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([2601:188:c102:b180:1f8b:71d0:77b1:1f6e]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-8846e54c84esm90318906d6.30.2025.11.23.20.31.47 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sun, 23 Nov 2025 20:31:48 -0800 (PST) From: Waiman Long X-Google-Original-From: Waiman Long Message-ID: Date: Sun, 23 Nov 2025 23:31:47 -0500 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] sched/deadline: Fix potential race in dl_add_task_root_domain() To: Pingfan Liu , Tejun Heo , cgroups@vger.kernel.org, linux-kernel@vger.kernel.org Cc: Juri Lelli , Chen Ridong , Peter Zijlstra , Pierre Gondois , Ingo Molnar , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider References: <20251124033415.10784-1-piliu@redhat.com> Content-Language: en-US In-Reply-To: <20251124033415.10784-1-piliu@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 11/23/25 10:34 PM, Pingfan Liu wrote: > The access rule for local_cpu_mask_dl requires it to be called on the > local CPU with preemption disabled. However, dl_add_task_root_domain() > currently violates this rule. > > Without preemption disabled, the following race can occur: > > 1. ThreadA calls dl_add_task_root_domain() on CPU 0 > 2. Gets pointer to CPU 0's local_cpu_mask_dl > 3. ThreadA is preempted and migrated to CPU 1 > 4. ThreadA continues using CPU 0's local_cpu_mask_dl > 5. Meanwhile, the scheduler on CPU 0 calls find_later_rq() which also > uses local_cpu_mask_dl (with preemption properly disabled) > 6. Both contexts now corrupt the same per-CPU buffer concurrently > > Fix this by moving the local_cpu_mask_dl access to the preemption > disabled section. > > Closes: https://lore.kernel.org/lkml/aSBjm3mN_uIy64nz@jlelli-thinkpadt14gen4.remote.csb > Fixes: 318e18ed22e8 ("sched/deadline: Walk up cpuset hierarchy to decide root domain when hot-unplug") > Reported-by: Juri Lelli > Signed-off-by: Pingfan Liu > To: Tejun Heo > Cc: Waiman Long > Cc: Chen Ridong > Cc: Peter Zijlstra > Cc: Juri Lelli > Cc: Pierre Gondois > Cc: Ingo Molnar > Cc: Vincent Guittot > Cc: Dietmar Eggemann > Cc: Steven Rostedt > Cc: Ben Segall > Cc: Mel Gorman > Cc: Valentin Schneider > To: cgroups@vger.kernel.org > To: linux-kernel@vger.kernel.org > --- > kernel/sched/deadline.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c > index 194a341e85864..e9153e86de0a7 100644 > --- a/kernel/sched/deadline.c > +++ b/kernel/sched/deadline.c > @@ -2944,7 +2944,7 @@ void dl_add_task_root_domain(struct task_struct *p) > struct rq *rq; > struct dl_bw *dl_b; > unsigned int cpu; > - struct cpumask *msk = this_cpu_cpumask_var_ptr(local_cpu_mask_dl); > + struct cpumask *msk; > > raw_spin_lock_irqsave(&p->pi_lock, rf.flags); > if (!dl_task(p) || dl_entity_is_special(&p->dl)) { > @@ -2952,6 +2952,7 @@ void dl_add_task_root_domain(struct task_struct *p) > return; > } > > + msk = this_cpu_cpumask_var_ptr(local_cpu_mask_dl); > /* > * Get an active rq, whose rq->rd traces the correct root > * domain. It will be clearerer by moving the statement down to before the dl_get_task_effective_cpus() call that uses msk. Please also update the comment as suggested by Juri. Thanks, Longman