/**
* When a ProfileTM transaction commits, we end up in this code, which
* calls the current policy's 'decider' to pick the new algorithm, and then
* sets up metadata and makes the switch.
*/
void profile_oncomplete(TxThread* tx)
{
// NB: This is subtle: When we switched /to/ ProfileTM, we installed
// begin_blocker, then changed algorithms via install_algorithm(),
// then uninstalled begin_blocker. We are about to call
// install_algorithm(), but the invariant that install_algorithm
// demands is that the caller of install_algorithm() must have
// installed begin_blocker. Failure to do so can result in a race
// with TxThread constructors.
//
// No transactions can start, so we don't need to wait for everyone
// to commit/abort. But we do need this, in case tmbegin is
// /already/ begin_blocker on account of a call to set_policy or
// TxThread()
while (!bcasptr(&TxThread::tmbegin, stms[curr_policy.ALG_ID].begin,
&begin_blocker))
{
spin64();
}
// Use the policy to decide what algorithm to switch to
uint32_t new_algorithm = pols[curr_policy.POL_ID].decider();
// adjust thresholds
adjust_thresholds(new_algorithm, curr_policy.PREPROFILE_ALG);
// update the instrumentation level and install the algorithm
install_algorithm(new_algorithm, tx);
}
/**
* TML requires this to be called before every write
*/
inline void beforewrite_TML(TxThread* tx) {
// acquire the lock, abort on failure
if (!bcasptr(×tamp.val, tx->start_time, tx->start_time + 1))
tx->tmabort(tx);
++tx->start_time;
tx->tmlHasLock = true;
}
/*** set a bit */
inline void rrec_t::setbit(unsigned slot)
{
uint32_t bucket = slot / BITS;
uintptr_t mask = 1lu<<(slot % BITS);
uintptr_t oldval = bits[bucket];
if (oldval & mask)
return;
while (true) {
if (bcasptr(&bits[bucket], oldval, (oldval | mask)))
return;
oldval = bits[bucket];
}
}
/*** MCS release */
inline void mcs_release(mcs_qnode_t** lock, mcs_qnode_t* mine)
{
// if my node is the only one, then if I can zero the lock, do so and I'm
// done
if (mine->next == 0) {
if (bcasptr(lock, mine, static_cast<mcs_qnode_t*>(NULL)))
return;
// uh-oh, someone arrived while I was zeroing... wait for arriver to
// initialize, fall out to other case
while (mine->next == 0) { } // spin
}
// other case: someone is waiting on me... set their flag to let them start
mine->next->flag = false;
}
/**
* On abort, get a timestamp if I exceed some threshold
*/
static void onAbort(TxThread* tx)
{
// if I'm already in the hourglass, just return
if (tx->strong_HG) {
tx->abort_hist.onHGAbort();
return;
}
// acquire a timestamp if consecutive aborts exceed a threshold
if (tx->consec_aborts > ABORT_THRESHOLD) {
if (bcasptr(&fcm_timestamp.val, 0ul, 1ul)) {
tx->strong_HG = true;
}
}
// NB: as before, some counting opportunities here
}
/**
* On abort, get a timestamp if I exceed some threshold
*/
static void onAbort(TxThread* tx)
{
// if I'm already in the hourglass, just return
if (tx->strong_HG) {
tx->abort_hist.onHGAbort();
return;
}
// acquire a timestamp if consecutive aborts exceed a threshold
if (tx->consec_aborts > ABORT_THRESHOLD) {
if (bcasptr(&fcm_timestamp.val, 0ul, 1ul)) {
tx->strong_HG = true;
}
} else {
// randomized exponential backoff
exp_backoff(tx);
}
}
/*** unset a bit */
inline void rrec_t::unsetbit(unsigned slot)
{
uint32_t bucket = slot / BITS;
uintptr_t mask = 1lu<<(slot % BITS);
uintptr_t unmask = ~mask;
uintptr_t oldval = bits[bucket];
if (!(oldval & mask))
return;
// NB: this GCC-specific code
#if defined(STM_CPU_X86) && defined(STM_CC_GCC)
__sync_fetch_and_and(&bits[bucket], unmask);
#else
while (true) {
if (bcasptr(&bits[bucket], oldval, (oldval & unmask)))
return;
oldval = bits[bucket];
}
#endif
}
/*** combine test and set */
inline bool rrec_t::setif(unsigned slot)
{
uint32_t bucket = slot / BITS;
uintptr_t mask = 1lu<<(slot % BITS);
uintptr_t oldval = bits[bucket];
if (oldval & mask)
return false;
// NB: We don't have suncc fetch_and_or, so there is an ifdef here that
// falls back to a costly CAS-based atomic or
#if defined(STM_CPU_X86) && defined(STM_CC_GCC) /* little endian */
__sync_fetch_and_or(&bits[bucket], mask);
return true;
#else
while (true) {
if (bcasptr(&bits[bucket], oldval, oldval | mask))
return true;
oldval = bits[bucket];
}
#endif
}
/**
* On abort, get a timestamp if I exceed some threshold
*/
static void onAbort(TxThread* tx)
{
// if I'm already in the hourglass, just return
if (tx->strong_HG) {
tx->abort_hist.onHGAbort();
return;
}
// acquire a timestamp if consecutive aborts exceed a threshold
if (tx->consec_aborts > ABORT_THRESHOLD) {
while (true) {
if (bcasptr(&fcm_timestamp.val, 0ul, 1ul)) {
tx->strong_HG = true;
return;
}
while (fcm_timestamp.val) { }
}
}
// NB: It would be good to explore what happens if I have a
// strong_HG already? Can we count how many times I abort with
// strong_HG?
}