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

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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>
This commit is contained in:
Uros Bizjak
2023-01-18 16:07:03 +01:00
committed by Andrew Morton
parent f3296f80c8
commit 030c6ff649
1 changed files with 8 additions and 10 deletions
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18
lib/genalloc.c
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@@ -40,32 +40,30 @@ static inline size_t chunk_size(const struct gen_pool_chunk *chunk)
return chunk->end_addr - chunk->start_addr + 1; 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 { do {
val = nval;
if (val & mask_to_set) if (val & mask_to_set)
return -EBUSY; return -EBUSY;
cpu_relax(); cpu_relax();
} while ((nval = cmpxchg(addr, val, val | mask_to_set)) != val); } while (!try_cmpxchg(addr, &val, val | mask_to_set));
return 0; 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 { do {
val = nval;
if ((val & mask_to_clear) != mask_to_clear) if ((val & mask_to_clear) != mask_to_clear)
return -EBUSY; return -EBUSY;
cpu_relax(); cpu_relax();
} while ((nval = cmpxchg(addr, val, val & ~mask_to_clear)) != val); } while (!try_cmpxchg(addr, &val, val & ~mask_to_clear));
return 0; return 0;
} }