+ revert-lib-plistc-enforce-memory-ordering-in-plist_check_list.patch added to mm-nonmm-unstable branch

+ revert-lib-plistc-enforce-memory-ordering-in-plist_check_list.patch added to mm-nonmm-unstable branch

[Andrew Morton]

The patch titled
     Subject: Revert "lib/plist.c: enforce memory ordering in plist_check_list"
has been added to the -mm mm-nonmm-unstable branch.  Its filename is
     revert-lib-plistc-enforce-memory-ordering-in-plist_check_list.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/revert-lib-plistc-enforce-memory-ordering-in-plist_check_list.patch

This patch will later appear in the mm-nonmm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***

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------------------------------------------------------
From: Kuan-Wei Chiu <visitorckw@gmail.com>
Subject: Revert "lib/plist.c: enforce memory ordering in plist_check_list"
Date: Thu, 13 Nov 2025 19:34:13 +0000

This reverts commit 7abcb84f953df037d40fad66f2109db318dd155b.

The introduction of WRITE_ONCE() calls for the 'prev' and 'next' variables
inside plist_check_list() was a misapplication.  WRITE_ONCE() is
fundamentally a compiler barrier designed to prevent compiler
optimizations (like caching or reordering) on shared memory locations. 
However, the variables 'prev' and 'next' are local, stack-allocated
pointers accessed only by the current thread's invocation of the function.

Since these pointers are thread-local and are never accessed concurrently,
applying WRITE_ONCE() to them is semantically incorrect and unnecessary. 
Furthermore, the use of WRITE_ONCE() on local variables prevents the
compiler from performing standard optimizations, such as keeping these
variables cached solely in CPU registers throughout the loop, potentially
introducing performance overhead.  Restore the conventional C assignment
for local loop variables, allowing the compiler to generate optimal code.

Link: https://lkml.kernel.org/r/20251113193413.499309-1-visitorckw@gmail.com
Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Cc: I Hsin Cheng <richard120310@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 lib/plist.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/lib/plist.c~revert-lib-plistc-enforce-memory-ordering-in-plist_check_list
+++ a/lib/plist.c
@@ -47,8 +47,8 @@ static void plist_check_list(struct list
 
 	plist_check_prev_next(top, prev, next);
 	while (next != top) {
-		WRITE_ONCE(prev, next);
-		WRITE_ONCE(next, prev->next);
+		prev = next;
+		next = prev->next;
 		plist_check_prev_next(top, prev, next);
 	}
 }
_

Patches currently in -mm which might be from visitorckw@gmail.com are

revert-lib-plistc-enforce-memory-ordering-in-plist_check_list.patch