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 00fc50d0a640e03bcd72bf382b2814397b90b897..0c883d6fbd44d06763ef037e5df0b55e44222c88 100644 (file)
--- 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;
 }