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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
This commit is contained in:
Uros Bizjak 2023-01-16 17:34:46 +01:00 committed by Ingo Molnar
parent 1d61659ced
commit 50fd4d5e69
1 changed files with 2 additions and 2 deletions
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4
arch/x86/mm/pat/memtype.c
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@ -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)