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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 <>

    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 <>
    Acked-by: Serge Hallyn <>
    Acked-by: Paul Clements <>
    Cc: Arnd Bergmann <>
    Cc: Jens Axboe <>
    Signed-off-by: Andrew Morton <>
    Signed-off-by: Linus Torvalds <>
    Signed-off-by: Greg Kroah-Hartman <>

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

    return error;

     \ /
      Last update: 2011-02-16 01:41    [W:0.021 / U:133.096 seconds]
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