linux-kernel - [PATCH] mm/page_alloc: replace local_lock with normal spinlock -fix -fix

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Message-ID: <20220708144406.GJ27531@techsingularity.net>
Date:   Fri, 8 Jul 2022 15:44:06 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Nicolas Saenz Julienne <nsaenzju@...hat.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...nel.org>,
        Hugh Dickins <hughd@...gle.com>, Yu Zhao <yuzhao@...gle.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: [PATCH] mm/page_alloc: replace local_lock with normal spinlock -fix
 -fix

pcpu_spin_unlock and pcpu_spin_unlock_irqrestore both unlock
pcp->lock and then enable preemption. This lacks symmetry against
both the pcpu_spin helpers and differs from how local_unlock_* is
implemented. While this is harmless, it's unnecessary and it's generally
better to unwind locks and preemption state in the reverse order as
they were acquired.

This is a fix on top of the mm-unstable patch
mm-page_alloc-replace-local_lock-with-normal-spinlock-fix.patch

Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>
---
 mm/page_alloc.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 934d1b5a5449..d0141e51e613 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -192,14 +192,14 @@ static DEFINE_MUTEX(pcp_batch_high_lock);
 
 #define pcpu_spin_unlock(member, ptr)					\
 ({									\
-	spin_unlock(&ptr->member);					\
 	pcpu_task_unpin();						\
+	spin_unlock(&ptr->member);					\
 })
 
 #define pcpu_spin_unlock_irqrestore(member, ptr, flags)			\
 ({									\
-	spin_unlock_irqrestore(&ptr->member, flags);			\
 	pcpu_task_unpin();						\
+	spin_unlock_irqrestore(&ptr->member, flags);			\
 })
 
 /* struct per_cpu_pages specific helpers. */