Use C11-like atomics instead of plain memory accesses in x86 lock elision.

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; therefore, we add
support for shorter-size atomic load and stores too.

	* include/atomic.h (__atomic_check_size_ls): New.
	(atomic_load_relaxed, atomic_load_acquire, atomic_store_relaxed,
	atomic_store_release): Use it.
	* sysdeps/x86/elide.h (ACCESS_ONCE): Remove.
	(elision_adapt, ELIDE_LOCK): Use atomics.
	* sysdeps/unix/sysv/linux/x86/elision-lock.c (__lll_lock_elision): Use
	atomics and improve code comments.
	* sysdeps/unix/sysv/linux/x86/elision-trylock.c
	(__lll_trylock_elision): Likewise.

3 files changed, 50 insertions, 26 deletions

diff --git a/sysdeps/unix/sysv/linux/x86/elision-lock.c b/sysdeps/unix/sysv/linux/x86/elision-lock.c
index 5e66960123..c05ade4722 100644
--- a/sysdeps/unix/sysv/linux/x86/elision-lock.c
+++ b/sysdeps/unix/sysv/linux/x86/elision-lock.c
@@ -44,7 +44,13 @@
 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)
     {
       unsigned status;
       int try_xbegin;
@@ -70,15 +76,20 @@ __lll_lock_elision (int *futex, short *adapt_count, EXTRAARG int private)
 			&& _XABORT_CODE (status) == _ABORT_LOCK_BUSY)
 	        {
 		  /* Right now we skip here.  Better would be to wait a bit
-		     and retry.  This likely needs some spinning.  */
-		  if (*adapt_count != aconf.skip_lock_busy)
-		    *adapt_count = aconf.skip_lock_busy;
+		     and retry.  This likely needs some spinning.  See
+		     above for why relaxed MO is sufficient.  */
+		  if (atomic_load_relaxed (adapt_count)
+		      != aconf.skip_lock_busy)
+		    atomic_store_relaxed (adapt_count, aconf.skip_lock_busy);
 		}
 	      /* Internal abort.  There is no chance for retry.
 		 Use the normal locking and next time use lock.
-		 Be careful to avoid writing to the lock.  */
-	      else if (*adapt_count != aconf.skip_lock_internal_abort)
-		*adapt_count = aconf.skip_lock_internal_abort;
+		 Be careful to avoid writing to the lock.  See above for why
+		 relaxed MO is sufficient.  */
+	      else if (atomic_load_relaxed (adapt_count)
+		  != aconf.skip_lock_internal_abort)
+		atomic_store_relaxed (adapt_count,
+		    aconf.skip_lock_internal_abort);
 	      break;
 	    }
 	}
@@ -87,7 +98,8 @@ __lll_lock_elision (int *futex, short *adapt_count, EXTRAARG int private)
     {
       /* Use a normal lock until the threshold counter runs out.
 	 Lost updates possible.  */
-      (*adapt_count)--;
+      atomic_store_relaxed (adapt_count,
+	  atomic_load_relaxed (adapt_count) - 1);
     }
 
   /* Use a normal lock here.  */
diff --git a/sysdeps/unix/sysv/linux/x86/elision-trylock.c b/sysdeps/unix/sysv/linux/x86/elision-trylock.c
index 65d9c18584..2d44f50426 100644
--- a/sysdeps/unix/sysv/linux/x86/elision-trylock.c
+++ b/sysdeps/unix/sysv/linux/x86/elision-trylock.c
@@ -36,8 +36,10 @@ __lll_trylock_elision (int *futex, short *adapt_count)
      return an error.  */
   _xabort (_ABORT_NESTED_TRYLOCK);
 
-  /* 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;
 
@@ -55,16 +57,18 @@ __lll_trylock_elision (int *futex, short *adapt_count)
       if (!(status & _XABORT_RETRY))
         {
           /* Internal abort.  No chance for retry.  For future
-             locks don't try speculation for some time.  */
-          if (*adapt_count != aconf.skip_trylock_internal_abort)
-            *adapt_count = aconf.skip_trylock_internal_abort;
+             locks don't try speculation for some time.  See above for MO.  */
+          if (atomic_load_relaxed (adapt_count)
+              != aconf.skip_lock_internal_abort)
+            atomic_store_relaxed (adapt_count, aconf.skip_lock_internal_abort);
         }
       /* Could do some retries here.  */
     }
   else
     {
-      /* Lost updates are possible, but harmless.  */
-      (*adapt_count)--;
+      /* Lost updates are possible but harmless (see above).  */
+      atomic_store_relaxed (adapt_count,
+	  atomic_load_relaxed (adapt_count) - 1);
     }
 
   return lll_trylock (*futex);
diff --git a/sysdeps/x86/elide.h b/sysdeps/x86/elide.h
index 8691e6673d..f7d5220c17 100644
--- a/sysdeps/x86/elide.h
+++ b/sysdeps/x86/elide.h
@@ -20,8 +20,8 @@
 
 #include <hle.h>
 #include <elision-conf.h>
+#include <atomic.h>
 
-#define ACCESS_ONCE(x) (* (volatile typeof(x) *) &(x))
 
 /* Adapt elision with ADAPT_COUNT and STATUS and decide retries.  */
 
@@ -35,28 +35,35 @@ elision_adapt(signed char *adapt_count, unsigned int status)
     {
       /* Right now we skip here.  Better would be to wait a bit
 	 and retry.  This likely needs some spinning. Be careful
-	 to avoid writing the lock.  */
-      if (*adapt_count != __elision_aconf.skip_lock_busy)
-	ACCESS_ONCE (*adapt_count) = __elision_aconf.skip_lock_busy;
+	 to avoid writing the lock.
+	 Using relaxed MO and separate atomic accesses is sufficient because
+	 adapt_count is just a hint.  */
+      if (atomic_load_relaxed (adapt_count) != __elision_aconf.skip_lock_busy)
+	atomic_store_relaxed (adapt_count, __elision_aconf.skip_lock_busy);
     }
   /* Internal abort.  There is no chance for retry.
      Use the normal locking and next time use lock.
-     Be careful to avoid writing to the lock.  */
-  else if (*adapt_count != __elision_aconf.skip_lock_internal_abort)
-    ACCESS_ONCE (*adapt_count) = __elision_aconf.skip_lock_internal_abort;
+     Be careful to avoid writing to the lock.  See above for MO.  */
+  else if (atomic_load_relaxed (adapt_count)
+      != __elision_aconf.skip_lock_internal_abort)
+    atomic_store_relaxed (adapt_count,
+	__elision_aconf.skip_lock_internal_abort);
   return true;
 }
 
 /* is_lock_free must be executed inside the transaction */
 
 /* Returns true if lock defined by IS_LOCK_FREE was elided.
-   ADAPT_COUNT is a pointer to per-lock state variable. */
+   ADAPT_COUNT is a per-lock state variable; it must be accessed atomically
+   to avoid data races but is just a hint, so using relaxed MO and separate
+   atomic loads and stores instead of atomic read-modify-write operations is
+   sufficient.  */
 
 #define ELIDE_LOCK(adapt_count, is_lock_free)			\
   ({								\
     int ret = 0;						\
 								\
-    if ((adapt_count) <= 0) 					\
+    if (atomic_load_relaxed (&(adapt_count)) <= 0)		\
       {								\
         for (int i = __elision_aconf.retry_try_xbegin; i > 0; i--) \
           {							\
@@ -75,12 +82,13 @@ elision_adapt(signed char *adapt_count, unsigned int status)
           }							\
       }								\
     else 							\
-      (adapt_count)--; /* missing updates ok */			\
+      atomic_store_relaxed (&(adapt_count),			\
+	  atomic_load_relaxed (&(adapt_count)) - 1);		\
     ret;							\
   })
 
 /* Returns true if lock defined by IS_LOCK_FREE was try-elided.
-   ADAPT_COUNT is a pointer to per-lock state variable.  */
+   ADAPT_COUNT is a per-lock state variable.  */
 
 #define ELIDE_TRYLOCK(adapt_count, is_lock_free, write) ({	\
   int ret = 0;						\