workqueue: fix state-dump console deadlock

Console drivers often queue work while holding locks also taken in their console write paths, something which can lead to deadlocks on SMP when dumping workqueue state (e.g. sysrq-t or on suspend failures).

For serial console drivers this could look like:

	CPU0				CPU1
	----				----

	show_workqueue_state();
	  lock(&pool->lock);		<IRQ>
	  				  lock(&port->lock);
					  schedule_work();
					    lock(&pool->lock);
	  printk();
	    lock(console_owner);
	    lock(&port->lock);

where workqueues are, for example, used to push data to the line discipline, process break signals and handle modem-status changes. Line disciplines and serdev drivers can also queue work on write-wakeup notifications, etc.

Reworking every console driver to avoid queuing work while holding locks also taken in their write paths would complicate drivers and is neither desirable or feasible.

Instead use the deferred-printk mechanism to avoid printing while holding pool locks when dumping workqueue state.

Note that there are a few WARN_ON() assertions in the workqueue code which could potentially also trigger a deadlock. Hopefully the ongoing printk rework will provide a general solution for this eventually.

This was originally reported after a lockdep splat when executing sysrq-t with the imx serial driver.

Fixes 3494fc30846d ("workqueue: dump workqueues on sysrq-t")

Cc [email protected] # 4.0

Reported-by Fabio Estevam <[email protected]>

Tested-by Fabio Estevam <[email protected]>

Signed-off-by Johan Hovold <[email protected]>

---

Changes in v2
- defer printing also of worker pool state (Peter Mladek)
- add Fabio's tested-by tag

 kernel/workqueue.c | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

kernel/workqueue.c +16 -2

index 33a6b4a2443d..1b3eb1e9531f 100644
@@ -4830,8 +4830,16 @@ void show_workqueue_state(void)
 
 		for_each_pwq(pwq, wq) {
 			raw_spin_lock_irqsave(&pwq->pool->lock, flags);
-			if (pwq->nr_active || !list_empty(&pwq->inactive_works))
+			if (pwq->nr_active || !list_empty(&pwq->inactive_works)) {
+				/*
+				 * Defer printing to avoid deadlocks in console
+				 * drivers that queue work while holding locks
+				 * also taken in their write paths.
+				 */
+				printk_deferred_enter();
 				show_pwq(pwq);
+				printk_deferred_exit();
+			}
 			raw_spin_unlock_irqrestore(&pwq->pool->lock, flags);
 			/*
 			 * We could be printing a lot from atomic context, e.g.

@@ -4849,7 +4857,12 @@ void show_workqueue_state(void)
 		raw_spin_lock_irqsave(&pool->lock, flags);
 		if (pool->nr_workers == pool->nr_idle)
 			goto next_pool;
-
+		/*
+		 * Defer printing to avoid deadlocks in console drivers that
+		 * queue work while holding locks also taken in their write
+		 * paths.
+		 */
+		printk_deferred_enter();
 		pr_info("pool %d:", pool->id);
 		pr_cont_pool_info(pool);
 		pr_cont(" hung=%us workers=%d",

@@ -4864,6 +4877,7 @@ void show_workqueue_state(void)
 			first = false;
 		}
 		pr_cont("\n");
+		printk_deferred_exit();
 	next_pool:
 		raw_spin_unlock_irqrestore(&pool->lock, flags);
 		/*

Signature · 1 line

2.32.0

Console drivers often queue work while holding locks also taken in their console write paths, something which can lead to deadlocks on SMP when dumping workqueue state (e.g. sysrq-t or on suspend failures).

For serial console drivers this could look like:

	CPU0				CPU1
	----				----

	show_workqueue_state();
	  lock(&pool->lock);		<IRQ>
	  				  lock(&port->lock);
					  schedule_work();
					    lock(&pool->lock);
	  printk();
	    lock(console_owner);
	    lock(&port->lock);

where workqueues are, for example, used to push data to the line discipline, process break signals and handle modem-status changes. Line disciplines and serdev drivers can also queue work on write-wakeup notifications, etc.

Reworking every console driver to avoid queuing work while holding locks also taken in their write paths would complicate drivers and is neither desirable or feasible.

Instead use the deferred-printk mechanism to avoid printing while holding pool locks when dumping workqueue state.

Note that there are a few WARN_ON() assertions in the workqueue code which could potentially also trigger a deadlock. Hopefully the ongoing printk rework will provide a general solution for this eventually.

This was originally reported after a lockdep splat when executing sysrq-t with the imx serial driver.

Fixes 3494fc30846d ("workqueue: dump workqueues on sysrq-t")

Cc [email protected] # 4.0

Reported-by Fabio Estevam <[email protected]>

Tested-by Fabio Estevam <[email protected]>

Signed-off-by Johan Hovold <[email protected]>