linux-kernel - Re: [PATCH] checkpatch: warn about flushing system-wide workqueues

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Message-ID: <6e4ed62e-888b-6e7a-c13d-67656f39ca94@I-love.SAKURA.ne.jp>
Date:   Mon, 25 Apr 2022 09:33:30 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     Joe Perches <joe@...ches.com>, Andy Whitcroft <apw@...onical.com>,
        Dwaipayan Ray <dwaipayanray1@...il.com>,
        Lukas Bulwahn <lukas.bulwahn@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] checkpatch: warn about flushing system-wide workqueues

On 2022/04/25 8:45, Joe Perches wrote:
> And it's probably more readable using separate lines and it looks as
> if the 3rd test is unnecessary as it could be combined with the 2nd.
> 
> 		if ($line =~ /\bflush_scheduled_work\s*\(/ ||
> 		    $line =~ /\bflush_workqueue\s*\(\s*system(_\w*)?_wq\s*\)/) {

We don't need to worry about possibility like

	flush_workqueue(system_module1_wq);

? Then, we can simplify like you suggested.

>From 06a4c861eed2062ba58c3023ec28b1b3020bd1f8 Mon Sep 17 00:00:00 2001
From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Date: Mon, 25 Apr 2022 09:28:27 +0900
Subject: [PATCH v2] checkpatch: warn about flushing system-wide workqueues

Flushing the system-wide WQ has possibility of deadlock, for the caller
waits for completion of all works in that WQ even if the caller cannot
wait for completion of one of works due to locking dependency. Since
it is difficult to catch such attempts using lockdep, try to catch also
using checkpatch.

Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
---
Changes in v2:
  Simplify matching, suggested by Joe Perches <joe@...ches.com>

 scripts/checkpatch.pl | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
index 577e02998701..88491cfcfd59 100755
--- a/scripts/checkpatch.pl
+++ b/scripts/checkpatch.pl
@@ -7226,6 +7226,13 @@ sub process {
 			     "Deprecated use of '$deprecated_api', prefer '$new_api' instead\n" . $herecurr);
 		}
 
+# check for flushing system-wide workqueues
+		if ($line =~ /\bflush_scheduled_work\s*\(/ ||
+		    $line =~ /\bflush_workqueue\s*\(\s*system(_\w*)?_wq\s*\)/) {
+			ERROR("DEPRECATED_API",
+			      "Flushing system-wide workqueues is dangerous and will be forbidden - see https://lkml.kernel.org/r/49925af7-78a8-a3dd-bce6-cfc02e1a9236\@I-love.SAKURA.ne.jp\n" . $herecurr);
+		}
+
 # check for various structs that are normally const (ops, kgdb, device_tree)
 # and avoid what seem like struct definitions 'struct foo {'
 		if (defined($const_structs) &&
-- 
2.34.1