Patch "mtd: blkdevs: fix potential deadlock + lockdep warnings" has been added to the 4.3-stable tree

[<gregkh@linuxfoundation.org>]

This is a note to let you know that I've just added the patch titled

    mtd: blkdevs: fix potential deadlock + lockdep warnings

to the 4.3-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     mtd-blkdevs-fix-potential-deadlock-lockdep-warnings.patch
and it can be found in the queue-4.3 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.


>From f3c63795e90f0c6238306883b6c72f14d5355721 Mon Sep 17 00:00:00 2001
From: Brian Norris <computersforpeace@gmail.com>
Date: Mon, 26 Oct 2015 10:20:23 -0700
Subject: mtd: blkdevs: fix potential deadlock + lockdep warnings

From: Brian Norris <computersforpeace@gmail.com>

commit f3c63795e90f0c6238306883b6c72f14d5355721 upstream.

Commit 073db4a51ee4 ("mtd: fix: avoid race condition when accessing
mtd->usecount") fixed a race condition but due to poor ordering of the
mutex acquisition, introduced a potential deadlock.

The deadlock can occur, for example, when rmmod'ing the m25p80 module, which
will delete one or more MTDs, along with any corresponding mtdblock
devices. This could potentially race with an acquisition of the block
device as follows.

 -> blktrans_open()
    ->  mutex_lock(&dev->lock);
    ->  mutex_lock(&mtd_table_mutex);

 -> del_mtd_device()
    ->  mutex_lock(&mtd_table_mutex);
    ->  blktrans_notify_remove() -> del_mtd_blktrans_dev()
       ->  mutex_lock(&dev->lock);

This is a classic (potential) ABBA deadlock, which can be fixed by
making the A->B ordering consistent everywhere. There was no real
purpose to the ordering in the original patch, AFAIR, so this shouldn't
be a problem. This ordering was actually already present in
del_mtd_blktrans_dev(), for one, where the function tried to ensure that
its caller already held mtd_table_mutex before it acquired &dev->lock:

        if (mutex_trylock(&mtd_table_mutex)) {
                mutex_unlock(&mtd_table_mutex);
                BUG();
        }

So, reverse the ordering of acquisition of &dev->lock and &mtd_table_mutex so
we always acquire mtd_table_mutex first.

Snippets of the lockdep output follow:

  # modprobe -r m25p80
  [   53.419251]
  [   53.420838] ======================================================
  [   53.427300] [ INFO: possible circular locking dependency detected ]
  [   53.433865] 4.3.0-rc6 #96 Not tainted
  [   53.437686] -------------------------------------------------------
  [   53.444220] modprobe/372 is trying to acquire lock:
  [   53.449320]  (&new->lock){+.+...}, at: [<c043fe4c>] del_mtd_blktrans_dev+0x80/0xdc
  [   53.457271]
  [   53.457271] but task is already holding lock:
  [   53.463372]  (mtd_table_mutex){+.+.+.}, at: [<c0439994>] del_mtd_device+0x18/0x100
  [   53.471321]
  [   53.471321] which lock already depends on the new lock.
  [   53.471321]
  [   53.479856]
  [   53.479856] the existing dependency chain (in reverse order) is:
  [   53.487660]
  -> #1 (mtd_table_mutex){+.+.+.}:
  [   53.492331]        [<c043fc5c>] blktrans_open+0x34/0x1a4
  [   53.497879]        [<c01afce0>] __blkdev_get+0xc4/0x3b0
  [   53.503364]        [<c01b0bb8>] blkdev_get+0x108/0x320
  [   53.508743]        [<c01713c0>] do_dentry_open+0x218/0x314
  [   53.514496]        [<c0180454>] path_openat+0x4c0/0xf9c
  [   53.519959]        [<c0182044>] do_filp_open+0x5c/0xc0
  [   53.525336]        [<c0172758>] do_sys_open+0xfc/0x1cc
  [   53.530716]        [<c000f740>] ret_fast_syscall+0x0/0x1c
  [   53.536375]
  -> #0 (&new->lock){+.+...}:
  [   53.540587]        [<c063f124>] mutex_lock_nested+0x38/0x3cc
  [   53.546504]        [<c043fe4c>] del_mtd_blktrans_dev+0x80/0xdc
  [   53.552606]        [<c043f164>] blktrans_notify_remove+0x7c/0x84
  [   53.558891]        [<c04399f0>] del_mtd_device+0x74/0x100
  [   53.564544]        [<c043c670>] del_mtd_partitions+0x80/0xc8
  [   53.570451]        [<c0439aa0>] mtd_device_unregister+0x24/0x48
  [   53.576637]        [<c046ce6c>] spi_drv_remove+0x1c/0x34
  [   53.582207]        [<c03de0f0>] __device_release_driver+0x88/0x114
  [   53.588663]        [<c03de19c>] device_release_driver+0x20/0x2c
  [   53.594843]        [<c03dd9e8>] bus_remove_device+0xd8/0x108
  [   53.600748]        [<c03dacc0>] device_del+0x10c/0x210
  [   53.606127]        [<c03dadd0>] device_unregister+0xc/0x20
  [   53.611849]        [<c046d878>] __unregister+0x10/0x20
  [   53.617211]        [<c03da868>] device_for_each_child+0x50/0x7c
  [   53.623387]        [<c046eae8>] spi_unregister_master+0x58/0x8c
  [   53.629578]        [<c03e12f0>] release_nodes+0x15c/0x1c8
  [   53.635223]        [<c03de0f8>] __device_release_driver+0x90/0x114
  [   53.641689]        [<c03de900>] driver_detach+0xb4/0xb8
  [   53.647147]        [<c03ddc78>] bus_remove_driver+0x4c/0xa0
  [   53.652970]        [<c00cab50>] SyS_delete_module+0x11c/0x1e4
  [   53.658976]        [<c000f740>] ret_fast_syscall+0x0/0x1c
  [   53.664621]
  [   53.664621] other info that might help us debug this:
  [   53.664621]
  [   53.672979]  Possible unsafe locking scenario:
  [   53.672979]
  [   53.679169]        CPU0                    CPU1
  [   53.683900]        ----                    ----
  [   53.688633]   lock(mtd_table_mutex);
  [   53.692383]                                lock(&new->lock);
  [   53.698306]                                lock(mtd_table_mutex);
  [   53.704658]   lock(&new->lock);
  [   53.707946]
  [   53.707946]  *** DEADLOCK ***

Fixes: 073db4a51ee4 ("mtd: fix: avoid race condition when accessing mtd->usecount")
Reported-by: Felipe Balbi <balbi@ti.com>
Tested-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 drivers/mtd/mtd_blkdevs.c |   10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

--- a/drivers/mtd/mtd_blkdevs.c
+++ b/drivers/mtd/mtd_blkdevs.c
@@ -192,8 +192,8 @@ static int blktrans_open(struct block_de
 	if (!dev)
 		return -ERESTARTSYS; /* FIXME: busy loop! -arnd*/
 
-	mutex_lock(&dev->lock);
 	mutex_lock(&mtd_table_mutex);
+	mutex_lock(&dev->lock);
 
 	if (dev->open)
 		goto unlock;
@@ -217,8 +217,8 @@ static int blktrans_open(struct block_de
 
 unlock:
 	dev->open++;
-	mutex_unlock(&mtd_table_mutex);
 	mutex_unlock(&dev->lock);
+	mutex_unlock(&mtd_table_mutex);
 	blktrans_dev_put(dev);
 	return ret;
 
@@ -228,8 +228,8 @@ error_release:
 error_put:
 	module_put(dev->tr->owner);
 	kref_put(&dev->ref, blktrans_dev_release);
-	mutex_unlock(&mtd_table_mutex);
 	mutex_unlock(&dev->lock);
+	mutex_unlock(&mtd_table_mutex);
 	blktrans_dev_put(dev);
 	return ret;
 }
@@ -241,8 +241,8 @@ static void blktrans_release(struct gend
 	if (!dev)
 		return;
 
-	mutex_lock(&dev->lock);
 	mutex_lock(&mtd_table_mutex);
+	mutex_lock(&dev->lock);
 
 	if (--dev->open)
 		goto unlock;
@@ -256,8 +256,8 @@ static void blktrans_release(struct gend
 		__put_mtd_device(dev->mtd);
 	}
 unlock:
-	mutex_unlock(&mtd_table_mutex);
 	mutex_unlock(&dev->lock);
+	mutex_unlock(&mtd_table_mutex);
 	blktrans_dev_put(dev);
 }
 


Patches currently in stable-queue which might be from computersforpeace@gmail.com are

queue-4.3/mtd-mtdpart-fix-add_mtd_partitions-error-path.patch
queue-4.3/mtd-blkdevs-fix-potential-deadlock-lockdep-warnings.patch
queue-4.3/mtd-nand-fix-shutdown-reboot-for-multi-chip-systems.patch
queue-4.3/mtd-jz4740_nand-fix-build-on-jz4740-after-removing-gpio.h.patch