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S390: Use C11-like atomics instead of plain memory accesses in lock elision code.

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This uses atomic operations to access lock elision metadata that is accessed
concurrently (ie, adapt_count fields).  The size of the data is less than a
word but accessed only with atomic loads and stores.

See also x86 commit ca6e601a9d:
"Use C11-like atomics instead of plain memory accesses in x86 lock elision."

ChangeLog:

	* sysdeps/unix/sysv/linux/s390/elision-lock.c
	(__lll_lock_elision): Use atomics to load / store adapt_count.
	* sysdeps/unix/sysv/linux/s390/elision-trylock.c
	(__lll_trylock_elision): Likewise.
This commit is contained in:
Stefan Liebler
2016-12-20 15:12:48 +01:00
parent 8d71242eb7
commit c813dae5d8
3 changed files with 34 additions and 12 deletions
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14
sysdeps/unix/sysv/linux/s390/elision-trylock.c
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@ -49,8 +49,10 @@ __lll_trylock_elision (int *futex, short *adapt_count)
__builtin_tabort (_HTM_FIRST_USER_ABORT_CODE | 1);
}
/* Only try a transaction if it's worth it. */
if (*adapt_count <= 0)
/* Only try a transaction if it's worth it. See __lll_lock_elision for
why we need atomic accesses. Relaxed MO is sufficient because this is
just a hint. */
if (atomic_load_relaxed (adapt_count) <= 0)
{
unsigned status;
@ -65,9 +67,10 @@ __lll_trylock_elision (int *futex, short *adapt_count)
__builtin_tend ();
/* Note: Changing the adapt_count here might abort a transaction on a
different cpu, but that could happen anyway when the futex is
acquired, so there's no need to check the nesting depth here. */
acquired, so there's no need to check the nesting depth here.
See above for why relaxed MO is sufficient. */
if (aconf.skip_lock_busy > 0)
*adapt_count = aconf.skip_lock_busy;
atomic_store_relaxed (adapt_count, aconf.skip_lock_busy);
}
else
{
@ -87,7 +90,8 @@ __lll_trylock_elision (int *futex, short *adapt_count)
{
/* Lost updates are possible, but harmless. Due to races this might lead
to *adapt_count becoming less than zero. */
(*adapt_count)--;
atomic_store_relaxed (adapt_count,
atomic_load_relaxed (adapt_count) - 1);
}
return lll_trylock (*futex);