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

   (Thread 1)                |      (Thread 2)
                             | rr_open()
rr_close()                   |  add_timer()
 spin_lock_irqsave() //(1)   |  (wait a time)
 ...                         | rr_timer()
 del_timer_sync()            |  spin_lock_irqsave() //(2)
 (wait timer to stop)        |  ...

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

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

Signed-off-by: Duoming Zhou <duoming@zju.edu.cn>
---
 drivers/net/hippi/rrunner.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/net/hippi/rrunner.c b/drivers/net/hippi/rrunner.c
index 16105292b14..74e845fa2e0 100644
--- a/drivers/net/hippi/rrunner.c
+++ b/drivers/net/hippi/rrunner.c
@@ -1355,7 +1355,9 @@ static int rr_close(struct net_device *dev)
 
 	rrpriv->fw_running = 0;
 
+	spin_unlock_irqrestore(&rrpriv->lock, flags);
 	del_timer_sync(&rrpriv->timer);
+	spin_lock_irqsave(&rrpriv->lock, flags);
 
 	writel(0, &regs->TxPi);
 	writel(0, &regs->IpRxPi);
-- 
2.17.1

Hello:

This patch was applied to netdev/net.git (master)
by Paolo Abeni <pabeni@redhat.com>:

On Sun, 17 Apr 2022 20:55:19 +0800 you wrote:
> There is a deadlock in rr_close(), which is shown below:
> 
>    (Thread 1)                |      (Thread 2)
>                              | rr_open()
> rr_close()                   |  add_timer()
>  spin_lock_irqsave() //(1)   |  (wait a time)
>  ...                         | rr_timer()
>  del_timer_sync()            |  spin_lock_irqsave() //(2)
>  (wait timer to stop)        |  ...
> 
> [...]

Here is the summary with links:
  - drivers: net: hippi: Fix deadlock in rr_close()
    https://git.kernel.org/netdev/net/c/bc6de2878429

You are awesome, thank you!
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