From nobody Fri Oct 3 06:37:01 2025 Received: from galois.linutronix.de (Galois.linutronix.de [193.142.43.55]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 052ED3090E6 for ; Thu, 4 Sep 2025 14:25:30 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=193.142.43.55 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1756995932; cv=none; b=CYgwBMq1uKYp/8VFKddZ6HIYqnuOb4nv0nF2S1Rx4ZKL9zKobgtSN6EHofg7/ppN1i1DMZhURkvE0Uq7eBMgDSXNqJJ5j6u3K5/5usjDJJi7gXqUf21xpadt3uu8hkIRY8IPZxH8W5T+KAZz5ZtptlN/Wn4wJSo9TS5TtM1QUe4= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1756995932; c=relaxed/simple; bh=OrS1GLE8fZzJL/5WRFTo0sr5TpCRl02BRmGnO/uO/II=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=TmzEXMyQ9ytk97diiomyfiEJq3fKdzFTKqTik7XzrY7oczU5tKsg6w48Ye+56u6zrCCQpIrKSyLIskjEzfpG2ini+yF40DCerXoc6tZ4GCCNafVEw/ZDHbdbbcde9A2dD+4e9zfI8zISxzL//muQbPfBvbwI5RWkNAk2s3v4Vjk= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linutronix.de; spf=pass smtp.mailfrom=linutronix.de; dkim=pass (2048-bit key) header.d=linutronix.de header.i=@linutronix.de header.b=xeuGdQQ4; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b=3oS3Ydrn; arc=none smtp.client-ip=193.142.43.55 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linutronix.de Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linutronix.de Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="xeuGdQQ4"; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="3oS3Ydrn" From: Sebastian Andrzej Siewior DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1756995929; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=0R+Kh082/1AWPhILklHbuMCbALmYooNHUNhKm2ZSP7E=; b=xeuGdQQ4erUNqEd/mG46YAsDkzcFHffKLonhcBKcGK4neVFyyo7rhGkICi4e2Wav/2Ovtb CDWbitnLvGZxh0FP8MVfLbzqu9fbMRpkRqYKbx+sc75gkk+PcpXoc8p069VEjWjQe7LL8x X/jQ42Vaia8IIM71h7wYdgstrsfP0MBycmHvKlgy8QYvD9hpdVIfMBrVhw2iCotVc13XKE W8iYr5YYkrUnpi2/wzsxSk55bYPpa8hjWpNON4498R8yK+R9fIuITqI9eDA8bEMLEjjQg7 aGULdU8/3rYyhNoWJVgfPUq/kQ3cQiVn0qUZtMUECuDM+iL8zxUI+XmLu6FAsA== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1756995929; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=0R+Kh082/1AWPhILklHbuMCbALmYooNHUNhKm2ZSP7E=; b=3oS3YdrnFYMfSXWkdXUVM0DuwMInxZoDLlolA1NE91851K6usc0Ey8Q2mrrqgT9JuOc/3m 82llLXgjMv3fBVCw== To: linux-rt-devel@lists.linux.dev, linux-kernel@vger.kernel.org Cc: Clark Williams , Ingo Molnar , Lai Jiangshan , Peter Zijlstra , Steven Rostedt , Tejun Heo , Thomas Gleixner , Sebastian Andrzej Siewior Subject: [PATCH v3 1/3] workqueue: Provide a handshake for canceling BH workers Date: Thu, 4 Sep 2025 16:25:23 +0200 Message-ID: <20250904142526.1845999-2-bigeasy@linutronix.de> In-Reply-To: <20250904142526.1845999-1-bigeasy@linutronix.de> References: <20250904142526.1845999-1-bigeasy@linutronix.de> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" While a BH work item is canceled, the core code spins until it determines that the item completed. On PREEMPT_RT the spinning relies on a lock in local_bh_disable() to avoid a live lock if the canceling thread has higher priority than the BH-worker and preempts it. This lock ensures that the BH-worker makes progress by PI-boosting it. This lock in local_bh_disable() is a central per-CPU BKL and about to be removed. To provide the required synchronisation add a per pool lock. The lock is acquired by the bh_worker at the begin while the individual callbacks are invoked. To enforce progress in case of interruption, __flush_work() needs to acquire the lock. This will flush all BH-work items assigned to that pool. Signed-off-by: Sebastian Andrzej Siewior --- kernel/workqueue.c | 50 +++++++++++++++++++++++++++++++++++++--------- 1 file changed, 41 insertions(+), 9 deletions(-) diff --git a/kernel/workqueue.c b/kernel/workqueue.c index c6b79b3675c31..d6c94ee8edfc5 100644 --- a/kernel/workqueue.c +++ b/kernel/workqueue.c @@ -222,7 +222,9 @@ struct worker_pool { struct workqueue_attrs *attrs; /* I: worker attributes */ struct hlist_node hash_node; /* PL: unbound_pool_hash node */ int refcnt; /* PL: refcnt for unbound pools */ - +#ifdef CONFIG_PREEMPT_RT + spinlock_t cb_lock; /* BH worker cancel lock */ +#endif /* * Destruction of pool is RCU protected to allow dereferences * from get_work_pool(). @@ -3078,6 +3080,31 @@ __acquires(&pool->lock) goto restart; } =20 +#ifdef CONFIG_PREEMPT_RT +static void worker_lock_callback(struct worker_pool *pool) +{ + spin_lock(&pool->cb_lock); +} + +static void worker_unlock_callback(struct worker_pool *pool) +{ + spin_unlock(&pool->cb_lock); +} + +static void workqueue_callback_cancel_wait_running(struct worker_pool *poo= l) +{ + spin_lock(&pool->cb_lock); + spin_unlock(&pool->cb_lock); +} + +#else + +static void worker_lock_callback(struct worker_pool *pool) { } +static void worker_unlock_callback(struct worker_pool *pool) { } +static void workqueue_callback_cancel_wait_running(struct worker_pool *poo= l) { } + +#endif + /** * manage_workers - manage worker pool * @worker: self @@ -3557,6 +3584,7 @@ static void bh_worker(struct worker *worker) int nr_restarts =3D BH_WORKER_RESTARTS; unsigned long end =3D jiffies + BH_WORKER_JIFFIES; =20 + worker_lock_callback(pool); raw_spin_lock_irq(&pool->lock); worker_leave_idle(worker); =20 @@ -3585,6 +3613,7 @@ static void bh_worker(struct worker *worker) worker_enter_idle(worker); kick_pool(pool); raw_spin_unlock_irq(&pool->lock); + worker_unlock_callback(pool); } =20 /* @@ -4222,17 +4251,17 @@ 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; + + guard(rcu)(); + pool =3D get_work_pool(work); + if (pool) + workqueue_callback_cancel_wait_running(pool); } else { cpu_relax(); } @@ -4782,6 +4811,9 @@ static int init_worker_pool(struct worker_pool *pool) ida_init(&pool->worker_ida); INIT_HLIST_NODE(&pool->hash_node); pool->refcnt =3D 1; +#ifdef CONFIG_PREEMPT_RT + spin_lock_init(&pool->cb_lock); +#endif =20 /* shouldn't fail above this point */ pool->attrs =3D alloc_workqueue_attrs(); --=20 2.51.0