[PATCH v4 4/8] block: honor IOCB_NOWAIT in the block device buffered read path

[Tal Zussman <tz2294@columbia.edu>]

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@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>
---
 block/fops.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/block/fops.c b/block/fops.c
index a51814821100..a3a709697b40 100644
--- a/block/fops.c
+++ b/block/fops.c
@@ -845,7 +845,15 @@ 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);
 

-- 
2.39.5