linux-kernel - [PATCH V2 10/11] arch: xtensa: platforms: Fix deadlock in iss_net

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Date:   Sat,  9 Apr 2022 13:51:35 +0800
From:   Duoming Zhou <duoming@....edu.cn>
To:     linux-kernel@...r.kernel.org
Cc:     linux-xtensa@...ux-xtensa.org, kuba@...nel.org,
        alexander.deucher@....com, gregkh@...uxfoundation.org,
        davem@...emloft.net, jcmvbkbc@...il.com, chris@...kel.net,
        Duoming Zhou <duoming@....edu.cn>
Subject: [PATCH V2 10/11] arch: xtensa: platforms: Fix deadlock in iss_net_close()

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

   (Thread 1)              |      (Thread 2)
                           | iss_net_open()
iss_net_close()            |  mod_timer()
 spin_lock_bh() //(1)      |  (wait a time)
 ...                       | iss_net_timer()
 del_timer_sync()          |  spin_lock() //(2)
 (wait timer to stop)      |  ...

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

This patch extracts del_timer_sync() from the protection of
spin_lock_bh(), which could let timer handler to obtain
the needed lock. What`s more, we should remove spin_lock()
in iss_net_timer(), because there is no resource need to
protect and spin_lock() may cause deadlock in timer handler.

Signed-off-by: Duoming Zhou <duoming@....edu.cn>
---
Changes in V2:
  - Remove spin_lock() in iss_net_timer().

 arch/xtensa/platforms/iss/network.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/xtensa/platforms/iss/network.c b/arch/xtensa/platforms/iss/network.c
index be3aaaad8be..1ae39c18c6e 100644
--- a/arch/xtensa/platforms/iss/network.c
+++ b/arch/xtensa/platforms/iss/network.c
@@ -352,9 +352,7 @@ static void iss_net_timer(struct timer_list *t)
 	struct iss_net_private *lp = from_timer(lp, t, timer);
 
 	iss_net_poll();
-	spin_lock(&lp->lock);
 	mod_timer(&lp->timer, jiffies + lp->timer_val);
-	spin_unlock(&lp->lock);
 }
 
 
@@ -403,7 +401,9 @@ static int iss_net_close(struct net_device *dev)
 	list_del(&opened);
 	spin_unlock(&opened_lock);
 
+	spin_unlock_bh(&lp->lock);
 	del_timer_sync(&lp->timer);
+	spin_lock_bh(&lp->lock);
 
 	lp->tp.close(lp);
 
-- 
2.17.1