 class_mutex_constructor include/linux/mutex.h:253 [inline]
 futex_pivot_pending kernel/futex/core.c:1789 [inline]
 futex_hash_allocate+0x7fb/0xf00 kernel/futex/core.c:1872
 __do_sys_prctl kernel/sys.c:2885 [inline]
 __se_sys_prctl+0x78c/0x1910 kernel/sys.c:2534

Fix this by reverting futex_pivot_pending() to a lockless implementation
using RCU and memory barriers, which is the idiomatic way to handle
conditions in wait_event loops. By reading the hash pointer first,
executing an smp_rmb() memory barrier, and then reading hash_new, we
leverage the Message Passing (MP) pattern to guarantee correctness without
blocking. This pairs with the rcu_assign_pointer() release barrier in
__futex_pivot_hash(). If the reader sees the new hash, it is guaranteed to
see the cleared hash_new and correctly return true. If the reader sees the
old hash, it will check futex_ref_is_dead(old), which will return true if
the writer has already completed the pivot. The old hash memory is
guaranteed to remain valid for the duration of the check in
futex_ref_is_dead() because futex_pivot_pending() executes within an RCU
read-side critical section and the old hash is freed using kvfree_rcu().

Fixes: 8e7ff730dd96 ("futex: Fix race in futex_pivot_pending() during private hash resize")
Assisted-by: Gemini:gemini-3.6-flash Gemini:gemini-3.1-pro-preview syzbot
Reported-by: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=350a93852ac854927f45
Link: https://syzkaller.appspot.com/ai_job?id=29771462-e030-4501-832d-adbf8cb167c2
To: <[email protected]>
To: "Ingo Molnar" <[email protected]>
To: "Thomas Gleixner" <[email protected]>
To: "Yao Kai" <[email protected]>
Cc: =?utf-8?q?Andr=C3=A9_Almeida?= <[email protected]>
Cc: "Davidlohr Bueso" <[email protected]>
Cc: "Darren Hart" <[email protected]>
Cc: "Peter Zijlstra" <[email protected]>

---
v5:
- Removed Cc: stable tag from the commit description.

v4:
- Removed the blank line before the Cc trailer tag to maintain a contiguous trailer block.
https://lore.kernel.org/all/[email protected]/T/

v3:
- Dropped paragraph discussing busy-loop behavior when mutex is contended.
- Added Cc: [email protected] tag.
https://lore.kernel.org/all/[email protected]/T/

v2:
- Use WRITE_ONCE() for stores to hash_new to complement READ_ONCE() in futex_pivot_pending().
https://lore.kernel.org/all/[email protected]/T/

v1:
https://lore.kernel.org/all/[email protected]/T/
---
diff --git a/kernel/futex/core.c b/kernel/futex/core.c
index 128c5752f..e84be5410 100644
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -202,7 +202,7 @@ static bool __futex_pivot_hash(struct mm_struct *mm, struct futex_private_hash *
 	fph = rcu_dereference_protected(mmph->hash, lockdep_is_held(&mmph->lock));
 	if (fph) {
 		if (!futex_ref_is_dead(fph)) {
-			mmph->hash_new = new;
+			WRITE_ONCE(mmph->hash_new, new);
 			return false;
 		}
 
@@ -224,7 +224,7 @@ static void futex_pivot_hash(struct mm_struct *mm)
 
 		fph = mm->futex.phash.hash_new;
 		if (fph) {
-			mm->futex.phash.hash_new = NULL;
+			WRITE_ONCE(mm->futex.phash.hash_new, NULL);
 			__futex_pivot_hash(mm, fph);
 		}
 	}
@@ -1786,12 +1786,18 @@ static bool futex_pivot_pending(struct mm_struct *mm)
 	struct futex_mm_phash *mmph = &mm->futex.phash;
 	struct futex_private_hash *fph;
 
-	guard(mutex)(&mmph->lock);
+	guard(rcu)();
 
-	if (!mmph->hash_new)
+	fph = rcu_dereference(mmph->hash);
+	/*
+	 * Ensure that if we see the new hash, we will also see the cleared
+	 * hash_new pointer. Pairs with rcu_assign_pointer() in
+	 * __futex_pivot_hash().
+	 */
+	smp_rmb();
+	if (!READ_ONCE(mmph->hash_new))
 		return true;
 
-	fph = rcu_dereference_raw(mmph->hash);
 	return futex_ref_is_dead(fph);
 }
 
@@ -1879,7 +1885,7 @@ static int futex_hash_allocate(unsigned int hash_slots, unsigned int flags)
 		cur = rcu_dereference_protected(mm->futex.phash.hash,
 						lockdep_is_held(&mm->futex.phash.lock));
 		new = mm->futex.phash.hash_new;
-		mm->futex.phash.hash_new = NULL;
+		WRITE_ONCE(mm->futex.phash.hash_new, NULL);
 
 		if (fph) {
 			if (cur && !cur->hash_mask) {
@@ -1889,7 +1895,7 @@ static int futex_hash_allocate(unsigned int hash_slots, unsigned int flags)
 				 * the second one returns here.
 				 */
 				free = fph;
-				mm->futex.phash.hash_new = new;
+				WRITE_ONCE(mm->futex.phash.hash_new, new);
 				return -EBUSY;
 			}
 			if (cur && !new) {


base-commit: db2ddb87143519e20a95aa36c60b36107b736a58
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