linux-kernel - Re: [PATCH v2 1/3] workqueue: Provide a handshake for canceling BH workers

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Message-ID: <aLiLJh4n80iKWzRe@slm.duckdns.org>
Date: Wed, 3 Sep 2025 08:38:30 -1000
From: Tejun Heo <tj@...nel.org>
To: Lai Jiangshan <jiangshanlai@...il.com>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	linux-rt-devel@...ts.linux.dev, linux-kernel@...r.kernel.org,
	Clark Williams <clrkwllms@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2 1/3] workqueue: Provide a handshake for canceling BH
 workers

On Wed, Sep 03, 2025 at 03:30:08PM +0800, Lai Jiangshan wrote:
> On Tue, Sep 2, 2025 at 12:38 AM Sebastian Andrzej Siewior
> <bigeasy@...utronix.de> wrote:
> 
> > @@ -4222,17 +4251,18 @@ static bool __flush_work(struct work_struct *work, bool from_cancel)
> >                     (data & WORK_OFFQ_BH)) {
> >                         /*
> >                          * On RT, prevent a live lock when %current preempted
> > -                        * soft interrupt processing or prevents ksoftirqd from
> > -                        * running by keeping flipping BH. If the BH work item
> > -                        * runs on a different CPU then this has no effect other
> > -                        * than doing the BH disable/enable dance for nothing.
> > -                        * This is copied from
> > -                        * kernel/softirq.c::tasklet_unlock_spin_wait().
> > +                        * soft interrupt processing by blocking on lock which
> > +                        * is owned by the thread invoking the callback.
> >                          */
> >                         while (!try_wait_for_completion(&barr.done)) {
> >                                 if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
> > -                                       local_bh_disable();
> > -                                       local_bh_enable();
> > +                                       struct worker_pool *pool;
> > +
> > +                                       mutex_lock(&wq_pool_mutex);
> > +                                       pool = get_work_pool(work);
> > +                                       if (pool)
> > +                                               workqueue_callback_cancel_wait_running(pool);
> > +                                       mutex_unlock(&wq_pool_mutex);
> 
> The goal is to avoid using a potentially sleeping function in
> __flush_work() for BH work items on PREEMPT_RT, but
> mutex_lock(&wq_pool_mutex) is not appropriate in this context.
> 
> To obtain the pool of a work item, the preferred approach is to use
> rcu_read_lock() together with get_work_pool(work), as is done in
> start_flush_work().

Yeah, Sebastian, can you please switch it to rcu_read_lock()?

Thanks.

-- 
tejun