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-rw-r--r--ChangeLog7
-rw-r--r--sysdeps/unix/sysv/linux/s390/elision-lock.c25
-rw-r--r--sysdeps/unix/sysv/linux/s390/elision-trylock.c14
3 files changed, 34 insertions, 12 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.
diff --git a/sysdeps/unix/sysv/linux/s390/elision-lock.c b/sysdeps/unix/sysv/linux/s390/elision-lock.c
index ecb507e989..1876d2128d 100644
--- a/sysdeps/unix/sysv/linux/s390/elision-lock.c
+++ b/sysdeps/unix/sysv/linux/s390/elision-lock.c
@@ -45,11 +45,18 @@
int
__lll_lock_elision (int *futex, short *adapt_count, EXTRAARG int private)
{
- if (*adapt_count > 0)
+ /* adapt_count can be accessed concurrently; these accesses can be both
+ inside of transactions (if critical sections are nested and the outer
+ critical section uses lock elision) and outside of transactions. Thus,
+ we need to use atomic accesses to avoid data races. However, the
+ value of adapt_count is just a hint, so relaxed MO accesses are
+ sufficient. */
+ if (atomic_load_relaxed (adapt_count) > 0)
{
/* 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);
goto use_lock;
}
@@ -74,8 +81,10 @@ __lll_lock_elision (int *futex, short *adapt_count, EXTRAARG int private)
/* In a non-nested transaction there is no need to abort,
which is expensive. */
__builtin_tend ();
+ /* Don't try to use transactions for the next couple of times.
+ 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);
goto use_lock;
}
else /* nesting depth is > 1 */
@@ -101,18 +110,20 @@ __lll_lock_elision (int *futex, short *adapt_count, EXTRAARG int private)
/* A persistent abort (cc 1 or 3) indicates that a retry is
probably futile. Use the normal locking now and for the
next couple of calls.
- Be careful to avoid writing to the lock. */
+ Be careful to avoid writing to the lock. See above for why
+ relaxed MO is sufficient. */
if (aconf.skip_lock_internal_abort > 0)
- *adapt_count = aconf.skip_lock_internal_abort;
+ atomic_store_relaxed (adapt_count,
+ aconf.skip_lock_internal_abort);
goto use_lock;
}
}
}
/* Same logic as above, but for for a number of temporary failures in a
- row. */
+ row. See above for why relaxed MO is sufficient. */
if (aconf.skip_lock_out_of_tbegin_retries > 0 && aconf.try_tbegin > 0)
- *adapt_count = aconf.skip_lock_out_of_tbegin_retries;
+ atomic_store_relaxed (adapt_count, aconf.skip_lock_out_of_tbegin_retries);
use_lock:
return LLL_LOCK ((*futex), private);
diff --git a/sysdeps/unix/sysv/linux/s390/elision-trylock.c b/sysdeps/unix/sysv/linux/s390/elision-trylock.c
index 3d5a994ad9..a3252b83ce 100644
--- a/sysdeps/unix/sysv/linux/s390/elision-trylock.c
+++ b/sysdeps/unix/sysv/linux/s390/elision-trylock.c
@@ -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);