Re: [PATCH v2 4/7] block: honor IOCB_NOWAIT in the block device buffered read path

[Hannes Reinecke]

On 8/28/26 3:49 PM, Tal Zussman wrote:
> blkdev_read_iter() takes inode_lock_shared() unconditionally around
> filemap_read(). Unlike blkdev_write_iter(), it does not reject
> IOCB_NOWAIT for buffered I/O, so a non-blocking read, or the buffered
> tail of a short IOCB_NOWAIT direct read, blocks behind set_blocksize()
> holding i_rwsem across sync_blockdev().
>
> A preadv2(RWF_NOWAIT) issued while another thread changes the block
> size with a dirty page cache blocks for as long as sync_blockdev()
> takes, 4 to 6 seconds on a scsi_debug device with delay=5.
>
> Use inode_trylock_shared() for IOCB_NOWAIT and return the bytes the
> direct path already read, or -EAGAIN if none, when the lock is
> contended, preserving NOWAIT semantics.
>
> Fixes: c0e473a0d226 ("block: fix race between set_blocksize and read paths")
> Reported-by: Sashiko <sashiko-bot@xxxxxxxxxx>
> Link: https://sashiko.dev/#/patchset/20260802-blkdev-fixes-v1-0-a82fc549fd74%40columbia.edu?part=2
> Assisted-by: Claude:claude-fable-5
> Signed-off-by: Tal Zussman <tz2294@xxxxxxxxxxxx>
> ---
>   block/fops.c | 10 +++++++++-
>   1 file changed, 9 insertions(+), 1 deletion(-)
Reviewed-by: Hannes Reinecke <hare@xxxxxxxxxx>

Cheers,

Hannes
--
Dr. Hannes Reinecke                  Kernel Storage Architect
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