From: Uros Bizjak <ubizjak@gmail.com>
Date: Wed, 18 Jan 2023 15:07:03 +0000 (+0100)
Subject: lib/genalloc: use try_cmpxchg in {set,clear}_bits_ll
X-Git-Tag: v6.3-rc1~112^2~28
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=030c6ff6498881ac842c7f6da5eec181202b29f4;p=thirdparty%2Fkernel%2Flinux.git

lib/genalloc: use try_cmpxchg in {set,clear}_bits_ll

Use try_cmpxchg instead of cmpxchg (*ptr, old, new) == old in
{set,clear}_bits_ll.  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.

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

The patch also declares these two functions inline, to ensure inlining.

No functional change intended.

Link: https://lkml.kernel.org/r/20230118150703.4024-1-ubizjak@gmail.com
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

diff --git a/lib/genalloc.c b/lib/genalloc.c
index 00fc50d0a640e..0c883d6fbd44d 100644
--- a/lib/genalloc.c
+++ b/lib/genalloc.c
@@ -40,32 +40,30 @@ static inline size_t chunk_size(const struct gen_pool_chunk *chunk)
 	return chunk->end_addr - chunk->start_addr + 1;
 }
 
-static int set_bits_ll(unsigned long *addr, unsigned long mask_to_set)
+static inline int
+set_bits_ll(unsigned long *addr, unsigned long mask_to_set)
 {
-	unsigned long val, nval;
+	unsigned long val = READ_ONCE(*addr);
 
-	nval = *addr;
 	do {
-		val = nval;
 		if (val & mask_to_set)
 			return -EBUSY;
 		cpu_relax();
-	} while ((nval = cmpxchg(addr, val, val | mask_to_set)) != val);
+	} while (!try_cmpxchg(addr, &val, val | mask_to_set));
 
 	return 0;
 }
 
-static int clear_bits_ll(unsigned long *addr, unsigned long mask_to_clear)
+static inline int
+clear_bits_ll(unsigned long *addr, unsigned long mask_to_clear)
 {
-	unsigned long val, nval;
+	unsigned long val = READ_ONCE(*addr);
 
-	nval = *addr;
 	do {
-		val = nval;
 		if ((val & mask_to_clear) != mask_to_clear)
 			return -EBUSY;
 		cpu_relax();
-	} while ((nval = cmpxchg(addr, val, val & ~mask_to_clear)) != val);
+	} while (!try_cmpxchg(addr, &val, val & ~mask_to_clear));
 
 	return 0;
 }