From: Sebastian Andrzej Siewior Date: Fri, 19 Aug 2022 11:24:45 +0200 Subject: [PATCH 347/353] workqueue: Use rcuwait for wq_manager_wait Origin: https://git.kernel.org/cgit/linux/kernel/git/rt/linux-stable-rt.git/commit?id=f194473966fbca2b59765c07b6ce08a7aa4579f1 Upstream commit d8bb65ab70f702531aaaa11d9710f9450078e295 The workqueue code has it's internal spinlock (pool::lock) and also implicit spinlock usage in the wq_manager waitqueue. These spinlocks are converted to 'sleeping' spinlocks on a RT-kernel. Workqueue functions can be invoked from contexts which are truly atomic even on a PREEMPT_RT enabled kernel. Taking sleeping locks from such contexts is forbidden. pool::lock can be converted to a raw spinlock as the lock held times are short. But the workqueue manager waitqueue is handled inside of pool::lock held regions which again violates the lock nesting rules of raw and regular spinlocks. The manager waitqueue has no special requirements like custom wakeup callbacks or mass wakeups. While it does not use exclusive wait mode explicitly there is no strict requirement to queue the waiters in a particular order as there is only one waiter at a time. This allows to replace the waitqueue with rcuwait which solves the locking problem because rcuwait relies on existing locking. Signed-off-by: Sebastian Andrzej Siewior Signed-off-by: Tejun Heo Signed-off-by: Sebastian Andrzej Siewior [wagi: Updated context as v4.19-rt was using swait] Signed-off-by: Daniel Wagner --- kernel/workqueue.c | 24 ++++++++++++++++++++---- 1 file changed, 20 insertions(+), 4 deletions(-) diff --git a/kernel/workqueue.c b/kernel/workqueue.c index e0ab9febe11f..a9f3cc02bdc1 100644 --- a/kernel/workqueue.c +++ b/kernel/workqueue.c @@ -50,6 +50,7 @@ #include #include #include +#include #include "workqueue_internal.h" @@ -299,7 +300,8 @@ static struct workqueue_attrs *wq_update_unbound_numa_attrs_buf; static DEFINE_MUTEX(wq_pool_mutex); /* protects pools and workqueues list */ static DEFINE_MUTEX(wq_pool_attach_mutex); /* protects worker attach/detach */ static DEFINE_RAW_SPINLOCK(wq_mayday_lock); /* protects wq->maydays list */ -static DECLARE_SWAIT_QUEUE_HEAD(wq_manager_wait); /* wait for manager to go away */ +/* wait for manager to go away */ +static struct rcuwait manager_wait = __RCUWAIT_INITIALIZER(manager_wait); static LIST_HEAD(workqueues); /* PR: list of all workqueues */ static bool workqueue_freezing; /* PL: have wqs started freezing? */ @@ -2023,7 +2025,7 @@ static bool manage_workers(struct worker *worker) pool->manager = NULL; pool->flags &= ~POOL_MANAGER_ACTIVE; - swake_up_one(&wq_manager_wait); + rcuwait_wake_up(&manager_wait); return true; } @@ -3342,6 +3344,18 @@ static void rcu_free_pool(struct rcu_head *rcu) kfree(pool); } +/* This returns with the lock held on success (pool manager is inactive). */ +static bool wq_manager_inactive(struct worker_pool *pool) +{ + raw_spin_lock_irq(&pool->lock); + + if (pool->flags & POOL_MANAGER_ACTIVE) { + raw_spin_unlock_irq(&pool->lock); + return false; + } + return true; +} + /** * put_unbound_pool - put a worker_pool * @pool: worker_pool to put @@ -3377,10 +3391,12 @@ static void put_unbound_pool(struct worker_pool *pool) * Become the manager and destroy all workers. This prevents * @pool's workers from blocking on attach_mutex. We're the last * manager and @pool gets freed with the flag set. + * Because of how wq_manager_inactive() works, we will hold the + * spinlock after a successful wait. */ raw_spin_lock_irq(&pool->lock); - swait_event_lock_irq(wq_manager_wait, - !(pool->flags & POOL_MANAGER_ACTIVE), pool->lock); + rcuwait_wait_event(&manager_wait, wq_manager_inactive(pool), + TASK_UNINTERRUPTIBLE); pool->flags |= POOL_MANAGER_ACTIVE; while ((worker = first_idle_worker(pool)))