From mboxrd@z Thu Jan 1 00:00:00 1970 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 C44383783BB for ; Mon, 12 Oct 2026 10:39:49 +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=1791801591; cv=none; b=orAWWPD05l2MEYvUBW4h/2cbFlMeH2r6OLKpvMslS+i+Sl0QXY/Yrd1xAZZR8HqqhtaelYAIwuYEYOiTyRw8XPyvQvEelUYYk2ijxmAl1waJsZ0zc0NcpemN2rLNeIFzzGJTRxHxnxzFTH4nseBcSxUSxtm9lwe+vtflWpRML0c= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791801591; c=relaxed/simple; bh=okcEIJH7GFF4AW5kN1B5wpZMQI+aGdR2ruVQdZ+dc7o=; h=Date:From:To:Cc:Subject:Message-ID:MIME-Version:Content-Type: Content-Disposition; b=cX06qwzHuJtvehmft9SOwUFr4cUYwdNfGpOM06+Q6P7lfJ8XsfHZXu5aqSwAyP3Qh8m0qRvDyiO1BAK3BOZY5YIaFAUpEl+YzFwatiwPOGFmWKF66CaqYUg2O7YOT5slYG/vEk9kic61HDm9qshRYuDHJz7Jn3Nqgl/VO4XmLLM= 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=PNUNI7Vq; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b=xndS3Oen; 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="PNUNI7Vq"; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="xndS3Oen" Date: Mon, 12 Oct 2026 12:39:46 +0200 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1791801587; h=from:from: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; bh=U6h7MCBN0PR5UEqXz+h1WBgI29H1YjPXsRqL1KmkGZU=; b=PNUNI7VqHBRPPNxdIGQES12R8jyqvweCldaOBQ/cfvMM7McQDCCbK6IarlwqNu7elceVto b+piJL27yhKcEJMCpp9EcY+sdW30SUxDa/Cq1O1dxYSkjaYOK5rgMVZzL6Yyk6pVOsRG3g EqLIUxBjFGB6Q4HtTZBi4wpSvB3e9d+DTABb6sxN3gMTrp7/WnQL6kARibZT5qpSgOUm1G IqkG3M7AZyFEHASV0TZLDz+J7oG45xNQGM3Zia+Q2wVG+6c//XrIL+MkomLgTywVBkYV/J u8wj5nTV0nteNqtNG4naTdWSvUUdACSH7G2Y81ZDJK9MbJsBxeNenp0UuzP9wQ== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1791801587; h=from:from: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; bh=U6h7MCBN0PR5UEqXz+h1WBgI29H1YjPXsRqL1KmkGZU=; b=xndS3Oen2HMfzwfTlJqnk25+OFzVwzMtpMzBCR5mwVo0ROHSjA/EtRVtv1Hq+QMKsc4O5V 8SpxW1I/GUwXlEBg== From: Sebastian Andrzej Siewior To: syzbot , linux-kernel@vger.kernel.org Cc: Thomas Gleixner , Radu Rendec , Ingo Molnar , Peter Zijlstra , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider , K Prateek Nayak Subject: [RFC PATCH v2] sched: irq: cpu-hotplug race vs set_cpus_allowed_ptr() Message-ID: <20261012103946.aOSAmrRe@linutronix.de> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: quoted-printable syzbot reported a race of assigning a CPU affinity during CPU hotplug operation. After the affinity of an IRQ has been changed, the IRQ-core sets IRQTF_AFFINITY and wakes the relevant interrupt threads which need to adjust their affinity mask. The thread will then invoke set_cpus_allowed_ptr() to update the mask. Based on the new mask a CPU is chosen on which the thread should run. It is verified that this CPU is online however there is no guarantee that this CPU remains online while affine_move_task() is moving _this_ task. Since the current task requests the migration the stopper/ migration_cpu_stop() is involved at which point the task pauses for a while. If the CPU goes offline (or is no longer cpu_online_mask) then __migrate_task() (due to is_cpu_allowed() reject) will return the rq of the CPU on which the task is currently running (which does not match it's task_struct::cpus_mask). The aftermath: The IRQ-thread's CPU and task's cpus_mask do not match. A migrate_disable()-> schedule() will change task_struct::cpus_ptr ensuring that the following migrate_enable() will update the task to the requested affinity mask. At this point, affine_move_task() expects task_struct::migration_pending set but it is NULL because no one requested an affinity change while the task was in migrate-disable section. I see two ways of fixing this: - Holding the cpus_read_lock while set_cpus_allowed_ptr() is invoked. This ensure that the CPU remains in the cpu_online_mask while the migration task moving the task over. Should the mask be already invalid, then it is rejected otherwise the operation completes. sashiko complains about deadlock of cpus_write_lock() in the CPU down path and calling irq_migrate_all_off_this_cpu() vs irq_thread_check_affinity() doing cpus_read_lock(). As far as I see, there is no deadlock because the mask is updated, the tread is woken, the tread blocks with cpus_read_lock() until the CPU is down. There is nothing that waits for the interrupt thread to complete, there is no synchronize_irq() invocation which would be required for the deadlock to happen. - Should __migrate_task() fail to return the requested rq make sure its CPU is part task_struct::cpus_mask. This ensures that the current CPU is still part of mask avoiding a possible push by migrate_enable(). sched_class::set_cpus_allowed did not see this mask. It might be a bit inconsistent and feels a bit like select_fallback_rq() without the printk. Sashiko complains: | If __migrate_task() fails (e.g., due to a concurrent CPU offline event), = this | fallback code forcibly modifies p->cpus_mask with cpumask_set_cpu(). This | skips the required p->sched_class->set_cpus_allowed() callback and fails = to | update p->nr_cpus_allowed. | Can this bypass global EDF real-time invariants for SCHED_DEADLINE (as | set_cpus_allowed_dl() is skipped), and allow unprivileged tasks to escape | affinity boundaries, giving them access to CPUs outside their cpusets? We did do_set_cpus_allowed() for CPU X and can't migrate there now. We also don't know if the invocation was successful. =20 Looking at this, it is not possible acquire the CPU-hotplug lock here and not every caller holds it. This seems to be limited to migrate-disable. Should this happen without the lock, then during the wake-up the scheduler will just print "process =E2=80=A6 no longer affine= to cpu" pick a CPU and be done with it. Maybe set the bit followed by do_set_cpus_allowed()? Both changes are implemented to illustrate, one is enough. I can reproduce this back on v6.8, therefore I assume we have this since day #1 of migrate-disable. Reported-by: syzbot+6835d5c11145e4f77057@syzkaller.appspotmail.com Closes: https://lore.kernel.org/all/6a9919ac.94649fcc.25487e.0005.GAE@googl= e.com/ Signed-off-by: Sebastian Andrzej Siewior --- v1=E2=80=A6v2: https://lore.kernel.org/all/20260907085825.f-CZ1Q5y@linutron= ix.de/ - Repost with collected notes. kernel/irq/manage.c | 6 ++++-- kernel/sched/core.c | 2 ++ 2 files changed, 6 insertions(+), 2 deletions(-) diff --git a/kernel/irq/manage.c b/kernel/irq/manage.c index 57eff26fa646a..75b6798c1d97c 100644 --- a/kernel/irq/manage.c +++ b/kernel/irq/manage.c @@ -8,6 +8,7 @@ =20 #define pr_fmt(fmt) "genirq: " fmt =20 +#include #include #include #include @@ -1044,8 +1045,9 @@ static void irq_thread_check_affinity(struct irq_desc= *desc, struct irqaction *a m =3D irq_data_get_effective_affinity_mask(&desc->irq_data); cpumask_copy(mask, m); } - - set_cpus_allowed_ptr(current, mask); + scoped_guard(cpus_read_lock) { + set_cpus_allowed_ptr(current, mask); + } free_cpumask_var(mask); } #else diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 1fe40de6ebe3c..5896be82e6ddc 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -2674,6 +2674,8 @@ static int migration_cpu_stop(void *data) if (task_on_rq_queued(p)) { update_rq_clock(rq); rq =3D __migrate_task(rq, &rf, p, arg->dest_cpu); + if (rq !=3D cpu_rq(arg->dest_cpu)) + cpumask_set_cpu(rq->cpu, &p->cpus_mask); } else { p->wake_cpu =3D arg->dest_cpu; } --=20 2.55.0