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block: honor IOCB_NOWAIT in the block device buffered read path

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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: c0e473a0d2 ("block: fix race between set_blocksize and read paths")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Link: https://sashiko.dev/#/patchset/20260802-blkdev-fixes-v1-0-a82fc549fd74%40columbia.edu?part=2
Assisted-by: Claude:claude-fable-5
Reviewed-by: Hannes Reinecke <hare@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Tal Zussman <tz2294@columbia.edu>
Link: https://patch.msgid.link/20261009-blkdev-fixes-v6-4-307939c387df@columbia.edu
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Tal Zussman 2026-10-09 12:50:12 -04:00 committed by Jens Axboe
parent 5574b81c64
commit d16519a561
1 changed files with 9 additions and 1 deletions
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10
block/fops.c
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@ -873,7 +873,15 @@ static ssize_t blkdev_read_iter(struct kiocb *iocb, struct iov_iter *to)
* Take i_rwsem and invalidate_lock to avoid racing with set_blocksize
* changing i_blkbits/folio order and punching out the pagecache.
*/
inode_lock_shared(bd_inode);
if (iocb->ki_flags & IOCB_NOWAIT) {
if (!inode_trylock_shared(bd_inode)) {
if (!ret)
ret = -EAGAIN;
goto reexpand;
}
} else {
inode_lock_shared(bd_inode);
}
ret = filemap_read(iocb, to, ret);
inode_unlock_shared(bd_inode);