linux-kernel - [PATCH] drivers: usb: host: Fix deadlock in oxu_bus

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Date:   Sun, 17 Apr 2022 20:03:05 +0800
From:   Duoming Zhou <duoming@....edu.cn>
To:     linux-kernel@...r.kernel.org
Cc:     gregkh@...uxfoundation.org, davem@...emloft.net,
        alexander.deucher@....com, akpm@...ux-foundation.org,
        broonie@...nel.org, linux-usb@...r.kernel.org,
        Duoming Zhou <duoming@....edu.cn>
Subject: [PATCH] drivers: usb: host: Fix deadlock in oxu_bus_suspend()

There is a deadlock in oxu_bus_suspend(), which is shown below:

   (Thread 1)              |      (Thread 2)
                           | timer_action()
oxu_bus_suspend()          |  mod_timer()
 spin_lock_irq() //(1)     |  (wait a time)
 ...                       | oxu_watchdog()
 del_timer_sync()          |  spin_lock_irq() //(2)
 (wait timer to stop)      |  ...

We hold oxu->lock in position (1) of thread 1, and use
del_timer_sync() to wait timer to stop, but timer handler
also need oxu->lock in position (2) of thread 2. As a result,
oxu_bus_suspend() will block forever.

This patch extracts del_timer_sync() from the protection of
spin_lock_irq(), which could let timer handler to obtain
the needed lock.

Signed-off-by: Duoming Zhou <duoming@....edu.cn>
---
 drivers/usb/host/oxu210hp-hcd.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/usb/host/oxu210hp-hcd.c b/drivers/usb/host/oxu210hp-hcd.c
index b741670525e..ee403df3309 100644
--- a/drivers/usb/host/oxu210hp-hcd.c
+++ b/drivers/usb/host/oxu210hp-hcd.c
@@ -3909,8 +3909,10 @@ static int oxu_bus_suspend(struct usb_hcd *hcd)
 		}
 	}
 
+	spin_unlock_irq(&oxu->lock);
 	/* turn off now-idle HC */
 	del_timer_sync(&oxu->watchdog);
+	spin_lock_irq(&oxu->lock);
 	ehci_halt(oxu);
 	hcd->state = HC_STATE_SUSPENDED;
 
-- 
2.17.1