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Wed, 18 Mar 2026 03:16:34 -0700 (PDT) Received: from jlelli-thinkpadt14gen4.remote.csb ([151.29.82.96]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4856ea98070sm123899475e9.6.2026.03.18.03.16.33 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 18 Mar 2026 03:16:34 -0700 (PDT) Date: Wed, 18 Mar 2026 11:16:31 +0100 From: Juri Lelli To: K Prateek Nayak Cc: John Stultz , LKML , Joel Fernandes , Qais Yousef , Ingo Molnar , Peter Zijlstra , Vincent Guittot , Dietmar Eggemann , Valentin Schneider , Steven Rostedt , Ben Segall , Zimuzo Ezeozue , Mel Gorman , Will Deacon , Waiman Long , Boqun Feng , "Paul E. McKenney" , Metin Kaya , Xuewen Yan , Thomas Gleixner , Daniel Lezcano , Suleiman Souhlal , kuyo chang , hupu , kernel-team@android.com Subject: Re: [PATCH v25 9/9] sched: Handle blocked-waiter migration (and return migration) Message-ID: References: <20260313023022.2902479-1-jstultz@google.com> <20260313023022.2902479-10-jstultz@google.com> <9b2d6615-6108-4271-bbd5-902261d72864@amd.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 Content-Transfer-Encoding: 8bit In-Reply-To: <9b2d6615-6108-4271-bbd5-902261d72864@amd.com> On 18/03/26 12:26, K Prateek Nayak wrote: > Hello Juri, > > On 3/18/2026 12:05 PM, Juri Lelli wrote: > >> + deactivate_task(rq, p, DEQUEUE_NOCLOCK); > >> + proxy_set_task_cpu(p, target_cpu); > >> + > >> + /* > >> + * We have to zap callbacks before unlocking the rq > >> + * as another CPU may jump in and call sched_balance_rq > >> + * which can trip the warning in rq_pin_lock() if we > >> + * leave callbacks set. > >> + */ > >> + zap_balance_callbacks(rq); > >> + rq_unpin_lock(rq, rf); > >> + raw_spin_rq_unlock(rq); > >> + > >> + attach_one_task(target_rq, p); > > > > We release rq lock between deactivate and attach (and we don't hold > > neither wait_lock nor blocked_lock as they are out of scope at this > > point). Can't something like the following happen? > > > > - Task A: blocked on mutex M, queued on CPU 0 > > - Task B: owns mutex M, running on CPU 1 > > > > CPU 0 (migrating A→CPU 1) CPU 1 (B finishes critical section) > > ------------------------- ------------------------------------ > > find_proxy_task(donor=A): > > owner = B, owner_cpu = 1 > > action = MIGRATE > > // guard releases wait_lock > > > > proxy_migrate_task(A, cpu=1): > > deactivate_task(rq0, A) > > → A->on_rq = 0 > > This sets TASK_ON_RQ_MIGRATING > before dequeuing. Right you are, I missed this! Sorry for the noise and thanks for the quick reply. Best, Juri