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Goncalves" To: Peter Zijlstra Cc: Sebastian Andrzej Siewior , Clark Williams , Steven Rostedt , Tejun Heo , David Vernet , Barret Rhoden , Josh Don , Crystal Wood , linux-kernel@vger.kernel.org, linux-rt-devel@lists.linux.dev, Juri Lelli , Ben Segall , DietmarEggemann@uudg.org, dietmar.eggemann@arm.com, Ingo Molnar , Mel Gorman , Valentin Schneider , Vincent Guittot , Thomas Gleixner , Wander Lairson Costa Subject: Re: [RESEND PATCH v4] sched: do not call __put_task_struct() on rt if pi_blocked_on is set Message-ID: References: <20250617092609.GR1613376@noisy.programming.kicks-ass.net> Precedence: bulk X-Mailing-List: linux-rt-devel@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250617092609.GR1613376@noisy.programming.kicks-ass.net> X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.4 On Tue, Jun 17, 2025 at 11:26:09AM +0200, Peter Zijlstra wrote: > On Fri, Jun 13, 2025 at 12:05:14PM -0300, Luis Claudio R. Goncalves wrote: > > With PREEMPT_RT enabled, some of the calls to put_task_struct() coming > > from rt_mutex_adjust_prio_chain() could happen in preemptible context and > > with a mutex enqueued. That could lead to this sequence: > > > > rt_mutex_adjust_prio_chain() > > put_task_struct() > > __put_task_struct() > > sched_ext_free() > > spin_lock_irqsave() > > rtlock_lock() ---> TRIGGERS > > lockdep_assert(!current->pi_blocked_on); > > > > Fix that by unconditionally resorting to the deferred call to > > __put_task_struct() if PREEMPT_RT is enabled. > > > > Should this have a Fixes: tag and go into /urgent? Makes sense! I will add the tag: Fixes: 893cdaaa3977b ("sched: avoid false lockdep splat in put_task_struct()") and resend. Thank you! > > Suggested-by: Crystal Wood > > Signed-off-by: Luis Claudio R. Goncalves > > --- > > > > Resent as a gentle reminder, because this issue results in scary backtraces, > > not obvious to debug and pinpoint root cause. > > > > v2: (Rostedt) remove the #ifdef from put_task_struct() and create > > tsk_is_pi_blocked_on() in sched.h to make the change cleaner. > > v3: (Sebastian, PeterZ) always call the deferred __put_task_struct() on RT. > > v4: Fix the implementation of what was requested on v3. > > > > include/linux/sched/task.h | 17 ++++++++--------- > > 1 file changed, 8 insertions(+), 9 deletions(-) > > > > diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h > > index 0f2aeb37bbb04..51678a541477a 100644 > > --- a/include/linux/sched/task.h > > +++ b/include/linux/sched/task.h > > @@ -134,11 +134,8 @@ static inline void put_task_struct(struct task_struct *t) > > if (!refcount_dec_and_test(&t->usage)) > > return; > > > > - /* > > - * In !RT, it is always safe to call __put_task_struct(). > > - * Under RT, we can only call it in preemptible context. > > - */ > > - if (!IS_ENABLED(CONFIG_PREEMPT_RT) || preemptible()) { > > + /* In !RT, it is always safe to call __put_task_struct(). */ > > + if (!IS_ENABLED(CONFIG_PREEMPT_RT)) { > > static DEFINE_WAIT_OVERRIDE_MAP(put_task_map, LD_WAIT_SLEEP); > > > > lock_map_acquire_try(&put_task_map); > > @@ -148,11 +145,13 @@ static inline void put_task_struct(struct task_struct *t) > > } > > > > /* > > - * under PREEMPT_RT, we can't call put_task_struct > > + * Under PREEMPT_RT, we can't call __put_task_struct > > * in atomic context because it will indirectly > > - * acquire sleeping locks. > > + * acquire sleeping locks. The same is true if the > > + * current process has a mutex enqueued (blocked on > > + * a PI chain). > > * > > - * call_rcu() will schedule delayed_put_task_struct_rcu() > > + * call_rcu() will schedule __put_task_struct_rcu_cb() > > * to be called in process context. > > * > > * __put_task_struct() is called when > > @@ -165,7 +164,7 @@ static inline void put_task_struct(struct task_struct *t) > > * > > * delayed_free_task() also uses ->rcu, but it is only called > > * when it fails to fork a process. Therefore, there is no > > - * way it can conflict with put_task_struct(). > > + * way it can conflict with __put_task_struct(). > > */ > > call_rcu(&t->rcu, __put_task_struct_rcu_cb); > > } > > > > ----- End forwarded message ----- > > > ---end quoted text---