Patchwork libitm: Fix race condition in dispatch choice at transaction begin.

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Submitter Torvald Riegel
Date Feb. 13, 2012, 11:03 p.m.
Message ID <1329174195.24846.775.camel@triegel.csb>
Download mbox | patch
Permalink /patch/141016/
State New
Headers show

Comments

Torvald Riegel - Feb. 13, 2012, 11:03 p.m.
This patch fixes a race condition in how transactions previously chose
the dispatch at transaction begin:  default_dispatch in retry.cc was
read by transaction before they became either serial or nonserial
transactions (with the serial_lock).  A concurrent change of
default_dispatch was possible to lead to a transaction starting with
some out-of-date dispatch that were potentially incompatible with the
actual current dispatch, leading in turn to all sorts of synchronization
failures.
This is fixed by this patch by moving the serial_lock acquisiton into
the dispatch choice code.

Tested on ppc64 with microbenchmarks.  OK for trunk?
commit ce52924dedca632b24ea931329e060959782f89a
Author: Torvald Riegel <triegel@redhat.com>
Date:   Mon Feb 13 23:49:55 2012 +0100

    libitm: Fix race condition in dispatch choice at transaction begin.
    
    	libitm/
    	* beginend.cc (GTM::gtm_thread::begin_transaction): Move serial lock
    	acquisition to ...
    	* retry.cc (GTM::gtm_thread::decide_begin_dispatch): ... here.
    	(default_dispatch): Make atomic.
    	(GTM::gtm_thread::decide_retry_strategy,
    	GTM::gtm_thread::set_default_dispatch): Access atomically.
    	(GTM::gtm_thread::number_of_threads_changed): Initialize
    	default_dispatch here.
Richard Henderson - Feb. 13, 2012, 11:12 p.m.
On 02/13/2012 03:03 PM, Torvald Riegel wrote:
> -// The default TM method used when starting a new transaction.
> -static GTM::abi_dispatch* default_dispatch = 0;
> +// The default TM method used when starting a new transaction.  Initialized
> +// in number_of_threads_changed() below.
> +static std::atomic<GTM::abi_dispatch*> default_dispatch;

I see nothing but memory_order_relaxed uses of default_dispatch?


r~

Patch

diff --git a/libitm/beginend.cc b/libitm/beginend.cc
index 08c2174..e6a84de 100644
--- a/libitm/beginend.cc
+++ b/libitm/beginend.cc
@@ -233,16 +233,6 @@  GTM::gtm_thread::begin_transaction (uint32_t prop, const gtm_jmpbuf *jb)
     {
       // Outermost transaction
       disp = tx->decide_begin_dispatch (prop);
-      if (disp == dispatch_serialirr() || disp == dispatch_serial())
-	{
-	  tx->state = STATE_SERIAL;
-	  if (disp == dispatch_serialirr())
-	    tx->state |= STATE_IRREVOCABLE;
-	  serial_lock.write_lock ();
-	}
-      else
-	serial_lock.read_lock (tx);
-
       set_abi_disp (disp);
     }
 
diff --git a/libitm/retry.cc b/libitm/retry.cc
index d57bba0..08c5d80 100644
--- a/libitm/retry.cc
+++ b/libitm/retry.cc
@@ -27,8 +27,9 @@ 
 #include <ctype.h>
 #include "libitm_i.h"
 
-// The default TM method used when starting a new transaction.
-static GTM::abi_dispatch* default_dispatch = 0;
+// The default TM method used when starting a new transaction.  Initialized
+// in number_of_threads_changed() below.
+static std::atomic<GTM::abi_dispatch*> default_dispatch;
 // The default TM method as requested by the user, if any.
 static GTM::abi_dispatch* default_dispatch_user = 0;
 
@@ -57,14 +58,18 @@  GTM::gtm_thread::decide_retry_strategy (gtm_restart_reason r)
 	  // given that re-inits should be very infrequent.
 	  serial_lock.read_unlock(this);
 	  serial_lock.write_lock();
-	  if (disp->get_method_group() == default_dispatch->get_method_group())
+	  if (disp->get_method_group()
+	      == default_dispatch.load(memory_order_relaxed)
+	      ->get_method_group())
 	    // Still the same method group.
 	    disp->get_method_group()->reinit();
 	  serial_lock.write_unlock();
 	  serial_lock.read_lock(this);
-	  if (disp->get_method_group() != default_dispatch->get_method_group())
+	  if (disp->get_method_group()
+	      != default_dispatch.load(memory_order_relaxed)
+	      ->get_method_group())
 	    {
-	      disp = default_dispatch;
+	      disp = default_dispatch.load(memory_order_relaxed);
 	      set_abi_disp(disp);
 	    }
 	}
@@ -124,48 +129,81 @@  GTM::gtm_thread::decide_retry_strategy (gtm_restart_reason r)
 
 
 // Decides which TM method should be used on the first attempt to run this
-// transaction.
+// transaction.  Acquires the serial lock and sets transaction state
+// according to the chosen TM method.
 GTM::abi_dispatch*
 GTM::gtm_thread::decide_begin_dispatch (uint32_t prop)
 {
+  abi_dispatch* dd;
   // TODO Pay more attention to prop flags (eg, *omitted) when selecting
   // dispatch.
+  // ??? We go irrevocable eagerly here, which is not always good for
+  // performance.  Don't do this?
   if ((prop & pr_doesGoIrrevocable) || !(prop & pr_instrumentedCode))
-    return dispatch_serialirr();
+    dd = dispatch_serialirr();
 
-  // If we might need closed nesting and the default dispatch has an
-  // alternative that supports closed nesting, use it.
-  // ??? We could choose another TM method that we know supports closed
-  // nesting but isn't the default (e.g., dispatch_serial()). However, we
-  // assume that aborts that need closed nesting are infrequent, so don't
-  // choose a non-default method until we have to actually restart the
-  // transaction.
-  if (!(prop & pr_hasNoAbort) && !default_dispatch->closed_nesting()
-      && default_dispatch->closed_nesting_alternative())
-    return default_dispatch->closed_nesting_alternative();
+  else
+    {
+      // Load the default dispatch.  We're not an active transaction and so it
+      // can change concurrently but will still be some valid dispatch.
+      abi_dispatch* dd_orig = default_dispatch.load(memory_order_relaxed);
+      dd = dd_orig;
+
+      // If we might need closed nesting and the default dispatch has an
+      // alternative that supports closed nesting, use it.
+      // ??? We could choose another TM method that we know supports closed
+      // nesting but isn't the default (e.g., dispatch_serial()). However, we
+      // assume that aborts that need closed nesting are infrequent, so don't
+      // choose a non-default method until we have to actually restart the
+      // transaction.
+      if (!(prop & pr_hasNoAbort) && !dd->closed_nesting()
+	  && dd->closed_nesting_alternative())
+	dd = dd->closed_nesting_alternative();
+
+      if (dd != dispatch_serial() && dd != dispatch_serialirr())
+	{
+	  // The current dispatch is supposedly a non-serial one.  Become an
+	  // active transaction and verify this.
+	  serial_lock.read_lock (this);
+	  if (default_dispatch.load(memory_order_relaxed) == dd_orig)
+	    return dd;
 
-  // No special case, just use the default dispatch.
-  return default_dispatch;
+	  // If we raced with a concurrent modification of default_dispatch,
+	  // just fall back to serialirr.  The dispatch choice might not be
+	  // up-to-date anymore, but this is harmless.
+	  serial_lock.read_unlock (this);
+	  dd = dispatch_serialirr();
+	}
+    }
+
+  // We are some kind of serial transaction.
+  serial_lock.write_lock();
+  if (dd == dispatch_serialirr())
+    state = STATE_SERIAL | STATE_IRREVOCABLE;
+  else
+    state = STATE_SERIAL;
+  return dd;
 }
 
 
 void
 GTM::gtm_thread::set_default_dispatch(GTM::abi_dispatch* disp)
 {
-  if (default_dispatch == disp)
+  abi_dispatch* dd = default_dispatch.load(memory_order_relaxed);
+  if (dd == disp)
     return;
-  if (default_dispatch)
+  if (dd)
     {
       // If we are switching method groups, initialize and shut down properly.
-      if (default_dispatch->get_method_group() != disp->get_method_group())
+      if (dd->get_method_group() != disp->get_method_group())
 	{
-	  default_dispatch->get_method_group()->fini();
+	  dd->get_method_group()->fini();
 	  disp->get_method_group()->init();
 	}
     }
   else
     disp->get_method_group()->init();
-  default_dispatch = disp;
+  default_dispatch.store(disp, memory_order_relaxed);
 }
 
 
@@ -233,6 +271,7 @@  GTM::gtm_thread::number_of_threads_changed(unsigned previous, unsigned now)
 	{
 	  initialized = true;
 	  // Check for user preferences here.
+	  default_dispatch = 0;
 	  default_dispatch_user = parse_default_method();
 	}
     }