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Fri, 21 Nov 2025 05:05:35 -0800 (PST) X-Google-Smtp-Source: AGHT+IFyj36IY8H0H5CVebP/iPiMzTN2r2ueDX4ua2UD3TpkLGK42ACEuxzEe6GE+nfw7yta26ow0Q== X-Received: by 2002:a05:600c:1f8f:b0:477:5c45:8100 with SMTP id 5b1f17b1804b1-477c01df155mr26900595e9.24.1763730334731; Fri, 21 Nov 2025 05:05:34 -0800 (PST) Received: from jlelli-thinkpadt14gen4.remote.csb ([151.29.129.40]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-477bf1f365fsm42599005e9.8.2025.11.21.05.05.33 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 21 Nov 2025 05:05:33 -0800 (PST) Date: Fri, 21 Nov 2025 14:05:31 +0100 From: Juri Lelli To: Pingfan Liu Cc: linux-kernel@vger.kernel.org, Waiman Long , Chen Ridong , Peter Zijlstra , Pierre Gondois , Ingo Molnar , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Valentin Schneider , Tejun Heo , Johannes Weiner , mkoutny@suse.com Subject: Re: [PATCHv7 2/2] sched/deadline: Walk up cpuset hierarchy to decide root domain when hot-unplug Message-ID: References: <20251119095525.12019-1-piliu@redhat.com> <20251119095525.12019-3-piliu@redhat.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=us-ascii Content-Disposition: inline In-Reply-To: <20251119095525.12019-3-piliu@redhat.com> Hi! On 19/11/25 17:55, Pingfan Liu wrote: ... > +/* Access rule: must be called on local CPU with preemption disabled */ > static DEFINE_PER_CPU(cpumask_var_t, local_cpu_mask_dl); ... > +/* The caller should hold cpuset_mutex */ Maybe we can add a lockdep explicit check? > void dl_add_task_root_domain(struct task_struct *p) > { > struct rq_flags rf; > struct rq *rq; > struct dl_bw *dl_b; > + unsigned int cpu; > + struct cpumask *msk = this_cpu_cpumask_var_ptr(local_cpu_mask_dl); Can this corrupt local_cpu_mask_dl? Without preemption being disabled, the following race can occur: 1. Thread calls dl_add_task_root_domain() on CPU 0 2. Gets pointer to CPU 0's local_cpu_mask_dl 3. Thread is preempted and migrated to CPU 1 4. Thread 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 > > raw_spin_lock_irqsave(&p->pi_lock, rf.flags); It's safe to get the pointer after this point. > if (!dl_task(p) || dl_entity_is_special(&p->dl)) { > @@ -2919,16 +2952,25 @@ void dl_add_task_root_domain(struct task_struct *p) > return; > } > > - rq = __task_rq_lock(p, &rf); > - > + /* > + * Get an active rq, whose rq->rd traces the correct root > + * domain. > + * Ideally this would be under cpuset reader lock until rq->rd is > + * fetched. However, sleepable locks cannot nest inside pi_lock, so we > + * rely on the caller of dl_add_task_root_domain() holds 'cpuset_mutex' > + * to guarantee the CPU stays in the cpuset. > + */ > + dl_get_task_effective_cpus(p, msk); > + cpu = cpumask_first_and(cpu_active_mask, msk); > + BUG_ON(cpu >= nr_cpu_ids); > + rq = cpu_rq(cpu); > dl_b = &rq->rd->dl_bw; > - raw_spin_lock(&dl_b->lock); > + /* End of fetching rd */ Not sure we need this comment above. :) > + raw_spin_lock(&dl_b->lock); > __dl_add(dl_b, p->dl.dl_bw, cpumask_weight(rq->rd->span)); > - > raw_spin_unlock(&dl_b->lock); > - > - task_rq_unlock(rq, p, &rf); > + raw_spin_unlock_irqrestore(&p->pi_lock, rf.flags); > } Thanks, Juri