From a19db3f2e39d927e1595bbb6b9f4470c57161c7d Mon Sep 17 00:00:00 2001
From: Torvald Riegel <triegel@redhat.com>
Date: Mon, 20 Feb 2012 13:06:07 +0000
Subject: [PATCH] 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::set_default_dispatch): Access atomically.
(GTM::gtm_thread::decide_retry_strategy): Access atomically and
use decide_begin_dispatch() if default_dispatch might have changed.
(GTM::gtm_thread::number_of_threads_changed): Initialize
default_dispatch here.
From-SVN: r184392
---
libitm/ChangeLog | 12 +++++
libitm/beginend.cc | 10 ----
libitm/retry.cc | 114 +++++++++++++++++++++++++++++++++------------
3 files changed, 96 insertions(+), 40 deletions(-)
diff --git a/libitm/ChangeLog b/libitm/ChangeLog
index e103ca0288bf..e0d94a1d1130 100644
--- a/libitm/ChangeLog
+++ b/libitm/ChangeLog
@@ -1,3 +1,15 @@
+2012-02-20 Torvald Riegel <triegel@redhat.com>
+
+ * 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::set_default_dispatch): Access atomically.
+ (GTM::gtm_thread::decide_retry_strategy): Access atomically and
+ use decide_begin_dispatch() if default_dispatch might have changed.
+ (GTM::gtm_thread::number_of_threads_changed): Initialize
+ default_dispatch here.
+
2012-02-15 Iain Sandoe <iains@gcc.gnu.org>
Patrick Marlier <patrick.marlier@gmail.com>
diff --git a/libitm/beginend.cc b/libitm/beginend.cc
index 08c2174ea676..e6a84de13e23 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 d59c1834ef00..2c1483eae5a0 100644
--- a/libitm/retry.cc
+++ b/libitm/retry.cc
@@ -27,8 +27,16 @@
#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.
+// Access to this variable is always synchronized with help of the serial
+// lock, except one read access that happens in decide_begin_dispatch() before
+// a transaction has become active (by acquiring the serial lock in read or
+// write mode). The default_dispatch is only changed and initialized in
+// serial mode. Transactions stay active when they restart (see beginend.cc),
+// thus decide_retry_strategy() can expect default_dispatch to be unmodified.
+// See decide_begin_dispatch() for further comments.
+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;
@@ -57,20 +65,24 @@ 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())
- {
- disp = default_dispatch;
- set_abi_disp(disp);
- }
+ // Also, we're making the transaction inactive, so when we become
+ // active again, some other thread might have changed the default
+ // dispatch, so we run the same code as for the first execution
+ // attempt.
+ disp = decide_begin_dispatch(prop);
+ set_abi_disp(disp);
}
else
// We are a serial transaction already, which makes things simple.
disp->get_method_group()->reinit();
+
+ return;
}
bool retry_irr = (r == RESTART_SERIAL_IRR);
@@ -124,48 +136,89 @@ 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();
-
- // 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();
-
- // No special case, just use the default dispatch.
- return default_dispatch;
+ dd = dispatch_serialirr();
+
+ else
+ {
+ // Load the default dispatch. We're not an active transaction and so it
+ // can change concurrently but will still be some valid dispatch.
+ // Relaxed memory order is okay because we expect each dispatch to be
+ // constructed properly already (at least that its closed_nesting() and
+ // closed_nesting_alternatives() will return sensible values). It is
+ // harmless if we incorrectly chose the serial or serialirr methods, and
+ // for all other methods we will acquire the serial lock in read mode
+ // and load the default dispatch again.
+ 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. Relaxed memory order is fine
+ // because the serial lock itself will have established
+ // happens-before for any change to the selected dispatch.
+ serial_lock.read_lock (this);
+ if (default_dispatch.load(memory_order_relaxed) == dd_orig)
+ return dd;
+
+ // 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 +286,7 @@ 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();
}
}
--
GitLab