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MDEV-26720: rw_lock: Prefer fetch_sub() to fetch_and()

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rw_lock::write_unlock(): Revert part of
commit d46b42489a (MDEV-24142)
to make the IA-32 and AMD64 implementation faster.
This commit is contained in:
Marko Mäkelä 2021-10-02 11:29:44 +03:00
parent d301cc8edb
commit 0144d1d2a6
1 changed files with 6 additions and 2 deletions
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8
storage/innobase/include/rw_lock.h
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@ -196,8 +196,12 @@ public:
/** Release an exclusive lock */
void write_unlock()
{
IF_DBUG_ASSERT(auto l=,)
lock.fetch_and(~WRITER, std::memory_order_release);
/* Below, we use fetch_sub(WRITER) instead of fetch_and(~WRITER).
The reason is that on IA-32 and AMD64 it translates into the 80486
instruction LOCK XADD, while fetch_and() translates into a loop
around LOCK CMPXCHG. For other ISA either form should be fine. */
static_assert(WRITER == 1U << 31, "compatibility");
IF_DBUG_ASSERT(auto l=,) lock.fetch_sub(WRITER, std::memory_order_release);
/* the write lock must have existed */
#ifdef SUX_LOCK_GENERIC
DBUG_ASSERT((l & (WRITER | UPDATER)) == WRITER);