diff options
| -rw-r--r-- | libitm/ChangeLog | 4 | ||||
| -rw-r--r-- | libitm/method-gl.cc | 28 |
2 files changed, 20 insertions, 12 deletions
diff --git a/libitm/ChangeLog b/libitm/ChangeLog index 977613d05be..da94906bcd3 100644 --- a/libitm/ChangeLog +++ b/libitm/ChangeLog @@ -1,5 +1,9 @@ 2012-02-20 Torvald Riegel <triegel@redhat.com> + * method-gl.cc (gl_wt_dispatch::rollback): Optimize memory orders. + +2012-02-20 Torvald Riegel <triegel@redhat.com> + * method-gl.cc (gl_wt_dispatch::trycommit): Remove handling of serial mode corner cases made obsolete by prior gtm_rwlock changes. (gl_wt_dispatch.rollback): Same. diff --git a/libitm/method-gl.cc b/libitm/method-gl.cc index 5bae22b90c1..4fd506ef154 100644 --- a/libitm/method-gl.cc +++ b/libitm/method-gl.cc @@ -314,22 +314,26 @@ public: // value that is correct wrt. privatization safety. if (gl_mg::is_locked(v)) { - // Release the global orec, increasing its version number / timestamp. - // See begin_or_restart() for why we need release memory order here. + // With our rollback, global time increases. v = gl_mg::clear_locked(v) + 1; - o_gl_mg.orec.store(v, memory_order_release); - // Also reset the timestamp published via shared_state. + // First reset the timestamp published via shared_state. Release + // memory order will make this happen after undoing prior data writes. + // This must also happen before we actually release the global orec + // next, so that future update transactions in other threads observe + // a meaningful snapshot time for our transaction; otherwise, they + // could read a shared_store value with the LOCK_BIT set, which can + // break privatization safety because it's larger than the actual + // snapshot time. Note that we only need to consider other update + // transactions because only those will potentially privatize data. tx->shared_state.store(v, memory_order_release); - // We need a store-load barrier after this store to prevent it - // from becoming visible after later data loads because the - // previous value of shared_state has been higher than the actual - // snapshot time (the lock bit had been set), which could break - // privatization safety. We do not need a barrier before this - // store (see pre_write() for an explanation). - // ??? What is the precise reasoning in the C++11 model? - atomic_thread_fence(memory_order_seq_cst); + // Release the global orec, increasing its version number / timestamp. + // See begin_or_restart() for why we need release memory order here, + // and we also need it to make future update transactions read the + // prior update to shared_state too (update transactions acquire the + // global orec with acquire memory order). + o_gl_mg.orec.store(v, memory_order_release); } } |
