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

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 ca6e601a9d4a72b3699cca15bad12ac1716bf49a:
"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.

1 files changed, 7 insertions, 0 deletions

diff --git a/ChangeLog b/ChangeLog
index 8cdcae68fb..cc21db7a77 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+2016-12-20  Stefan Liebler  <stli@linux.vnet.ibm.com>
+
+	* 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.
+
 2016-12-20  Florian Weimer  <fweimer@redhat.com>
 
 	Do not require memset elimination in explicit_bzero test.