From a329844ff8adaffc25343a6f9bb12a3a9e841018 Mon Sep 17 00:00:00 2001 From: Chris Metcalf Date: Thu, 10 Nov 2016 20:08:24 -0500 Subject: 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. --- ChangeLog | 5 +++++ sysdeps/tile/tilepro/atomic-machine.h | 10 ++++++++++ 2 files changed, 15 insertions(+) diff --git a/ChangeLog b/ChangeLog index 40ef733564..262069d694 100644 --- a/ChangeLog +++ b/ChangeLog @@ -1,3 +1,8 @@ +2016-11-10 Chris Metcalf + + * sysdeps/tile/tilepro/atomic-machine.h (atomic_store_relaxed) + (atomic_store_release): Provide tilepro-specific implementations. + 2016-11-10 Joseph Myers * math/math.h (__MATH_TG): New macro. diff --git a/sysdeps/tile/tilepro/atomic-machine.h b/sysdeps/tile/tilepro/atomic-machine.h index 702e17d77d..5365929c94 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 #endif /* atomic-machine.h */ -- cgit v1.2.3