From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=158.69.130.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1794164058; b=RQcbk4im+3BJuwetcSoCxoeADOMIL6vTm0sMWsz13zBMJELvFBfMhlgIzJo6ybe3kEfL 5u9dbrnD5aq4W3loACx5OaWuCpUn5lNZXea6I2FU8xHXvcekpnlegsxaSLavAL970GmrC u1MybDuRoK1LRHVyJAYiP4P5nswXbOSkhEC+N9fRtQVMXdQvhzL71yPruDTnkeaPY6QS+ UruKiyji1fTC82dFpLHOdOEdm4uk4ZoPb3le+Cy/4ybfktKZfwx/7fmtwv1tEHK5uM4iG 9KoM2S6Jg+0mfV98W0gjVHhdEw6zkUv8MIKOy2lcmCju86KJxV4GS52u+eUDouoX5cQ== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794164058; h=DMARC-Filter:DKIM-Signature:Received:From:To:Cc:Subject:Date: Message-Id:X-Mailer:In-Reply-To:References:MIME-Version: Content-Transfer-Encoding; bh=86euM9Gfx67Z/jCzCvP+9zClJRgbdRmi1RRM/bKJC7A=; b=mh4XyAYMwOaIoAi/BQIH1ZnA33z7+x53eHQvaDriQtj94jmItr4s3DASB/h39T1Nh0SE uYtPKy6E6manDKtsoM/ogv08ZzT6N+DTchUEhnEqXbtFL/vgfQN/Pu/BqqMRTXvBtL6Sb rShtbQ/il8Twe79ycJuKwLOtcMhpLbyutCVUuUIKRnB+94RcqK/kBbguiDKpq1buEbPl0 PeqG5NhpwSZst6hgJmrAb17lEailc4NC4DKmVrg6Of5Ni5QyDjuBtxcd++HTye5WJ7OAJ tGEuBVFqyHCOkklA/4wx9BLmVpA7XbT2nazMGvSWNfGjNyMS5HMIPO71eMZRPWL9/cw== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass header.from=efficios.com; spf=pass smtp.mailfrom=efficios.com; dkim=pass header.d=efficios.com header.i=@efficios.com header.a=rsa-sha256 header.s=smtpout1 header.b=XLwDVfxX; arc=none smtp.remote-ip=158.69.130.18 Received: from smtpout.efficios.com (smtpout.efficios.com [158.69.130.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id C13E9606A322; Sun, 8 Nov 2026 18:54:17 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.2 smtp.subspace.kernel.org C13E9606A322 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=efficios.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=efficios.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=efficios.com header.i=@efficios.com header.a=rsa-sha256 header.s=smtpout1 header.b=XLwDVfxX DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=efficios.com; s=smtpout1; t=1794164056; bh=86euM9Gfx67Z/jCzCvP+9zClJRgbdRmi1RRM/bKJC7A=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=XLwDVfxX9AiDDInUiRtFjFwl/7zrrebQYcO3snoBQ1K+ZN0L/JLDc7zq4QXqmLV90 ifWrHgzKYGNHdRmyAPltc+isnGO6/nnbT1BEXrBsWfu+VNdYtdtgMSApxc9SItPJp5 SmxUWdqoMdFZQ6Dcu015Hvc3fKI2AxNHv+KJcClOp/f5l/X/LnDunP+3PIKUZim3TE 8PCBh2DLlc9+IrTg3fJGVoATeHomhLkJLylzctDGwg80I4Wgev6gmkeaAppHFgh/UY SMZ1h0wTGfBy0TOHgV0QNNZK1HcBSJUC/vPC0H856gei9VeCsQHhUieiVS0pvy6HjX z3VlbvYat2v/g== Received: from thinkos.internal.efficios.com (unknown [64.58.40.251]) by smtpout.efficios.com (Postfix) with ESMTPSA id 4jN2Bw4cBczMkC; Sun, 08 Nov 2026 13:54:16 -0500 (EST) From: Mathieu Desnoyers To: juri.lelli@redhat.com, peterz@infradead.org, mingo@redhat.com, vincent.guittot@linaro.org Cc: linux-kernel@vger.kernel.org, Mathieu Desnoyers , tglx@kernel.org, will@kernel.org, boqun@kernel.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, vschneid@redhat.com, kprateek.nayak@amd.com, luca.abeni@santannapisa.it, mathieu.poirier@linaro.org, longman@redhat.com, ridong.chen@linux.dev, tj@kernel.org, hannes@cmpxchg.org, mkoutny@suse.com, shuah@kernel.org, bigeasy@linutronix.de, clrkwllms@kernel.org, parri.andrea@gmail.com, m.szyprowski@samsung.com, kayracizmeci@gmail.com, weihengqiuu@gmail.com, cgroups@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-rt-devel@lists.linux.dev, Sashiko , stable@vger.kernel.org Subject: [PATCH v5 01/29] sched: Fix cpuset_mutex double unlock in __sched_setscheduler() Date: Sun, 8 Nov 2026 13:53:38 -0500 Message-Id: <20261108185406.1549770-2-mathieu.desnoyers@efficios.com> X-Mailer: git-send-email 2.39.5 In-Reply-To: <20261108185406.1549770-1-mathieu.desnoyers@efficios.com> References: <20261108185406.1549770-1-mathieu.desnoyers@efficios.com> X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Andrea Parri A sched_setattr() with SCHED_FLAG_KEEP_POLICY on a SCHED_DEADLINE task unlocks cpuset_mutex twice if the task's policy changes concurrently: WARNING: bad unlock balance detected! 7.4.0-rc1 #1 Not tainted ------------------------------------- repro/126 is trying to release lock (cpuset_mutex) at: [] __sched_setscheduler+0x9cc/0xc40 but there are no more locks to release! ... Call Trace: print_unlock_imbalance_bug.part.0+0xfb/0x110 lock_release+0x286/0x2e0 __mutex_unlock_slowpath+0x1a/0x400 __sched_setscheduler+0x9cc/0xc40 __x64_sys_sched_setattr+0xa3/0x1b0 do_syscall_64+0x106/0x5f0 entry_SYSCALL_64_after_hwframe+0x77/0x7f __sched_setscheduler() samples p->policy before taking any lock, and restarts from the recheck label if p->policy differs under the rq lock. The restart drops cpuset_mutex but leaves cpuset_locked set, so a pass that no longer needs the mutex still unlocks it on its way out: CPU 1 CPU 2 __sched_setscheduler(KEEP_POLICY) policy = p->policy /* DEADLINE */ __sched_setscheduler(SCHED_OTHER) p->policy = SCHED_OTHER cpuset_locked = true cpuset_lock() rq = task_rq_lock(p) oldpolicy != p->policy cpuset_unlock() goto recheck /* no cpuset_lock() this time */ ... if (cpuset_locked) cpuset_unlock() Without CONFIG_DEBUG_MUTEXES, or CONFIG_DEBUG_RT_MUTEXES on PREEMPT_RT, nothing warns. The second unlock still clears the owner field regardless of who holds the mutex, so another task can lose cpuset_mutex inside its critical section. Entering SCHED_DEADLINE needs CAP_SYS_NICE, but a racing caller that owns the task does not need it if it samples a policy other than SCHED_DEADLINE. Clear cpuset_locked when dropping the mutex for the restart. This should be safe because the flag is only tested on the paths leaving the next pass, after that pass has set it again if it took the mutex. The following program, run as root, reproduces the splat, which no longer appears with this applied. On PREEMPT_RT it crashes the kernel within seconds, which this also fixes: #define _GNU_SOURCE #include #include #include #include struct attr { uint32_t size, policy; uint64_t flags; int32_t nice; uint32_t prio; uint64_t runtime, deadline, period; }; static volatile int stop; static volatile pid_t target; static void set(struct attr a) { a.size = sizeof(a); syscall(SYS_sched_setattr, target, &a, 0); } static void *sleeper(void *arg) { target = syscall(SYS_gettid); while (!stop) usleep(1000); return arg; } static void *switcher(void *arg) { while (!stop) { /* SCHED_DEADLINE, 2us every 1s */ set((struct attr){ .policy = 6, .runtime = 2000, .deadline = 1000000000, .period = 1000000000 }); set((struct attr){ .policy = 0 }); /* SCHED_OTHER */ } return arg; } static void *racer(void *arg) { /* KEEP_POLICY | KEEP_PARAMS */ while (!stop) set((struct attr){ .flags = 0x18 }); return arg; } int main(void) { pthread_t t[5]; int i; pthread_create(&t[0], NULL, sleeper, NULL); while (!target) usleep(100); pthread_create(&t[1], NULL, switcher, NULL); for (i = 2; i < 5; i++) pthread_create(&t[i], NULL, racer, NULL); sleep(30); stop = 1; for (i = 0; i < 5; i++) pthread_join(t[i], NULL); return 0; } An LLM helped find the PREEMPT_RT crash and write this patch and its test. Fixes: 111cd11bbc54 ("sched/cpuset: Bring back cpuset_mutex") Reported-by: Sashiko Link: https://sashiko.dev/#/patchset/20261016062629.2102883-1-sh_def%40163.com Signed-off-by: Andrea Parri Cc: stable@vger.kernel.org Reviewed-by: Waiman Long Reviewed-by: Kayra Cizmeci Reviewed-by: Juri Lelli Signed-off-by: Mathieu Desnoyers --- kernel/sched/syscalls.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c index 45a127deeadd..2af550f92ac6 100644 --- a/kernel/sched/syscalls.c +++ b/kernel/sched/syscalls.c @@ -640,8 +640,10 @@ int __sched_setscheduler(struct task_struct *p, if (unlikely(oldpolicy != -1 && oldpolicy != p->policy)) { policy = oldpolicy = -1; task_rq_unlock(rq, p, &rf); - if (cpuset_locked) + if (cpuset_locked) { cpuset_unlock(); + cpuset_locked = false; + } goto recheck; } -- 2.47.3