Re: [PATCH] null_blk: Fix scheduling in atomic with zoned mode

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Tue, Nov 10, 2020 at 04:36:05PM +0900, Damien Le Moal wrote:
> commit e1777d099728a76a8f8090f89649aac961e7e530 upstream.
> 
> Commit aa1c09cb65e2 ("null_blk: Fix locking in zoned mode") changed
> zone locking to using the potentially sleeping wait_on_bit_io()
> function. This is acceptable when memory backing is enabled as the
> device queue is in that case marked as blocking, but this triggers a
> scheduling while in atomic context with memory backing disabled.
> 
> Fix this by relying solely on the device zone spinlock for zone
> information protection without temporarily releasing this lock around
> null_process_cmd() execution in null_zone_write(). This is OK to do
> since when memory backing is disabled, command processing does not
> block and the memory backing lock nullb->lock is unused. This solution
> avoids the overhead of having to mark a zoned null_blk device queue as
> blocking when memory backing is unused.
> 
> This patch also adds comments to the zone locking code to explain the
> unusual locking scheme.
> 
> Fixes: aa1c09cb65e2 ("null_blk: Fix locking in zoned mode")
> Reported-by: kernel test robot <lkp@intel.com>
> Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> Cc: stable@vger.kernel.org
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> ---
>  drivers/block/null_blk.h       |  1 +
>  drivers/block/null_blk_zoned.c | 31 +++++++++++++++++++++++++------
>  2 files changed, 26 insertions(+), 6 deletions(-)

Now queued up, thanks.

greg k-h