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@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20230116163446.4734-1-ubizjak@gmail.com

diff --git a/arch/x86/mm/pat/memtype.c b/arch/x86/mm/pat/memtype.c
index fb4b1b5..6d1ba2d 100644 (file)
--- 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)