linux-kernel - [PATCH] driver/dma/ioat: Call del_timer_sync() without holding prep

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Date:   Fri, 14 Sep 2018 14:53:32 -0400
From:   Waiman Long <longman@...hat.com>
To:     Vinod Koul <vkoul@...nel.org>
Cc:     linux-kernel@...r.kernel.org, dmaengine@...r.kernel.org,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Kees Cook <keescook@...omium.org>,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        Waiman Long <longman@...hat.com>
Subject: [PATCH] driver/dma/ioat: Call del_timer_sync() without holding prep_lock

The following lockdep splat was observed:

[ 1222.241750] ======================================================
[ 1222.271301] WARNING: possible circular locking dependency detected
[ 1222.301060] 4.16.0-10.el8+5.x86_64+debug #1 Not tainted
[ 1222.326659] ------------------------------------------------------
[ 1222.356565] systemd-shutdow/1 is trying to acquire lock:
[ 1222.382660]  ((&ioat_chan->timer)){+.-.}, at: [<00000000f71e1a28>] del_timer_sync+0x5/0xf0
[ 1222.422928]
[ 1222.422928] but task is already holding lock:
[ 1222.451743]  (&(&ioat_chan->prep_lock)->rlock){+.-.}, at: [<000000008ea98b12>] ioat_shutdown+0x86/0x100 [ioatdma]
   :
[ 1223.524987] Chain exists of:
[ 1223.524987]   (&ioat_chan->timer) --> &(&ioat_chan->cleanup_lock)->rlock --> &(&ioat_chan->prep_lock)->rlock
[ 1223.524987]
[ 1223.594082]  Possible unsafe locking scenario:
[ 1223.594082]
[ 1223.622630]        CPU0                    CPU1
[ 1223.645080]        ----                    ----
[ 1223.667404]   lock(&(&ioat_chan->prep_lock)->rlock);
[ 1223.691535]                                lock(&(&ioat_chan->cleanup_lock)->rlock);
[ 1223.728657]                                lock(&(&ioat_chan->prep_lock)->rlock);
[ 1223.765122]   lock((&ioat_chan->timer));
[ 1223.784095]
[ 1223.784095]  *** DEADLOCK ***
[ 1223.784095]
[ 1223.813492] 4 locks held by systemd-shutdow/1:
[ 1223.834677]  #0:  (reboot_mutex){+.+.}, at: [<0000000056d33456>] SYSC_reboot+0x10f/0x300
[ 1223.873310]  #1:  (&dev->mutex){....}, at: [<00000000258dfdd7>] device_shutdown+0x1c8/0x660
[ 1223.913604]  #2:  (&dev->mutex){....}, at: [<0000000068331147>] device_shutdown+0x1d6/0x660
[ 1223.954000]  #3:  (&(&ioat_chan->prep_lock)->rlock){+.-.}, at: [<000000008ea98b12>] ioat_shutdown+0x86/0x100 [ioatdma]

In the ioat_shutdown() function:

	spin_lock_bh(&ioat_chan->prep_lock);
	set_bit(IOAT_CHAN_DOWN, &ioat_chan->state);
	del_timer_sync(&ioat_chan->timer);
	spin_unlock_bh(&ioat_chan->prep_lock);

According to the synchronization rule for the del_timer_sync() function,
the caller must not hold locks which would prevent completion of the
timer's handler.

The timer structure has its own lock that manages its synchronization.
Setting the IOAT_CHAN_DOWN bit should prevent other CPUs from
trying to use that device anyway, there is probably no need to call
del_timer_sync() while holding the prep_lock. So the del_timer_sync()
call is now moved outside of the prep_lock critical section to prevent
the circular lock dependency.

Signed-off-by: Waiman Long <longman@...hat.com>
---
 drivers/dma/ioat/init.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/dma/ioat/init.c b/drivers/dma/ioat/init.c
index 4fa4c06..21a5708 100644
--- a/drivers/dma/ioat/init.c
+++ b/drivers/dma/ioat/init.c
@@ -1205,8 +1205,15 @@ static void ioat_shutdown(struct pci_dev *pdev)
 
 		spin_lock_bh(&ioat_chan->prep_lock);
 		set_bit(IOAT_CHAN_DOWN, &ioat_chan->state);
-		del_timer_sync(&ioat_chan->timer);
 		spin_unlock_bh(&ioat_chan->prep_lock);
+		/*
+		 * Synchronization rule for del_timer_sync():
+		 *  - The caller must not hold locks which would prevent
+		 *    completion of the timer's handler.
+		 * So prep_lock cannot be held before calling it.
+		 */
+		del_timer_sync(&ioat_chan->timer);
+
 		/* this should quiesce then reset */
 		ioat_reset_hw(ioat_chan);
 	}
-- 
1.8.3.1