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Subject[209/272] nbd: remove module-level ioctl mutex
2.6.37-stable review patch.  If anyone has any objections, please let us know.

------------------
From: Soren Hansen <soren@linux2go.dk>

commit de1f016f882e52facc3c8609599f827bcdd14af9 upstream.

Commit 2a48fc0ab242417 ("block: autoconvert trivial BKL users to private
mutex") 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
[<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
[<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.

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@linux2go.dk>
Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Acked-by: Paul Clements <paul.clements@steeleye.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
drivers/block/nbd.c | 3 ---
1 file changed, 3 deletions(-)
--- 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
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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