[PATCH v2 3/4] PM: hibernate: allow wait_for_device_probe() to timeout when resuming from hibernation

[Tetsuo Handa]

syzbot is reporting hung task at misc_open() [1], for there is a race
window of AB-BA deadlock which involves probe_count variable.

Even with "char: misc: allow calling open() callback without misc_mtx
held" and "PM: hibernate: call wait_for_device_probe() without
system_transition_mutex held", wait_for_device_probe() from snapshot_open()
can sleep forever if probe_count cannot become 0.

Since snapshot_open() is a userland-driven hibernation/resume request,
it should be acceptable to fail if something is wrong. Users would not
want to wait for hours if device stopped responding. Therefore, introduce
killable version of wait_for_device_probe() with timeout.

According to Oliver Neukum, there are SCSI commands that can run for more
than 60 seconds. Therefore, this patch choose 5 minutes for timeout.

Link: https://syzkaller.appspot.com/bug?extid=358c9ab4c93da7b7238c [1]
Reported-by: syzbot <syzbot+358c9ab4c93da7b7238c@xxxxxxxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Cc: Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Oliver Neukum <oneukum@xxxxxxxx>
Cc: Wedson Almeida Filho <wedsonaf@xxxxxxxxxx>
Cc: Rafael J. Wysocki <rjw@xxxxxxx>
Cc: Arjan van de Ven <arjan@xxxxxxxxxxxxxxx>
---
 drivers/base/dd.c             | 14 ++++++++++++++
 include/linux/device/driver.h |  1 +
 kernel/power/user.c           |  9 +++++++--
 3 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/drivers/base/dd.c b/drivers/base/dd.c
index 70f79fc71539..3136b8403bb0 100644
--- a/drivers/base/dd.c
+++ b/drivers/base/dd.c
@@ -724,6 +724,20 @@ void wait_for_device_probe(void)
 }
 EXPORT_SYMBOL_GPL(wait_for_device_probe);
 
+void wait_for_device_probe_killable_timeout(unsigned long timeout)
+{
+	/* wait for probe timeout */
+	wait_event(probe_timeout_waitqueue, !driver_deferred_probe_timeout);
+
+	/* wait for the deferred probe workqueue to finish */
+	flush_work(&deferred_probe_work);
+
+	/* wait for the known devices to complete their probing */
+	wait_event_killable_timeout(probe_waitqueue,
+				    atomic_read(&probe_count) == 0, timeout);
+	async_synchronize_full();
+}
+
 static int __driver_probe_device(struct device_driver *drv, struct device *dev)
 {
 	int ret = 0;
diff --git a/include/linux/device/driver.h b/include/linux/device/driver.h
index 7acaabde5396..4ee909144470 100644
--- a/include/linux/device/driver.h
+++ b/include/linux/device/driver.h
@@ -129,6 +129,7 @@ extern struct device_driver *driver_find(const char *name,
 					 struct bus_type *bus);
 extern int driver_probe_done(void);
 extern void wait_for_device_probe(void);
+extern void wait_for_device_probe_killable_timeout(unsigned long timeout);
 void __init wait_for_init_devices_probe(void);
 
 /* sysfs interface for exporting driver attributes */
diff --git a/kernel/power/user.c b/kernel/power/user.c
index db98a028dfdd..32dd5a855e8c 100644
--- a/kernel/power/user.c
+++ b/kernel/power/user.c
@@ -58,8 +58,13 @@ static int snapshot_open(struct inode *inode, struct file *filp)
 		/* The image device should be already ready. */
 		break;
 	default: /* Resuming */
-		/* We may need to wait for the image device to appear. */
-		wait_for_device_probe();
+		/*
+		 * Since the image device might not be ready, try to wait up to
+		 * 5 minutes. We should not wait forever, for we might get stuck
+		 * due to unresponsive devices and/or new probe events which
+		 * are irrelevant to the image device keep coming in.
+		 */
+		wait_for_device_probe_killable_timeout(300 * HZ);
 		break;
 	}
 
-- 
2.18.4