The patch titled
Subject: lib/genalloc: use try_cmpxchg in {set,clear}_bits_ll
has been added to the -mm mm-nonmm-unstable branch. Its filename is
lib-genalloc-use-try_cmpxchg-in-setclear_bits_ll.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/lib-genalloc-use-try_cmpxchg-in-setclear_bits_ll.patch
This patch will later appear in the mm-nonmm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Uros Bizjak <ubizjak@xxxxxxxxx>
Subject: lib/genalloc: use try_cmpxchg in {set,clear}_bits_ll
Date: Wed, 18 Jan 2023 16:07:03 +0100
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@xxxxxxxxx
Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
lib/genalloc.c | 18 ++++++++----------
1 file changed, 8 insertions(+), 10 deletions(-)
--- a/lib/genalloc.c~lib-genalloc-use-try_cmpxchg-in-setclear_bits_ll
+++ a/lib/genalloc.c
@@ -40,32 +40,30 @@ static inline size_t chunk_size(const st
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;
}
_
Patches currently in -mm which might be from ubizjak@xxxxxxxxx are
lib-genalloc-use-try_cmpxchg-in-setclear_bits_ll.patch
