From nobody Tue Sep 29 13:20:16 2026 Received: from galois.linutronix.de (Galois.linutronix.de [193.142.43.55]) (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 711C337F8C2; Fri, 7 Aug 2026 15:54:41 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=193.142.43.55 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786118084; cv=none; b=PwNNpKNE/3C1cuNf6KvdqGbkoH4ya0iR47iTrs36Xjb6nZMkQbMv/PgQJzrGaaS+E/mcoeapCsrcL2AWFwYNDAbC3yVwQHSHAmEgpPf2ndBrJm5ylGFtMf9mKa0rAeoT8agoWoLeimTARbzvhy7U7xEI/7Nc8OGSzKPc+XJyFjQ= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786118084; c=relaxed/simple; bh=Yhsjm/yXiH/XDWnDEcsjh6syHvFTqp10KqxKrbi2Wvk=; h=Date:From:To:Subject:Cc:In-Reply-To:References:MIME-Version: Message-ID:Content-Type; b=QVOoxn70KWWD7QrYpdIVAj6KBQqEUeQpEDrT69+a7NB8GZLuiy4ndLZMDAeDgfrMS2MSKXbOzCuSyGJxb63bEXHYGej/wKRmat3lW/VNb3NcHufVGaL4jqOP9qlY9WH8b869V3UErFRQyRd6VssuAcqIHI+jxPJ+pVOMB8edqcI= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linutronix.de; spf=pass smtp.mailfrom=linutronix.de; dkim=pass (2048-bit key) header.d=linutronix.de header.i=@linutronix.de header.b=N9o9kJis; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b=El2GYPn6; arc=none smtp.client-ip=193.142.43.55 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linutronix.de Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linutronix.de Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="N9o9kJis"; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="El2GYPn6" Date: Fri, 07 Aug 2026 15:54:37 -0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1786118079; h=from:from:sender:sender:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=aorsr+bc2/wkK7RC+86R/h/VTILkW5/D9G235zqy0So=; b=N9o9kJisPlVZR3rzll8w+U5ph39o2WPi50UpOcGDtRXTrwRKEAE7T+HNFxUcO4FlN5ipCl s4ijwqwUPosh97D0cdjuUKWvqPwdbXua+rNak2jaFtGpN7vpmjIm5w8eK/cIS4NrH5Uq/7 biAGBwQDwccnFs4h2Shsk8VSU+IoBkzJBjG2cw1Obs4usHYE0to2U+59q1ngRt3NokklAY 87RieiLsMOGzt5RRLz/vjVgVgD9JYrahvfdIfcG5HCPY3fmFJuk1AhBuO3rTX3dcMQpSJo Thxl5OmThpqzLM5Tur6MFBh2N8Zfz7WA13sLbzTzEoi4O+GNOnldlST4Kla7MA== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1786118079; h=from:from:sender:sender:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=aorsr+bc2/wkK7RC+86R/h/VTILkW5/D9G235zqy0So=; b=El2GYPn6/swyxUoEAMsLeN2ehHcxekxlTCT83O9GPp43XZnu6l71KqCznt7LddOfB1KoPr Pv5Zsq/XfheMrhAQ== From: "tip-bot2 for Yao Kai" Sender: tip-bot2@linutronix.de Reply-to: linux-kernel@vger.kernel.org To: linux-tip-commits@vger.kernel.org Subject: [tip: locking/urgent] futex: Fix race in futex_pivot_pending() during private hash resize Cc: Peter Zijlstra , Yao Kai , Thomas Gleixner , stable@vger.kernel.org, x86@kernel.org, linux-kernel@vger.kernel.org In-Reply-To: <20260804125530.3933754-1-yaokai34@huawei.com> References: <20260804125530.3933754-1-yaokai34@huawei.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Message-ID: <178611807751.708.88477594766113221.tip-bot2@tip-bot2> Robot-ID: Robot-Unsubscribe: Contact to get blacklisted from these emails Precedence: bulk Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable The following commit has been merged into the locking/urgent branch of tip: Commit-ID: 8e7ff730dd96519a333d1570edf1c3fabb6d3629 Gitweb: https://git.kernel.org/tip/8e7ff730dd96519a333d1570edf1c3fab= b6d3629 Author: Yao Kai AuthorDate: Tue, 04 Aug 2026 20:55:30 +08:00 Committer: Thomas Gleixner CommitterDate: Fri, 07 Aug 2026 17:46:30 +02:00 futex: Fix race in futex_pivot_pending() during private hash resize A task performing a custom private hash resize can remain blocked in uninterruptible sleep indefinitely. The hung-task detector reports: INFO: task futex-resizer:314 blocked for more than 10 seconds. task:futex-resizer state:D stack:14824 pid:314 tgid:312 ppid:311 Call Trace: __schedule+0x521/0xf30 schedule+0x22/0xa0 futex_hash_allocate+0x3db/0x490 __do_sys_prctl+0x6f5/0xbd0 do_syscall_64+0xf9/0x530 entry_SYSCALL_64_after_hwframe+0x77/0x7f Kernel panic - not syncing: hung_task: blocked tasks futex_pivot_pending() allows the resize request to continue when either no replacement hash is pending (hash_new =3D=3D NULL) or the current hash reference count has reached zero. After the final-reference wake, another futex task can complete the pivot between the two observations: T1 T2 futex_hash_allocate() wait_var_event(mm, ...) futex_pivot_pending(mm) hash_new !=3D NULL futex_hash() futex_ref_get(old) -> false futex_pivot_hash(mm) hash_new =3D NULL __futex_pivot_hash(mm, new) rcu_assign_pointer(hash, new) fph =3D rcu_dereference(hash) /* new */ futex_ref_is_dead(fph) -> false schedule() The pivot changes the state from hash_new !=3D NULL with a dead current hash to hash_new =3D=3D NULL with a live current hash. Because futex_pivot_pending() reads hash_new and hash without serialization, the resize task can observe hash_new in the pre-pivot state and hash in the post-pivot state, causing futex_pivot_pending() to return false even though the pivot has completed. The task then goes to sleep after the wakeup has already been consumed. Serialize state reads in futex_pivot_pending() using futex_mm_phash::lock. This guarantees that futex_pivot_pending() observes hash_new and hash atomically, eliminating the race condition. Fixes: bd54df5ea7ca ("futex: Allow to resize the private local hash") Suggested-by: Peter Zijlstra Signed-off-by: Yao Kai Signed-off-by: Thomas Gleixner Cc: stable@vger.kernel.org Link: https://patch.msgid.link/20260804125530.3933754-1-yaokai34@huawei.com --- kernel/futex/core.c | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-) diff --git a/kernel/futex/core.c b/kernel/futex/core.c index 2650d1e..128c575 100644 --- a/kernel/futex/core.c +++ b/kernel/futex/core.c @@ -1783,14 +1783,15 @@ void futex_hash_free(struct mm_struct *mm) =20 static bool futex_pivot_pending(struct mm_struct *mm) { + struct futex_mm_phash *mmph =3D &mm->futex.phash; struct futex_private_hash *fph; =20 - guard(rcu)(); + guard(mutex)(&mmph->lock); =20 - if (!mm->futex.phash.hash_new) + if (!mmph->hash_new) return true; =20 - fph =3D rcu_dereference(mm->futex.phash.hash); + fph =3D rcu_dereference_raw(mmph->hash); return futex_ref_is_dead(fph); } =20