From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 5B5D23043DB for ; Thu, 13 Aug 2026 03:15:50 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786590951; cv=none; b=LBBwaSqTaup8avu9C9bs+EoKKLsdg4plbF3LDeHAwG8scMARzfv7gk5AX5iJVt2w6+mkHYNaR7qh69XFpdvwAsZJlOYpkYxSYm2v0EBx5fAJrNaBEO6ci80WZWFOQeOo0XA55dnxjDc7LgIhHQhGClAUYV4UsMS5KTr4T3MvmIc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786590951; c=relaxed/simple; bh=ElBdleGmxaH4J2tvMQS2HK6VLaSlqP4G5F3/5KrerVs=; h=From:To:Cc:Subject:Message-ID:MIME-Version:Content-Type:Date; b=i4v1ssUYfw6VX7P04P3CdqhWY6lujIXWRNozHSdBEVH593GHqEgG0OZO8XLS2QFyH1qUyg7oUOyzCFY1Ujr/PNvBHW/3SmhwRY771ZxecL7Q2kL4sNQkd/p8zyBFcvRusCxmxHS6pB/+l+3oKK8reW277WP3KMV7/1zpt1+H6J8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=dNHu45qt; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="dNHu45qt" Received: by smtp.kernel.org (Postfix) with UTF8SMTPSA id 8ACAC1F000E9; Thu, 13 Aug 2026 03:15:49 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1786590949; bh=01onj4hVEvyxCw3VVjHWGVpFbu9CRriloTXP7HSS69U=; h=From:To:Cc:Subject:Date; b=dNHu45qtgAReIcm+qDwsGBZ4qh0tl+BkbqBPoPQW88xb4jYxxuz07+FwxatGa8L8F SjEZyzqK9pXQOKeAJSTsXVGd47I0sotpUmppvy8at7pK0ONZMAinIqsUsLkxvXwuy8 vQnWxlZmlShXP3V4MjK2PJNalYkw91xDwcXvc8hlyMgAhuWOCMX7VWSYQ8cz1e+zF/ TehM96zwV7y9iO8t5KZlgktOI6ZKXNc1VlZdz/t1zxXlSEaRUicWL4Rl+i5SBoNckv o50rhM2Mg3z+qtfFhNQ1StqauCJal1PIgQJfLAc6954ls16wisd1BhL4LOJ1mMXQ5Z pWRL50QpZJVzQ== From: "syzbot" To: syzkaller-upstream-moderation@googlegroups.com Cc: liuyongqiang13@huawei.com, syzbot@lists.linux.dev, yaokai34@huawei.com Subject: [PATCH RFC v2] futex: Fix might_sleep() warning in futex_pivot_pending() Message-ID: <8adeeded-1ded-4780-aed1-739d54729566@mail.kernel.org> Precedence: bulk X-Mailing-List: syzbot@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Date: Thu, 13 Aug 2026 03:15:49 +0000 (UTC) A recent change in commit 8e7ff730dd96 ("futex: Fix race in futex_pivot_pending() during private hash resize") 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 [] 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: __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 Furthermore, if the mutex is contended, mutex_lock() will block. When it acquires the lock and returns, the task state will be reset to TASK_RUNNING. This causes the subsequent schedule() in the wait loop to return immediately, leading to a busy loop that consumes 100% CPU until the condition is met. 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: syzbot+350a93852ac854927f45@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=350a93852ac854927f45 Link: https://syzkaller.appspot.com/ai_job?id=d25bd376-be2e-491a-a0a7-ef39dc13a9a5 To: To: "Ingo Molnar" To: "Thomas Gleixner" To: "Yao Kai" Cc: =?utf-8?q?Andr=C3=A9_Almeida?= Cc: "Davidlohr Bueso" Cc: "Darren Hart" Cc: "Peter Zijlstra" --- v2: - Use WRITE_ONCE() for stores to hash_new to complement READ_ONCE() in futex_pivot_pending(). v1: https://lore.kernel.org/all/1f966921-7789-4c70-92cd-ae6c4f8d5be4@mail.kernel.org/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 -- This is an AI-generated patch subject to moderation. Reply with '#syz upstream' to Sign-off the patch as a human author and send it to the upstream kernel mailing lists. Reply with '#syz reject' to reject it ('#syz unreject' to undo). See https://goo.gle/syzbot-ai-patches for information about AI-generated patches. You can comment on the patch as usual, syzbot will try to address the comments and send a new version of the patch if necessary. syzbot engineers can be reached at syzkaller@googlegroups.com.