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Date:	Mon, 16 Jun 2014 21:29:54 -0700 (PDT)
From:	David Miller <>
Subject: Re: [PATCH 1/2] slip: Fix deadlock in write_wakeup

From: Tyler Hall <>
Date: Sun, 15 Jun 2014 22:23:16 -0400

> Use schedule_work() to avoid potentially taking the spinlock in
> interrupt context.
> Commit cc9fa74e2a ("slip/slcan: added locking in wakeup function") added
> necessary locking to the wakeup function and 367525c8c2/ddcde142be ("can:
> slcan: Fix spinlock variant") converted it to spin_lock_bh() because the lock
> is also taken in timers.
> Disabling softirqs is not sufficient, however, as tty drivers may call
> write_wakeup from interrupt context. This driver calls tty->ops->write() with
> its spinlock held, which may immediately cause an interrupt on the same CPU and
> subsequent spin_bug().
> Simply converting to spin_lock_irq/irqsave() prevents this deadlock, but
> causes lockdep to point out a possible circular locking dependency
> between these locks:
> (&(&sl->lock)->rlock){-.....}, at: slip_write_wakeup
> (&port_lock_key){-.....}, at: serial8250_handle_irq.part.13
> The slip transmit is holding the slip spinlock when calling the tty write.
> This grabs the port lock. On an interrupt, the handler grabs the port
> lock and calls write_wakeup which grabs the slip lock. This could be a
> problem if a serial interrupt occurs on another CPU during the slip
> transmit.
> To deal with these issues, don't grab the lock in the wakeup function by
> deferring the writeout to a workqueue. Also hold the lock during close
> when de-assigning the tty pointer to safely disarm the worker and
> timers.
> This bug is easily reproducible on the first transmit when slip is
> used with the standard 8250 serial driver.
> Signed-off-by: Tyler Hall <>

Applied and queued up for -stable.
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