From mboxrd@z Thu Jan 1 00:00:00 1970 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 07ED224339D for ; Wed, 20 Aug 2025 10:37:00 +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=1755686222; cv=none; b=eVqfZmrj31M+MFjIaItfRzDkt0ZKZODPRYRnfsQFf2zEay/Boh6Z16uThRIbWMzAhRE+NyOrTHLm15zYZOQ1cMQ7u/5YCIokB1jn2gKR60rp50oZWgWwyOVkalo90DKtJOhK83ZVJMOAA24JTtUnrPGl25scHtEmPOcgLYemBsA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1755686222; c=relaxed/simple; bh=kv3W/G1ARmOcJJTz/9sGIEr4fqmIvswblhhL9o40NPA=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=h5Z32HiXW7ptSwGehindOxZEG4DPvMK4JJ5Pa3yor1hK1aSHC4gAzFCUZ838D4DF6hkSPK6pKHmiYWkg79UOTe0+yNkgJYYXXWTNXvsIbiN9llNK60KVjZ8Vx3LtC5H4WySZuXzRvu+pR3CCHWxbgjOeePwNG83xm/N/qUal9xY= 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=n1keIyXP; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b=b8KVD0rv; 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="n1keIyXP"; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="b8KVD0rv" Date: Wed, 20 Aug 2025 12:36:57 +0200 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1755686219; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=yUAcx2/jO8NfEFGgnI4HZ0vELTMmQeqDK1jIGQqE9hE=; b=n1keIyXP4kJDwb2X6G/j4BoY+EWoUdgKQs1UzgEhRMjD6wnbIahKcscOgTiZX2Nyn/kOa6 gaUdCDRguSYujJ/m5vSsEC+f+s8dAlgveIMUAVxHFEMFPe6eXfLltqWSLNzAuzs00bDrLb HFQDBp+xpGYdOJlAyKIlkXGq34aBdhH9fD1vvvbBEa4XN3FuStiROsw9CxMOG9I6hscytY gymqdKy3MYyQlWaWt/rPbv5Ha6s8l5xBYEuO4Wyi7DM6+jh2g/e0f++8zxZRDkJpNxn+UI +4L9NVp43rdlixKlD3hFF1b7slcVE95iqb9Vp3fINhlXKwS4M4v8jNsrNf+FYA== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1755686219; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=yUAcx2/jO8NfEFGgnI4HZ0vELTMmQeqDK1jIGQqE9hE=; b=b8KVD0rvSA6QxdRuN7k4RM6lpdvanrn106bPzO9cPmxBCEKD1olS2bTNpxPg7fis0/4TyQ IX4tZoltKy+L31Dw== From: Sebastian Andrzej Siewior To: Tejun Heo Cc: linux-kernel@vger.kernel.org, linux-rt-devel@lists.linux.dev, Lai Jiangshan , Ingo Molnar , Peter Zijlstra , Steven Rostedt , Thomas Gleixner Subject: Re: [PATCH] softirq: Provide a handshake for canceling tasklets via polling on PREEMPT_RT Message-ID: <20250820103657.vDuDuLx6@linutronix.de> References: <20250812143930.22RBn5BW@linutronix.de> <20250812145359.QMcaYh9g@linutronix.de> <20250813063311.33m0TDKl@linutronix.de> <20250818125242.vJ4wGk20@linutronix.de> <20250819150105.DYeV89fa@linutronix.de> 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 In-Reply-To: <20250819150105.DYeV89fa@linutronix.de> On 2025-08-19 17:01:07 [+0200], To Tejun Heo wrote: > Okay. Then let me repost the tasklet patch and make one for workqueue to > stay in sync. > I do hope that we end up with a requirement that any kind of teardown > does not happen from an atomic context ;) That would be ------------->8------------- Subject: [PATCH] workqueue: Provide a handshake for canceling BH workers 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 | 51 ++++++++++++++++++++++++++++++++++++++-------- 1 file changed, 42 insertions(+), 9 deletions(-) diff --git a/kernel/workqueue.c b/kernel/workqueue.c index c6b79b3675c31..94e226f637992 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; } +#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 *pool) +{ + 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 *pool) { } + +#endif + /** * manage_workers - manage worker pool * @worker: self @@ -3557,6 +3584,7 @@ static void bh_worker(struct worker *worker) int nr_restarts = BH_WORKER_RESTARTS; unsigned long end = jiffies + BH_WORKER_JIFFIES; + worker_lock_callback(pool); raw_spin_lock_irq(&pool->lock); worker_leave_idle(worker); @@ -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); } /* @@ -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); } else { cpu_relax(); } @@ -4782,6 +4812,9 @@ static int init_worker_pool(struct worker_pool *pool) ida_init(&pool->worker_ida); INIT_HLIST_NODE(&pool->hash_node); pool->refcnt = 1; +#ifdef CONFIG_PREEMPT_RT + spin_lock_init(&pool->cb_lock); +#endif /* shouldn't fail above this point */ pool->attrs = alloc_workqueue_attrs(); -- 2.50.1