Re: [PATCH] Use C11 atomics instead of atomic_bit_set/bit_test_set

[Wilco Dijkstra <Wilco.Dijkstra@arm.com>]

Hi Florian,

> Why is acquire MO required here?  I don't see any synchronizing store.
> Isn't this mostly a compiler barrier for use-after-free detection?

You're right, it looks like the only reason for atomic is to ensure memory
is only freed once. A few cancelhandling accesses use acquire, and there
are various relaxed loads and even non-atomic loads of it, but not a
single release store, so there is no use for acquire MO here.

Cheers,
Wilco


v2: Use relaxed atomics since there is no MO dependence

Replace the 3 uses of atomic_bit_set and atomic_bit_test_set with
atomic_fetch_or_relaxed.

---

diff --git a/malloc/malloc.c b/malloc/malloc.c
index 29fa71b3b2a3d0a671149eaf619e4d518c56aef5..ecec901b14f602e3c93da1a847f043ffee41a1f4 100644
--- a/malloc/malloc.c
+++ b/malloc/malloc.c
@@ -2460,11 +2460,11 @@ sysmalloc_mmap (INTERNAL_SIZE_T nb, size_t pagesize, int extra_flags, mstate av)
     }
 
   /* update statistics */
-  int new = atomic_exchange_and_add (&mp_.n_mmaps, 1) + 1;
+  int new = atomic_fetch_add_relaxed (&mp_.n_mmaps, 1) + 1;
   atomic_max (&mp_.max_n_mmaps, new);
 
   unsigned long sum;
-  sum = atomic_exchange_and_add (&mp_.mmapped_mem, size) + size;
+  sum = atomic_fetch_add_relaxed (&mp_.mmapped_mem, size) + size;
   atomic_max (&mp_.max_mmapped_mem, sum);
 
   check_chunk (av, p);
@@ -3084,7 +3084,7 @@ mremap_chunk (mchunkptr p, size_t new_size)
   set_head (p, (new_size - offset) | IS_MMAPPED);
 
   INTERNAL_SIZE_T new;
-  new = atomic_exchange_and_add (&mp_.mmapped_mem, new_size - size - offset)
+  new = atomic_fetch_add_relaxed (&mp_.mmapped_mem, new_size - size - offset)
         + new_size - size - offset;
   atomic_max (&mp_.max_mmapped_mem, new);
   return p;