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From:   Hannes Reinecke <hare@suse.de>
To:     Jens Axboe <axboe@kernel.dk>
Cc:     Christoph Hellwig <hch@lst.de>, linux-block@vger.kernel.org,
        Linux Kernel Mailinglist <linux-kernel@vger.kernel.org>,
        Hannes Reinecke <hare@suse.de>
Subject: [PATCH] block/genhd: use atomic_t for disk_event->block
Date:   Tue,  1 Jun 2021 13:01:45 +0200
Message-Id: <20210601110145.113365-1-hare@suse.de>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

__disk_unblock_events() will call queue_delayed_work() with a '0' argument
under a spin lock. This might cause the queue_work item to be executed
immediately, and run into a deadlock in disk_check_events() waiting for
the lock to be released.

This patch converts the 'blocked' counter into an atomic variable, so we don't
need to hold a spinlock anymore when scheduling the workqueue function.

Signed-off-by: Hannes Reinecke <hare@suse.de>
---
 block/genhd.c | 36 +++++++++++++-----------------------
 1 file changed, 13 insertions(+), 23 deletions(-)

diff --git a/block/genhd.c b/block/genhd.c
index 9f8cb7beaad1..07e70f0c9c25 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -1379,7 +1379,7 @@ struct disk_events {
 	spinlock_t		lock;
 
 	struct mutex		block_mutex;	/* protects blocking */
-	int			block;		/* event blocking depth */
+	atomic_t		block;		/* event blocking depth */
 	unsigned int		pending;	/* events already sent out */
 	unsigned int		clearing;	/* events being cleared */
 
@@ -1439,8 +1439,6 @@ static unsigned long disk_events_poll_jiffies(struct gendisk *disk)
 void disk_block_events(struct gendisk *disk)
 {
 	struct disk_events *ev = disk->ev;
-	unsigned long flags;
-	bool cancel;
 
 	if (!ev)
 		return;
@@ -1451,11 +1449,7 @@ void disk_block_events(struct gendisk *disk)
 	 */
 	mutex_lock(&ev->block_mutex);
 
-	spin_lock_irqsave(&ev->lock, flags);
-	cancel = !ev->block++;
-	spin_unlock_irqrestore(&ev->lock, flags);
-
-	if (cancel)
+	if (atomic_inc_return(&ev->block) == 1)
 		cancel_delayed_work_sync(&disk->ev->dwork);
 
 	mutex_unlock(&ev->block_mutex);
@@ -1467,23 +1461,19 @@ static void __disk_unblock_events(struct gendisk *disk, bool check_now)
 	unsigned long intv;
 	unsigned long flags;
 
+	if (atomic_dec_return(&ev->block) <= 0) {
+		mutex_unlock(&ev->block_mutex);
+		return;
+	}
 	spin_lock_irqsave(&ev->lock, flags);
-
-	if (WARN_ON_ONCE(ev->block <= 0))
-		goto out_unlock;
-
-	if (--ev->block)
-		goto out_unlock;
-
 	intv = disk_events_poll_jiffies(disk);
+	spin_unlock_irqrestore(&ev->lock, flags);
 	if (check_now)
 		queue_delayed_work(system_freezable_power_efficient_wq,
 				&ev->dwork, 0);
 	else if (intv)
 		queue_delayed_work(system_freezable_power_efficient_wq,
 				&ev->dwork, intv);
-out_unlock:
-	spin_unlock_irqrestore(&ev->lock, flags);
 }
 
 /**
@@ -1523,10 +1513,10 @@ void disk_flush_events(struct gendisk *disk, unsigned int mask)
 
 	spin_lock_irq(&ev->lock);
 	ev->clearing |= mask;
-	if (!ev->block)
+	spin_unlock_irq(&ev->lock);
+	if (!atomic_read(&ev->block))
 		mod_delayed_work(system_freezable_power_efficient_wq,
 				&ev->dwork, 0);
-	spin_unlock_irq(&ev->lock);
 }
 
 /**
@@ -1638,11 +1628,11 @@ static void disk_check_events(struct disk_events *ev,
 	*clearing_ptr &= ~clearing;
 
 	intv = disk_events_poll_jiffies(disk);
-	if (!ev->block && intv)
+	spin_unlock_irq(&ev->lock);
+	if (!atomic_read(&ev->block) && intv)
 		queue_delayed_work(system_freezable_power_efficient_wq,
 				&ev->dwork, intv);
 
-	spin_unlock_irq(&ev->lock);
 
 	/*
 	 * Tell userland about new events.  Only the events listed in
@@ -1807,7 +1797,7 @@ static void disk_alloc_events(struct gendisk *disk)
 	ev->disk = disk;
 	spin_lock_init(&ev->lock);
 	mutex_init(&ev->block_mutex);
-	ev->block = 1;
+	atomic_set(&ev->block, 1);
 	ev->poll_msecs = -1;
 	INIT_DELAYED_WORK(&ev->dwork, disk_events_workfn);
 
@@ -1851,6 +1841,6 @@ static void disk_del_events(struct gendisk *disk)
 static void disk_release_events(struct gendisk *disk)
 {
 	/* the block count should be 1 from disk_del_events() */
-	WARN_ON_ONCE(disk->ev && disk->ev->block != 1);
+	WARN_ON_ONCE(disk->ev && atomic_read(&disk->ev->block) != 1);
 	kfree(disk->ev);
 }
-- 
2.29.2