GNU C Library master sources branch master updated. glibc-2.24-293-gabff18c

[carlos@sourceware.org]

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- Log -----------------------------------------------------------------
http://sourceware.org/git/gitweb.cgi?p=glibc.git;a=commitdiff;h=abff18c0c6055ca5d1cd46923fd1205c057139a5

commit abff18c0c6055ca5d1cd46923fd1205c057139a5
Author: Carlos O'Donell <carlos@systemhalted.org>
Date:   Wed Oct 26 00:00:13 2016 -0400

    Fix atomic_fetch_xor_release.
    
    No code uses atomic_fetch_xor_release except for the upcoming
    conditional variable rewrite. Therefore there is no user
    visible bug here. The use of atomic_compare_and_exchange_bool_rel
    is removed (since it doesn't exist anymore), and is replaced
    by atomic_compare_exchange_weak_release.
    
    We use weak_release because it provides better performance in
    the loop (the weak semantic) and because the xor is release MO
    (the release semantic). We don't reload expected in the loop
    because atomic_compare_and_exchange_weak_release does this for
    us as part of the CAS failure.
    
    It is otherwise a fairly plain conversion that fixes building
    the new condvar for 32-bit x86. Passes all regression tests
    for x86.

diff --git a/ChangeLog b/ChangeLog
index 6b8763f..9c4d06f 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+2016-10-26  Carlos O'Donell  <carlos@redhat.com>
+
+	* include/atomic.h
+	[USE_COMPILER_ATOMIC_BUILTINS && !atomic_fetch_xor_release]
+	(atomic_fetch_xor_release): Use	atomic_compare_exchange_weak_release.
+
 2016-10-25  Rajalakshmi Srinivasaraghavan  <raji@linux.vnet.ibm.com>
 
 	* stdlib/Makefile (tests): Add tst-strfrom and tst-strfrom-locale.
diff --git a/include/atomic.h b/include/atomic.h
index 5a8e7e7..c8b4664 100644
--- a/include/atomic.h
+++ b/include/atomic.h
@@ -777,18 +777,21 @@ void __atomic_link_error (void);
 # endif
 
 # ifndef atomic_fetch_xor_release
+/* Failing the atomic_compare_exchange_weak_release reloads the value in
+   __atg104_expected, so we need only do the XOR again and retry.  */
 # define atomic_fetch_xor_release(mem, operand) \
-  ({ __typeof (*(mem)) __atg104_old;					      \
-     __typeof (mem) __atg104_memp = (mem);				      \
+  ({ __typeof (mem) __atg104_memp = (mem);				      \
+     __typeof (*(mem)) __atg104_expected = (*__atg104_memp);		      \
+     __typeof (*(mem)) __atg104_desired;				      \
      __typeof (*(mem)) __atg104_op = (operand);				      \
 									      \
      do									      \
-       __atg104_old = (*__atg104_memp);					      \
-     while (__builtin_expect						      \
-	    (atomic_compare_and_exchange_bool_rel (			      \
-		__atg104_memp, __atg104_old ^ __atg104_op, __atg104_old), 0));\
-									      \
-     __atg104_old; })
+       __atg104_desired = __atg104_expected ^ __atg104_op;		      \
+     while (__glibc_unlikely						      \
+	    (atomic_compare_exchange_weak_release (			      \
+	       __atg104_memp, &__atg104_expected, __atg104_desired)	      \
+	     == 0));							      \
+     __atg104_expected; })
 #endif
 
 #endif /* !USE_ATOMIC_COMPILER_BUILTINS  */

-----------------------------------------------------------------------

Summary of changes:
 ChangeLog        |    6 ++++++
 include/atomic.h |   19 +++++++++++--------
 2 files changed, 17 insertions(+), 8 deletions(-)


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