[PATCH] xfs_scrub_all: fix deadlock if lsblk produces a lot of output

[PATCH] xfs_scrub_all: fix deadlock if lsblk produces a lot of output

[Darrick J. Wong]

From: Darrick J. Wong <djwong@kernel.org>

Patrick Fischer reported a deadlock in find_mounts() that is the result
of lsblk producing so much output that it fills the pipe buffer.  When
this happens, lsblk blocks on write()ing the pipe, at which point
waiting for lsblk to exit will also block forever.

Now that we can be reasonably assured that everyone has Python 3.5
(because RHEL6 is long dead), we can replace this whole mess with a call
to subprocess.run that captures the output.  The json library can
convert a byte array directly to a python dict, which means we don't
need to concatenate iterated lines or any of that stuff anymore.

Reported-by: patrick.fischer@siedl.net
Link: https://lore.kernel.org/linux-xfs/323580211.1220195.1777554001363.JavaMail.zimbra@siedl.net/
Cc: <linux-xfs@vger.kernel.org> # v4.15.0
Fixes: f1dca11cad1308 ("xfs_scrub: create a script to scrub all xfs filesystems")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
---
 scrub/xfs_scrub_all.py.in |   13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/scrub/xfs_scrub_all.py.in b/scrub/xfs_scrub_all.py.in
index 9f861639a43ce4..dbb6c36e467d3f 100644
--- a/scrub/xfs_scrub_all.py.in
+++ b/scrub/xfs_scrub_all.py.in
@@ -53,15 +53,16 @@ def find_mounts():
 
 	fs = {}
 	cmd=['lsblk', '-o', 'NAME,KNAME,TYPE,FSTYPE,MOUNTPOINT', '-J']
-	result = subprocess.Popen(cmd, stdout=subprocess.PIPE)
-	result.wait()
-	if result.returncode != 0:
+	try:
+		proc = subprocess.run(cmd, capture_output = True, text = True, check = True)
+	except Exception as e:
+		print(e)
+		return fs
+	if proc.returncode != 0:
 		return fs
-	sarray = [x.decode(sys.stdout.encoding) for x in result.stdout.readlines()]
-	output = ' '.join(sarray)
-	bdevdata = json.loads(output)
 
 	# The lsblk output had better be in disks-then-partitions order
+	bdevdata = json.loads(proc.stdout)
 	for bdev in bdevdata['blockdevices']:
 		lastdisk = bdev['kname']
 		find_xfs_mounts(bdev, fs, lastdisk)

Re: [PATCH] xfs_scrub_all: fix deadlock if lsblk produces a lot of output

[Andrey Albershteyn]

On 2026-04-30 08:54:06, Darrick J. Wong wrote:
> From: Darrick J. Wong <djwong@kernel.org>
> 
> Patrick Fischer reported a deadlock in find_mounts() that is the result
> of lsblk producing so much output that it fills the pipe buffer.  When
> this happens, lsblk blocks on write()ing the pipe, at which point
> waiting for lsblk to exit will also block forever.
> 
> Now that we can be reasonably assured that everyone has Python 3.5
> (because RHEL6 is long dead), we can replace this whole mess with a call
> to subprocess.run that captures the output.  The json library can
> convert a byte array directly to a python dict, which means we don't
> need to concatenate iterated lines or any of that stuff anymore.
> 
> Reported-by: patrick.fischer@siedl.net
> Link: https://lore.kernel.org/linux-xfs/323580211.1220195.1777554001363.JavaMail.zimbra@siedl.net/
> Cc: <linux-xfs@vger.kernel.org> # v4.15.0
> Fixes: f1dca11cad1308 ("xfs_scrub: create a script to scrub all xfs filesystems")
> Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>

Looks good to me
Reviewed-by: Andrey Albershteyn <aalbersh@kernel.org>

-- 
- Andrey

Re: [PATCH] xfs_scrub_all: fix deadlock if lsblk produces a lot of output

[Christoph Hellwig]

Looks good:

Reviewed-by: Christoph Hellwig <hch@lst.de>