Patchwork libitm: Fix wake-up of readers in futex-based serial lock.

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Submitter Torvald Riegel
Date Jan. 24, 2012, 5:43 p.m.
Message ID <1327427009.15992.684.camel@triegel.csb>
Download mbox | patch
Permalink /patch/137604/
State New
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Comments

Torvald Riegel - Jan. 24, 2012, 5:43 p.m.
This patch fixes a lost wake-up bug in the futex-based serial lock
implementation.  The previous code lacked some pieces of synchronization
(a seq_cst barrier and resetting a flag that is checked by the
respective futex_wait()), which led to those lost wake-ups when waking
and waiting were racing against each other.
Tested on ppc64 with microbenchmarks/STAMP and gl_wt.

OK for trunk?
commit a04fadca7c74f35d6d1d4f12d745c0ab44cc315a
Author: Torvald Riegel <triegel@redhat.com>
Date:   Tue Jan 24 15:05:42 2012 +0100

    libitm: Fix wake-up of readers in futex-based serial lock.
    
    	libitm/
    	* config/linux/rwlock.cc (GTM::gtm_rwlock::write_unlock): Fix reader
    	wake-up.
Richard Henderson - Jan. 24, 2012, 6:49 p.m.
On 01/25/2012 04:43 AM, Torvald Riegel wrote:
>     libitm: Fix wake-up of readers in futex-based serial lock.
>     
>     	libitm/
>     	* config/linux/rwlock.cc (GTM::gtm_rwlock::write_unlock): Fix reader
>     	wake-up.

Ok.


r~

Patch

diff --git a/libitm/config/linux/rwlock.cc b/libitm/config/linux/rwlock.cc
index 24e7042..ad1b042 100644
--- a/libitm/config/linux/rwlock.cc
+++ b/libitm/config/linux/rwlock.cc
@@ -231,10 +231,13 @@  gtm_rwlock::write_unlock ()
 	  // last writer (this can happen because write_lock_generic()
 	  // exchanges 0 or 1 to 2 and thus might go to contended mode even if
 	  // no other thread holds the write lock currently). Therefore, we
-	  // have to wake up readers here as well.
-	  futex_wake(&readers, INT_MAX);
+	  // have to wake up readers here as well.  Execute a barrier after
+	  // the previous relaxed reset of writers (Dekker-style), and fall
+	  // through to the normal reader wake-up code.
+	  atomic_thread_fence (memory_order_seq_cst);
 	}
-      return;
+      else
+	return;
     }
   // No waiting writers, so wake up all waiting readers.
   // Because the fetch_and_sub is a full barrier already, we don't need