From ab96bbab467c884ad684c5f669c91272a0455087 Mon Sep 17 00:00:00 2001 From: John Ogness Date: Fri, 19 Jun 2020 17:23:18 +0206 Subject: [PATCH] block: remove retry loop in ioc_release_fn() The reverse-order double lock dance in ioc_release_fn() is using a retry loop. This is a problem on PREEMPT_RT because it could preempt the task that would release q->queue_lock and thus live lock in the retry loop. RCU is already managing the freeing of the request queue and icq. If the trylock fails, use RCU to guarantee that the request queue and icq are not freed and re-acquire the locks in the correct order, allowing forward progress. Signed-off-by: John Ogness Reviewed-by: Daniel Wagner Signed-off-by: Jens Axboe --- block/blk-ioc.c | 20 +++++++++++++++++--- 1 file changed, 17 insertions(+), 3 deletions(-) diff --git a/block/blk-ioc.c b/block/blk-ioc.c index 5dbcfa1b872e..57299f860d41 100644 --- a/block/blk-ioc.c +++ b/block/blk-ioc.c @@ -107,9 +107,23 @@ static void ioc_release_fn(struct work_struct *work) ioc_destroy_icq(icq); spin_unlock(&q->queue_lock); } else { - spin_unlock_irq(&ioc->lock); - cpu_relax(); - spin_lock_irq(&ioc->lock); + /* Make sure q and icq cannot be freed. */ + rcu_read_lock(); + + /* Re-acquire the locks in the correct order. */ + spin_unlock(&ioc->lock); + spin_lock(&q->queue_lock); + spin_lock(&ioc->lock); + + /* + * The icq may have been destroyed when the ioc lock + * was released. + */ + if (!(icq->flags & ICQ_DESTROYED)) + ioc_destroy_icq(icq); + + spin_unlock(&q->queue_lock); + rcu_read_unlock(); } }