get rid of mfence instructions around atomic loads: not necessary on x86.

get rid of __sync_synchronize in gc_begin_epoch, since a LOCK instruction acts as an mfence. useful ref: http://www.cl.cam.ac.uk/~pes20/weakmemory/cacm.pdf

get rid of mfence instructions around atomic loads: not necessary on x86.
1153b3c3 · Nickolai Zeldovich · d7defe8f · 1153b3c3 · 1153b3c3 · 1153b3c3
--- a/include/atomic.hh
+++ b/include/atomic.hh
 #pragma once
-#if 0
-#define __sync_synchronize() do { __asm__ __volatile__("" ::: "memory"); } while (0)
-#endif
 #define _GLIBCXX_VISIBILITY(x)
 #define _GLIBCXX_BEGIN_NAMESPACE_VERSION
 #define _GLIBCXX_END_NAMESPACE_VERSION

--- a/include/atomic_2.h
+++ b/include/atomic_2.h
@@ -284,9 +284,9 @@ namespace __atomic2
 	__glibcxx_assert(__m != memory_order_release);
 	__glibcxx_assert(__m != memory_order_acq_rel);
-	__sync_synchronize();
+	// __sync_synchronize();
 	__int_type __ret = _M_i;
-	__sync_synchronize();
+	// __sync_synchronize();
 	return __ret;
      }
@@ -296,9 +296,9 @@ namespace __atomic2
 	__glibcxx_assert(__m != memory_order_release);
 	__glibcxx_assert(__m != memory_order_acq_rel);
-	__sync_synchronize();
+	// __sync_synchronize();
 	__int_type __ret = _M_i;
-	__sync_synchronize();
+	// __sync_synchronize();
 	return __ret;
      }
@@ -576,9 +576,9 @@ namespace __atomic2
 	__glibcxx_assert(__m != memory_order_release);
 	__glibcxx_assert(__m != memory_order_acq_rel);
-	__sync_synchronize();
+	// __sync_synchronize();
 	__pointer_type __ret = _M_p;
-	__sync_synchronize();
+	// __sync_synchronize();
 	return __ret;
      }
@@ -588,9 +588,9 @@ namespace __atomic2
 	__glibcxx_assert(__m != memory_order_release);
 	__glibcxx_assert(__m != memory_order_acq_rel);
-	__sync_synchronize();
+	// __sync_synchronize();
 	__pointer_type __ret = _M_p;
-	__sync_synchronize();
+	// __sync_synchronize();
 	return __ret;
      }

--- a/kernel/gc.cc
+++ b/kernel/gc.cc
@@ -183,7 +183,8 @@ gc_begin_epoch(void)
    return;
  cmpxch(&myproc()->epoch, v+1, (global_epoch.load()<<8)+1);
-  // __sync_synchronize();
+  // We effectively need an mfence here, and cmpxch provides one
+  // by virtue of being a LOCK instuction.
 }
 void