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 1946C18DB2A for ; Mon, 7 Sep 2026 08:58:29 +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=1788771511; cv=none; b=MGSmb9npUed7oe/QTGuSAAEThmQzYsWWnvNaZKrpa+93YxBnxNKd1pkhAv218Jat2C/i09bAg97JQDRVLuAjxke3oj56qz9SpOoTAkkQVQHec/VdJtvJTttt5xZDEe7BcAKT7y3eTxSj4OyLoYcVDC5sbPseQyrnnvBED2ik3OE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788771511; c=relaxed/simple; bh=eXWy9KsnkQKWsibdmTwCSWnahP6gUOLu80uexQgNmd8=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=d+srVIIAjvQsTbKhAJBXHNSwAqvkTqh993DdxnYBmaP86CFkjtDKgdpu7XEMBc5alAtJSkbl3IMpW2LGkGIB+8rx4j2rF51hiDDFdUrqmvMC3WpJ8Wg+PAI10M4A4MTprDcwe7a28DHWjbTC5ixFrMk4iKTtjDoDC/jp9u3/pZ4= 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=rZZgbnQq; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b=ASCaIR4P; 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="rZZgbnQq"; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="ASCaIR4P" Date: Mon, 7 Sep 2026 10:58:25 +0200 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1788771507; 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: in-reply-to:in-reply-to:references:references; bh=IWShzSpNemoixNCKQ1tHGMIBJaTjZdIz7azemrEJ/1A=; b=rZZgbnQqldGMwax8BJQwUu7sIAj6JLfxe/390bXxDGcAsqDdJJih2NM5TxqeA7NT4CX3Es shjCSQqMyopdZkNnOE17ZByTlLDB/zv/MqROtFVs57Y6IfXxOUkMUkajUw1CZazY6XTFYI C+xFfM4QmnhW0CrSyqU0GeP05/2CpAVGJvYXn4s5ia0Lw1i7V6rcm6QJs5x4RuzlDUbUGD PQriX6JZD8xNQnKtZDFrpuBST09rM8Ty/bhE3Q4IJDydjFgnqwTQrVjCJv787rvysHTMun KkdRqL9qY2U319t0FPx0mUpQdwteYYqLLGLKWbXtrEf7PhwgiKHJGhCN54xI8Q== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1788771507; 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: in-reply-to:in-reply-to:references:references; bh=IWShzSpNemoixNCKQ1tHGMIBJaTjZdIz7azemrEJ/1A=; b=ASCaIR4PEgRgy1L7ObRxOXTJYbF/c8WL696tYVfpaFrzlhcW0Yl3iqXrk0tm/6qoROf380 tq+f7pQKDTY/GWAg== From: Sebastian Andrzej Siewior To: syzbot , linux-kernel@vger.kernel.org Cc: syzkaller-bugs@googlegroups.com, 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] sched: irq: cpu-hotplug race vs set_cpus_allowed_ptr() Message-ID: <20260907085825.f-CZ1Q5y@linutronix.de> References: <6a9919ac.94649fcc.25487e.0005.GAE@google.com> 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 In-Reply-To: <6a9919ac.94649fcc.25487e.0005.GAE@google.com> 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 noone 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 operaton completes. Maybe we should also check if cpus_read_lock is held during the invocation of set_cpus_allowed_ptr() so we don't get this problem from other callers. - 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. 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@google.com/ Signed-off-by: Sebastian Andrzej Siewior --- 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 @@ #define pr_fmt(fmt) "genirq: " fmt +#include #include #include #include @@ -1044,8 +1045,9 @@ static void irq_thread_check_affinity(struct irq_desc *desc, struct irqaction *a m = 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 b998ef6b87af4..b090a374fa728 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 = __migrate_task(rq, &rf, p, arg->dest_cpu); + if (rq != cpu_rq(arg->dest_cpu)) + cpumask_set_cpu(rq->cpu, &p->cpus_mask); } else { p->wake_cpu = arg->dest_cpu; } -- 2.55.0