Re: [Gc] test_stack on powerpc (power7)

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On Wednesday, July 27, 2016 9:26:14 PM CEST Ivan Maidanski wrote:
>  Hello Pavel and Hans,
> 
> To Pavel:
> > I've seen Fedora maintainer applied this workaround downstream.
> Please give a reference? (sorry I failed to access the patch in ML)

Rex Dieter posted correct link -> that probably originally comes from this
thread, but link to archives is here:
https://lists.opendylan.org/pipermail/bdwgc/2014-January/005841.html

> To Hans:
> What do you think of the failure reason? Could you please better description of stack_aux?
> On Mon, 2012-12-17 at 15:34 +0100, Pavel Raiskup wrote:
> > ... 
> > I need to understand more deeply the "stack_aux" structure and how you are
> > using it, I still can't see whether there is guaranteed that the background
> > algorithm may not have collisions (considering 2+ threads trying to make a
> > pop and 2+ threads trying to make a push operation in parallel).

I was unable to download the patch from archives, so I'm re-attaching the
"attempt" to move this forward again.

Pavel

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From 6cd2f947eb6204d617ecbe3ee19dff8f8348beb8 Mon Sep 17 00:00:00 2001
From: Pavel Raiskup <[email protected]>
Date: Tue, 18 Dec 2012 14:31:20 +0100
Subject: [PATCH] This works OK.

---
 src/atomic_ops_stack.c | 25 +++++++++----------------
 src/atomic_ops_stack.h |  2 +-
 2 files changed, 10 insertions(+), 17 deletions(-)

diff --git a/src/atomic_ops_stack.c b/src/atomic_ops_stack.c
index 642bac0..e1077ce 100644
--- a/src/atomic_ops_stack.c
+++ b/src/atomic_ops_stack.c
@@ -1,4 +1,4 @@
-/*
+/* vi: ts=2 expandtab
  * Copyright (c) 2005 Hewlett-Packard Development Company, L.P.
  *
  * This file may be redistributed and/or modified under the
@@ -76,18 +76,8 @@ AO_stack_push_explicit_aux_release(volatile AO_t *list, AO_t *x,
   {
     int i;
     for (i = 0; i < AO_BL_SIZE; ++i)
-      {
-        if (AO_load(a -> AO_stack_bl + i) == x_bits)
-          {
-            /* Entry is currently being removed.  Change it a little.   */
-              ++x_bits;
-              if ((x_bits & AO_BIT_MASK) == 0)
-                /* Version count overflowed;         */
-                /* EXTREMELY unlikely, but possible. */
-                x_bits = (AO_t)x;
-            goto retry;
-          }
-      }
+      if (!AO_compare_and_swap_acquire(a->AO_stack_bl, 0, 1))
+        goto retry;
   }
 # endif
   /* x_bits is not currently being deleted */
@@ -97,6 +87,9 @@ AO_stack_push_explicit_aux_release(volatile AO_t *list, AO_t *x,
       *x = next;
     }
   while (AO_EXPECT_FALSE(!AO_compare_and_swap_release(list, next, x_bits)));
+
+  /* must success */
+  AO_compare_and_swap_release(a->AO_stack_bl, 1, 0);
 }
 
 /*
@@ -132,8 +125,8 @@ AO_stack_pop_explicit_aux_acquire(volatile AO_t *list, AO_stack_aux * a)
   /* structure a are currently in progress.                             */
   for (i = 0; ; )
     {
-      if (PRECHECK(a -> AO_stack_bl[i])
-          AO_compare_and_swap_acquire(a->AO_stack_bl+i, 0, first))
+      if (PRECHECK(a -> AO_stack_bl[0])
+          AO_compare_and_swap_acquire(a->AO_stack_bl, 0, 1))
         break;
       ++i;
       if ( i >= AO_BL_SIZE )
@@ -143,7 +136,7 @@ AO_stack_pop_explicit_aux_acquire(volatile AO_t *list, AO_stack_aux * a)
         }
     }
   assert(i < AO_BL_SIZE);
-  assert(a -> AO_stack_bl[i] == first);
+  // assert(a -> AO_stack_bl[i] == first);
   /* First is on the auxiliary black list.  It may be removed by        */
   /* another thread before we get to it, but a new insertion of x       */
   /* cannot be started here.                                            */
diff --git a/src/atomic_ops_stack.h b/src/atomic_ops_stack.h
index 1ca5f40..f2bace3 100644
--- a/src/atomic_ops_stack.h
+++ b/src/atomic_ops_stack.h
@@ -83,7 +83,7 @@
  * debugging.
  */
 #ifndef AO_BL_SIZE
-#  define AO_BL_SIZE 2
+#  define AO_BL_SIZE 1
 #endif
 
 #if AO_BL_SIZE > (1 << AO_N_BITS)
-- 
1.7.11.7


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