Re: nbd locking problems

[Paul Clements]

2a48fc0ab24241755dc93bfd4f01d68efab47f5a replaced uses of the BKL in
the nbd driver with mutex operations. Since then, I've been been seeing
these lock ups:

 INFO: task qemu-nbd:16115 blocked for more than 120 seconds.
 "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
 qemu-nbd      D 0000000000000001     0 16115  16114 0x00000004
  ffff88007d775d98 0000000000000082 ffff88007d775fd8 ffff88007d774000
  0000000000013a80 ffff8800020347e0 ffff88007d775fd8 0000000000013a80
  ffff880133730000 ffff880002034440 ffffea0004333db8 ffffffffa071c020
 Call Trace:
  [<ffffffff815b9997>] __mutex_lock_slowpath+0xf7/0x180
  [<ffffffff81132f13>] ? handle_mm_fault+0x1c3/0x410
  [<ffffffff815b93eb>] mutex_lock+0x2b/0x50
  [<ffffffffa071a21c>] nbd_ioctl+0x6c/0x1c0 [nbd]
  [<ffffffff812cb970>] blkdev_ioctl+0x230/0x730
  [<ffffffff811967a1>] block_ioctl+0x41/0x50
  [<ffffffff81175c03>] do_vfs_ioctl+0x93/0x370
  [<ffffffff8116fa83>] ? putname+0x33/0x50
  [<ffffffff81175f61>] sys_ioctl+0x81/0xa0
  [<ffffffff8100c0c2>] system_call_fastpath+0x16/0x1b
Instrumenting the nbd module's ioctl handler with some extra logging
clearly shows the NBD_DO_IT ioctl being invoked which is a long-lived
ioctl in the sense that it doesn't return until another ioctl asks the
driver to disconnect. However, that other ioctl blocks, waiting for the
module-level mutex that replaced the BKL, and then we're stuck.

Soren, just getting to this, but your analysis is correct. Since
Pavel's patch of about 2 years ago (putting tx_lock on the ioctls),
the BKL or any other locking around ioctls is not needed, so your
patch looks correct to me.

Thanks very much for this.

--
Paul


This patch removes the module-level mutex altogether. It's clearly
wrong, and as far as I can see, it's entirely unnecessary, since the
nbd driver maintains per-device mutexes, and I don't see anything that
would require a module-level (or kernel-level, for that matter) mutex.

Signed-off-by: Soren Hansen <soren@xxxxxxxxxxx>
---
 drivers/block/nbd.c |    3 ---
 1 files changed, 0 insertions(+), 3 deletions(-)
diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index a32fb41..e6fc716 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -53,7 +53,6 @@
 #define DBG_BLKDEV      0x0100
 #define DBG_RX          0x0200
 #define DBG_TX          0x0400
-static DEFINE_MUTEX(nbd_mutex);
 static unsigned int debugflags;
 #endif /* NDEBUG */

@@ -718,11 +717,9 @@ static int nbd_ioctl(struct block_device *bdev,
fmode_t mode,
 	dprintk(DBG_IOCTL, "%s: nbd_ioctl cmd=%s(0x%x) arg=%lu\n",
 			lo->disk->disk_name, ioctl_cmd_to_ascii(cmd), cmd, arg);

-	mutex_lock(&nbd_mutex);
 	mutex_lock(&lo->tx_lock);
 	error = __nbd_ioctl(bdev, lo, cmd, arg);
 	mutex_unlock(&lo->tx_lock);
-	mutex_unlock(&nbd_mutex);

 	return error;
 }
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