commit bfa6b5c51e110ca38c0ce5aa7234e7b719eee5a6
Author: Torvald Riegel <triegel@redhat.com>
Date: Wed Jan 6 15:34:57 2016 +0100
libitm: Fix HTM fastpath.
@@ -32,7 +32,11 @@ using namespace GTM;
extern __thread gtm_thread_tls _gtm_thr_tls;
#endif
-gtm_rwlock GTM::gtm_thread::serial_lock;
+// Put this at the start of a cacheline so that serial_lock's writers and
+// htm_fastpath fields are on the same cacheline, so that HW transactions
+// only have to pay one cacheline capacity to monitor both.
+gtm_rwlock GTM::gtm_thread::serial_lock
+ __attribute__((aligned(HW_CACHELINE_SIZE)));
gtm_thread *GTM::gtm_thread::list_of_threads = 0;
unsigned GTM::gtm_thread::number_of_threads = 0;
@@ -54,9 +58,6 @@ static pthread_mutex_t global_tid_lock = PTHREAD_MUTEX_INITIALIZER;
static pthread_key_t thr_release_key;
static pthread_once_t thr_release_once = PTHREAD_ONCE_INIT;
-// See gtm_thread::begin_transaction.
-uint32_t GTM::htm_fastpath = 0;
-
/* Allocate a transaction structure. */
void *
GTM::gtm_thread::operator new (size_t s)
@@ -176,9 +177,11 @@ GTM::gtm_thread::begin_transaction (uint32_t prop, const gtm_jmpbuf *jb)
// lock's writer flag and thus abort if another thread is or becomes a
// serial transaction. Therefore, if the fastpath is enabled, then a
// transaction is not executing as a HW transaction iff the serial lock is
- // write-locked. This allows us to use htm_fastpath and the serial lock's
- // writer flag to reliable determine whether the current thread runs a HW
- // transaction, and thus we do not need to maintain this information in
+ // write-locked. Also, HW transactions monitor the fastpath control
+ // variable, so that they will only execute if dispatch_htm is still the
+ // current method group. This allows us to use htm_fastpath and the serial
+ // lock's writers flag to reliable determine whether the current thread runs
+ // a HW transaction, and thus we do not need to maintain this information in
// per-thread state.
// If an uninstrumented code path is not available, we can still run
// instrumented code from a HW transaction because the HTM fastpath kicks
@@ -190,9 +193,14 @@ GTM::gtm_thread::begin_transaction (uint32_t prop, const gtm_jmpbuf *jb)
// for any internal changes (e.g., they never abort visibly to the STM code
// and thus do not trigger the standard retry handling).
#ifndef HTM_CUSTOM_FASTPATH
- if (likely(htm_fastpath && (prop & pr_hasNoAbort)))
+ if (likely(serial_lock.get_htm_fastpath() && (prop & pr_hasNoAbort)))
{
- for (uint32_t t = htm_fastpath; t; t--)
+ // Note that the snapshot of htm_fastpath that we take here could be
+ // outdated, and a different method group than dispatch_htm may have
+ // been chosen in the meantime. Therefore, take care not not touch
+ // anything besides the serial lock, which is independent of method
+ // groups.
+ for (uint32_t t = serial_lock.get_htm_fastpath(); t; t--)
{
uint32_t ret = htm_begin();
if (htm_begin_success(ret))
@@ -200,9 +208,11 @@ GTM::gtm_thread::begin_transaction (uint32_t prop, const gtm_jmpbuf *jb)
// We are executing a transaction now.
// Monitor the writer flag in the serial-mode lock, and abort
// if there is an active or waiting serial-mode transaction.
+ // Also checks that htm_fastpath is still nonzero and thus
+ // HW transactions are allowed to run.
// Note that this can also happen due to an enclosing
// serial-mode transaction; we handle this case below.
- if (unlikely(serial_lock.is_write_locked()))
+ if (unlikely(serial_lock.htm_fastpath_disabled()))
htm_abort();
else
// We do not need to set a_saveLiveVariables because of HTM.
@@ -213,9 +223,12 @@ GTM::gtm_thread::begin_transaction (uint32_t prop, const gtm_jmpbuf *jb)
// retrying the transaction will be successful.
if (!htm_abort_should_retry(ret))
break;
+ // Check whether the HTM fastpath has been disabled.
+ if (!serial_lock.get_htm_fastpath())
+ break;
// Wait until any concurrent serial-mode transactions have finished.
// This is an empty critical section, but won't be elided.
- if (serial_lock.is_write_locked())
+ if (serial_lock.htm_fastpath_disabled())
{
tx = gtm_thr();
if (unlikely(tx == NULL))
@@ -250,7 +263,7 @@ GTM::gtm_thread::begin_transaction (uint32_t prop, const gtm_jmpbuf *jb)
// HTM fastpath aborted, and that we thus have to decide whether to retry
// the fastpath (returning a_tryHTMFastPath) or just proceed with the
// fallback method.
- if (likely(htm_fastpath && (prop & pr_HTMRetryableAbort)))
+ if (likely(serial_lock.get_htm_fastpath() && (prop & pr_HTMRetryableAbort)))
{
tx = gtm_thr();
if (unlikely(tx == NULL))
@@ -264,13 +277,15 @@ GTM::gtm_thread::begin_transaction (uint32_t prop, const gtm_jmpbuf *jb)
// other fallback will use serial transactions, which don't use
// restart_total but will reset it when committing.
if (!(prop & pr_HTMRetriedAfterAbort))
- tx->restart_total = htm_fastpath;
+ tx->restart_total = gtm_thread::serial_lock.get_htm_fastpath();
if (--tx->restart_total > 0)
{
// Wait until any concurrent serial-mode transactions have finished.
// Essentially the same code as above.
- if (serial_lock.is_write_locked())
+ if (!serial_lock.get_htm_fastpath())
+ goto stop_custom_htm_fastpath;
+ if (serial_lock.htm_fastpath_disabled())
{
if (tx->nesting > 0)
goto stop_custom_htm_fastpath;
@@ -668,7 +683,7 @@ _ITM_commitTransaction(void)
// a serial-mode transaction. If we are, then there will be no other
// concurrent serial-mode transaction.
// See gtm_thread::begin_transaction.
- if (likely(htm_fastpath && !gtm_thread::serial_lock.is_write_locked()))
+ if (likely(!gtm_thread::serial_lock.htm_fastpath_disabled()))
{
htm_commit();
return;
@@ -684,7 +699,7 @@ _ITM_commitTransactionEH(void *exc_ptr)
{
#if defined(USE_HTM_FASTPATH)
// See _ITM_commitTransaction.
- if (likely(htm_fastpath && !gtm_thread::serial_lock.is_write_locked()))
+ if (likely(!gtm_thread::serial_lock.htm_fastpath_disabled()))
{
htm_commit();
return;
@@ -41,19 +41,27 @@ struct gtm_thread;
// read-to-write upgrades do not have a higher priority than writers.
//
// Do not change the layout of this class; it must remain a POD type with
-// standard layout, and the WRITERS field must be first (i.e., so the
+// standard layout, and the writers field must be first (i.e., so the
// assembler code can assume that its address is equal to the address of the
-// respective instance of the class).
+// respective instance of the class), and htm_fastpath must be second.
class gtm_rwlock
{
- // TODO Put futexes on different cachelines?
std::atomic<int> writers; // Writers' futex.
+ // We put the HTM fastpath control variable here so that HTM fastpath
+ // transactions can check efficiently whether they are allowed to run.
+ // This must be accessed atomically because threads can load this value
+ // when they are neither a registered reader nor writer (i.e., when they
+ // attempt to execute the HTM fastpath).
+ std::atomic<uint32_t> htm_fastpath;
+ // TODO Put these futexes on different cachelines? (writers and htm_fastpath
+ // should remain on the same cacheline.
std::atomic<int> writer_readers;// A confirmed writer waits here for readers.
std::atomic<int> readers; // Readers wait here for writers (iff true).
public:
- gtm_rwlock() : writers(0), writer_readers(0), readers(0) {};
+ gtm_rwlock() : writers(0), htm_fastpath(0), writer_readers(0), readers(0)
+ { }
void read_lock (gtm_thread *tx);
void read_unlock (gtm_thread *tx);
@@ -64,12 +72,28 @@ class gtm_rwlock
bool write_upgrade (gtm_thread *tx);
void write_upgrade_finish (gtm_thread *tx);
- // Returns true iff there is a concurrent active or waiting writer.
- // This is primarily useful for simple HyTM approaches, and the value being
- // checked is loaded with memory_order_relaxed.
- bool is_write_locked()
+ // Returns true iff there is a concurrent active or waiting writer, or
+ // htm_fastpath is zero. This is primarily useful for simple HyTM
+ // approaches, and the values being checked are loaded with
+ // memory_order_relaxed.
+ bool htm_fastpath_disabled ()
{
- return writers.load (memory_order_relaxed) != 0;
+ return writers.load (memory_order_relaxed) != 0
+ || htm_fastpath.load (memory_order_relaxed) == 0;
+ }
+
+ // This does not need to return an exact value, hence relaxed MO is
+ // sufficient.
+ uint32_t get_htm_fastpath ()
+ {
+ return htm_fastpath.load (memory_order_relaxed);
+ }
+ // This must only be called while having acquired the write lock, and other
+ // threads do not need to load an exact value; hence relaxed MO is
+ // sufficient.
+ void set_htm_fastpath (uint32_t val)
+ {
+ htm_fastpath.store (val, memory_order_relaxed);
}
protected:
@@ -31,6 +31,7 @@ namespace GTM HIDDEN {
gtm_rwlock::gtm_rwlock()
: summary (0),
+ htm_fastpath (0),
mutex (PTHREAD_MUTEX_INITIALIZER),
c_readers (PTHREAD_COND_INITIALIZER),
c_writers (PTHREAD_COND_INITIALIZER),
@@ -46,9 +46,9 @@ struct gtm_thread;
// read-to-write upgrades do not have a higher priority than writers.
//
// Do not change the layout of this class; it must remain a POD type with
-// standard layout, and the SUMMARY field must be first (i.e., so the
+// standard layout, and the summary field must be first (i.e., so the
// assembler code can assume that its address is equal to the address of the
-// respective instance of the class).
+// respective instance of the class), and htm_fastpath must be second.
class gtm_rwlock
{
@@ -58,6 +58,13 @@ class gtm_rwlock
std::atomic<unsigned int> summary; // Bitmask of the above.
+ // We put the HTM fastpath control variable here so that HTM fastpath
+ // transactions can check efficiently whether they are allowed to run.
+ // This must be accessed atomically because threads can load this value
+ // when they are neither a registered reader nor writer (i.e., when they
+ // attempt to execute the HTM fastpath).
+ std::atomic<uint32_t> htm_fastpath;
+
pthread_mutex_t mutex; // Held if manipulating any field.
pthread_cond_t c_readers; // Readers wait here
pthread_cond_t c_writers; // Writers wait here for writers
@@ -80,12 +87,28 @@ class gtm_rwlock
bool write_upgrade (gtm_thread *tx);
void write_upgrade_finish (gtm_thread *tx);
- // Returns true iff there is a concurrent active or waiting writer.
- // This is primarily useful for simple HyTM approaches, and the value being
- // checked is loaded with memory_order_relaxed.
- bool is_write_locked()
+ // Returns true iff there is a concurrent active or waiting writer, or
+ // htm_fastpath is zero. This is primarily useful for simple HyTM
+ // approaches, and the values being checked are loaded with
+ // memory_order_relaxed.
+ bool htm_fastpath_disabled ()
+ {
+ return (summary.load (memory_order_relaxed) & (a_writer | w_writer))
+ || htm_fastpath.load (memory_order_relaxed) == 0;
+ }
+
+ // This does not need to return an exact value, hence relaxed MO is
+ // sufficient.
+ uint32_t get_htm_fastpath ()
+ {
+ return htm_fastpath.load (memory_order_relaxed);
+ }
+ // This must only be called while having acquired the write lock, and other
+ // threads do not need to load an exact value; hence relaxed MO is
+ // sufficient.
+ void set_htm_fastpath (uint32_t val)
{
- return summary.load (memory_order_relaxed) & (a_writer | w_writer);
+ htm_fastpath.store (val, memory_order_relaxed);
}
protected:
@@ -81,8 +81,9 @@ SYM(_ITM_beginTransaction):
back to another execution method. RTM restores all registers after
a HW transaction abort, so we can do the SW setjmp after aborts,
and we have to because we might choose a SW fall back. However,
- we have to explicitly save/restore the first argument (edi). */
- cmpl $0, SYM(gtm_htm_fastpath)(%rip)
+ we have to explicitly save/restore the first argument (edi).
+ The htm_fastpath field is the second int in gtm_rwlock. */
+ cmpl $0, (SYM(gtm_serial_lock)+4)(%rip)
jz .Lno_htm
testl $pr_hasNoAbort, %edi
jz .Lno_htm
@@ -95,6 +96,10 @@ SYM(_ITM_beginTransaction):
this case in the retry policy implementation. */
cmpl $0, SYM(gtm_serial_lock)(%rip)
jnz 1f
+ /* Now also check that HW transactions are still allowed to run (see
+ gtm_thread::begin_transaction for why this is necessary). */
+ cmpl $0, (SYM(gtm_serial_lock)+4)(%rip)
+ jz 1f
/* Everything is good. Run the transaction, preferably using the
uninstrumented code path. Note that the following works because
pr_uninstrumentedCode == a_runUninstrumentedCode. */
@@ -102,8 +107,9 @@ SYM(_ITM_beginTransaction):
mov $a_runInstrumentedCode, %eax
cmovnz %edi, %eax
ret
- /* There is a serial-mode transaction, so abort (see htm_abort()
- regarding the abort code). */
+ /* There is a serial-mode transaction or HW transactions are not
+ allowed anymore, so abort (see htm_abort() regarding the abort
+ code). */
1: xabort $0xff
.Ltxn_abort:
/* If it might make sense to retry the HTM fast path, let the C++
@@ -356,12 +356,6 @@ extern abi_dispatch *dispatch_htm();
extern gtm_cacheline_mask gtm_mask_stack(gtm_cacheline *, gtm_cacheline_mask);
-// Control variable for the HTM fastpath that uses serial mode as fallback.
-// Non-zero if the HTM fastpath is enabled. See gtm_thread::begin_transaction.
-// Accessed from assembly language, thus the "asm" specifier on
-// the name, avoiding complex name mangling.
-extern uint32_t htm_fastpath __asm__(UPFX "gtm_htm_fastpath");
-
} // namespace GTM
#endif // LIBITM_I_H
@@ -222,13 +222,13 @@ struct htm_mg : public method_group
// Enable the HTM fastpath if the HW is available. The fastpath is
// initially disabled.
#ifdef USE_HTM_FASTPATH
- htm_fastpath = htm_init();
+ gtm_thread::serial_lock.set_htm_fastpath(htm_init());
#endif
}
virtual void fini()
{
// Disable the HTM fastpath.
- htm_fastpath = 0;
+ gtm_thread::serial_lock.set_htm_fastpath(0);
}
};
@@ -288,7 +288,7 @@ GTM::gtm_thread::serialirr_mode ()
#if defined(USE_HTM_FASTPATH)
// HTM fastpath. If we are executing a HW transaction, don't go serial but
// continue. See gtm_thread::begin_transaction.
- if (likely(htm_fastpath && !gtm_thread::serial_lock.is_write_locked()))
+ if (likely(!gtm_thread::serial_lock.htm_fastpath_disabled()))
return;
#endif
@@ -49,7 +49,7 @@ _ITM_inTransaction (void)
// a transaction and thus we can't deduce this by looking at just the serial
// lock. This function isn't used in practice currently, so the easiest
// way to handle it is to just abort.
- if (htm_fastpath && htm_transaction_active())
+ if (gtm_thread::serial_lock.get_htm_fastpath() && htm_transaction_active())
htm_abort();
#endif
struct gtm_thread *tx = gtm_thr();
@@ -69,7 +69,7 @@ _ITM_getTransactionId (void)
{
#if defined(USE_HTM_FASTPATH)
// See ITM_inTransaction.
- if (htm_fastpath && htm_transaction_active())
+ if (gtm_thread::serial_lock.get_htm_fastpath() && htm_transaction_active())
htm_abort();
#endif
struct gtm_thread *tx = gtm_thr();