linux-kernel - [PATCH] x86/PAT: use try_cmpxchg in set_page

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Date:   Mon, 16 Jan 2023 17:34:46 +0100
From:   Uros Bizjak <ubizjak@...il.com>
To:     x86@...nel.org, linux-kernel@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: [PATCH] x86/PAT: use try_cmpxchg in set_page_memtype

Use try_cmpxchg instead of cmpxchg (*ptr, old, new) == old in
set_page_memtype.  x86 CMPXCHG instruction returns success in ZF flag,
so this change saves a compare after cmpxchg (and related move
instruction in front of cmpxchg).

Also, try_cmpxchg implicitly assigns old *ptr value to "old" when cmpxchg
fails. There is no need to re-read the value in the loop.

Note that the value from *ptr should be read using READ_ONCE to prevent
the compiler from merging, refetching or reordering the read.

No functional change intended.

Signed-off-by: Uros Bizjak <ubizjak@...il.com>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Borislav Petkov <bp@...en8.de>
Cc: "H. Peter Anvin" <hpa@...or.com>
---
 arch/x86/mm/pat/memtype.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/mm/pat/memtype.c b/arch/x86/mm/pat/memtype.c
index fb4b1b5e0dea..6d1ba2dda35d 100644
--- a/arch/x86/mm/pat/memtype.c
+++ b/arch/x86/mm/pat/memtype.c
@@ -159,10 +159,10 @@ static inline void set_page_memtype(struct page *pg,
 		break;
 	}
 
+	old_flags = READ_ONCE(pg->flags);
 	do {
-		old_flags = pg->flags;
 		new_flags = (old_flags & _PGMT_CLEAR_MASK) | memtype_flags;
-	} while (cmpxchg(&pg->flags, old_flags, new_flags) != old_flags);
+	} while (!try_cmpxchg(&pg->flags, &old_flags, new_flags));
 }
 #else
 static inline enum page_cache_mode get_page_memtype(struct page *pg)
-- 
2.39.0