[PATCH] rapidio/mport_cdev: Fix race in mport_cdev_release()

[Alexander X Sverdlin <alexander.sverdlin@nokia.com>]

From: Alexander Sverdlin <alexander.sverdlin@nokia.com>

While get_dma_channel() is protected against concurrent calls, there is a
race against kref_put() in mport_cdev_release():

CPU0                                        CPU1

get_dma_channel()
 kref_init(&priv->md->dma_ref);
 ...
mport_cdev_release_dma()
 kref_put(&md->dma_ref,
          mport_release_def_dma);
                                            get_dma_channel()
                                             if (priv->md->dma_chan) {
                                              ...
                                              kref_get(&priv->md->dma_ref);
  mport_release_def_dma()
   md->dma_chan = NULL;

which may appear like this:

------------[ cut here ]------------
WARNING: CPU: 1 PID: 12057 at .../linux/include/linux/kref.h:46 rio_dma_transfer.isra.12+0x8e0/0xbe8 [rio_mport_cdev]
 ...
CPU: 1 PID: 12057 Comm: ... Tainted: G           O    4.9.109-... #1
Stack : ...

Call Trace:
[<ffffffff80140040>] show_stack+0x90/0xb0
[<ffffffff803eeeb8>] dump_stack+0x88/0xc0
[<ffffffff80159670>] __warn+0x108/0x120
[<ffffffffc0541df0>] rio_dma_transfer.isra.12+0x8e0/0xbe8 [rio_mport_cdev]
[<ffffffffc05426fc>] mport_cdev_ioctl+0x604/0x2988 [rio_mport_cdev]
[<ffffffff802881e8>] do_vfs_ioctl+0xb8/0x780
[<ffffffff80288938>] SyS_ioctl+0x88/0xc0
[<ffffffff80146ae8>] syscall_common+0x34/0x58
---[ end trace 78842d4915cfc502 ]---

Fixes: e8de370188d0 ("rapidio: add mport char device driver")
Cc: stable@vger.kernel.org
Signed-off-by: Alexander Sverdlin <alexander.sverdlin@nokia.com>
---
 drivers/rapidio/devices/rio_mport_cdev.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/rapidio/devices/rio_mport_cdev.c b/drivers/rapidio/devices/rio_mport_cdev.c
index 8155f59..a6276dc 100644
--- a/drivers/rapidio/devices/rio_mport_cdev.c
+++ b/drivers/rapidio/devices/rio_mport_cdev.c
@@ -1980,6 +1980,7 @@ static void mport_cdev_release_dma(struct file *filp)
 			current->comm, task_pid_nr(current), wret);
 	}
 
+	mutex_lock(&priv->dma_lock);
 	if (priv->dmach != priv->md->dma_chan) {
 		rmcd_debug(EXIT, "Release DMA channel for filp=%p %s(%d)",
 			   filp, current->comm, task_pid_nr(current));
@@ -1990,6 +1991,7 @@ static void mport_cdev_release_dma(struct file *filp)
 	}
 
 	priv->dmach = NULL;
+	mutex_unlock(&priv->dma_lock);
 }
 #else
 #define mport_cdev_release_dma(priv) do {} while (0)
-- 
2.4.6