GNU C Library master sources branch master updated. glibc-2.24-362-ga329844

[cmetcalf at sourceware dot org]

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http://sourceware.org/git/gitweb.cgi?p=glibc.git;a=commitdiff;h=a329844ff8adaffc25343a6f9bb12a3a9e841018

commit a329844ff8adaffc25343a6f9bb12a3a9e841018
Author: Chris Metcalf <cmetcalf@mellanox.com>
Date:   Thu Nov 10 20:08:24 2016 -0500

    Make sure tilepro uses kernel atomics fo atomic_store
    
    It's not legal for raw stores to be mixed with atomic operations
    on tilepro, since the atomics are managed by kernel fast syscalls.
    It's possible for a hardware store and a kernel fast atomic to race
    with each other in such a way that the hardware store is lost.
    
    Suppose you have an initial zero value, and you race with a store
    of 2 and a kernel cmpxchg from 0 to 1.  The legal output is only 2:
    either the store hit first and the cmpxchg failed, or the cmpxchg
    hit first and succeeded, then was overwritten by the 2.  But if
    the kernel cmpxchg starts first and loads the zero, then the store
    hits and sets the value to 2, the cmpxchg will still decide it was
    successful and write the 1, leaving the value illegally set to 1.
    
    Using atomic_exchange variants to implement atomic_store fixes this
    problem for tilepro.

diff --git a/ChangeLog b/ChangeLog
index 40ef733..262069d 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+2016-11-10  Chris Metcalf  <cmetcalf@mellanox.com>
+
+	* sysdeps/tile/tilepro/atomic-machine.h (atomic_store_relaxed)
+	(atomic_store_release): Provide tilepro-specific implementations.
+
 2016-11-10  Joseph Myers  <joseph@codesourcery.com>
 
 	* math/math.h (__MATH_TG): New macro.
diff --git a/sysdeps/tile/tilepro/atomic-machine.h b/sysdeps/tile/tilepro/atomic-machine.h
index 702e17d..5365929 100644
--- a/sysdeps/tile/tilepro/atomic-machine.h
+++ b/sysdeps/tile/tilepro/atomic-machine.h
@@ -83,6 +83,16 @@ int __atomic_update_32 (volatile int *mem, int mask, int addend)
   ({ __typeof (mask) __att1_v = (mask);                 \
     __atomic_update ((mem), ~__att1_v, __att1_v); })
 
+/*
+ * We must use the kernel atomics for atomic_store, since otherwise an
+ * unsynchronized store could become visible after another core's
+ * kernel-atomic implementation had read the memory word in question,
+ * but before it had written the updated value to it, which would
+ * cause the unsynchronized store to be lost.
+ */
+#define atomic_store_relaxed(mem, val) atomic_exchange_acq (mem, val)
+#define atomic_store_release(mem, val) atomic_exchange_rel (mem, val)
+
 #include <sysdeps/tile/atomic-machine.h>
 
 #endif /* atomic-machine.h */

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

Summary of changes:
 ChangeLog                             |    5 +++++
 sysdeps/tile/tilepro/atomic-machine.h |   10 ++++++++++
 2 files changed, 15 insertions(+), 0 deletions(-)


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