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T-save ObjectMonitor T-deflate
------------------------------- -- +-----------------------+ ------------------------------------------
save_om_ptr() { | owner=DEFLATER_MARKER | deflate_monitor_using_JT() {
atomic inc ref_count | ref_count=-max_jint+1 | cmpxchg(DEFLATER_MARKER, &owner, NULL)
1> if (owner == DEFLATER_MARKER && +-----------------------+ :
ref_count <= 0) { || prev = cmpxchg(-max_jint, &ref_count, 0)
restore obj header \/ 1> if (prev == 0 &&
atomic dec ref_count +-----------------------+ owner == DEFLATER_MARKER) {
2> return false to force retry | owner=DEFLATER_MARKER | restore obj header
} | ref_count=-max_jint | 2> finish the deflation
+-----------------------+ }
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T-save ObjectMonitor T-deflate
--------------------------------- +-----------------------+ ------------------------------------------
save_om_ptr() { | owner=DEFLATER_MARKER | deflate_monitor_using_JT() {
atomic inc ref_count | ref_count=1 | cmpxchg(DEFLATER_MARKER, &owner, NULL)
1> if (owner == DEFLATER_MARKER && +-----------------------+ :
ref_count <= 0) { || prev = cmpxchg(-max_jint, &ref_count, 0)
} else { \/ 1> if (prev == 0 &&
save om_ptr in the +-----------------------+ owner == DEFLATER_MARKER) {
ObjectMonitorHandle | owner=NULL | } else {
2> return true | ref_count=1 | cmpxchg(NULL, &owner, DEFLATER_MARKER)
+-----------------------+ 2> return
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T-enter ObjectMonitor T-deflate
-------------------------------------------- +-------------------------+ ------------------------------------------
ObjectMonitor::enter() { | owner=DEFLATER_MARKER | deflate_monitor_using_JT() {
<owner is contended> | ref_count=1 | cmpxchg(DEFLATER_MARKER, &owner, NULL)
1> EnterI() { +-------------------------+ 1> :
if (owner == DEFLATER_MARKER && || 2> : <thread_stalls>
cmpxchg(Self, &owner, \/ :
DEFLATER_MARKER) +-------------------------+ :
== DEFLATER_MARKER) { | owner=Self/T-enter | :
// EnterI is done | ref_count=0 | : <thread_resumes>
return +-------------------------+ prev = cmpxchg(-max_jint, &ref_count, 0)
} || if (prev == 0 &&
} // enter() is done \/ 3> owner == DEFLATER_MARKER) {
~OMH: atomic dec ref_count +-------------------------+ } else {
2> : <does app work> | owner=Self/T-enter|NULL | cmpxchg(NULL, &owner, DEFLATER_MARKER)
3> : | ref_count=-max_jint | atomic add max_jint to ref_count
exit() monitor +-------------------------+ 4> bailout on deflation
4> owner = NULL || }
\/
+-------------------------+
| owner=Self/T-enter|NULL |
| ref_count=0 |
+-------------------------+
- T-deflate has executed cmpxchg() and set owner to DEFLATE_MARKER.
- T-enter has called ObjectMonitor::enter() with "ref_count == 1", noticed that the owner is contended and is about to call ObjectMonitor::EnterI().
- The first ObjectMonitor box is showing the fields at this point and the "1>" markers are showing where each thread is at for that ObjectMonitor box.
- T-deflate stalls after setting the owner field to DEFLATER_MARKER.
- T-enter calls EnterI() to do the contended enter work.:
- EnterI() observes owner == DEFLATER_MARKER and uses cmpxchg() to set the owner field to Self/T-enter.
- T-enter owns the monitor, returns from EnterI(), and returns from enter().
- The ObjectMonitorHandle destructor decrements the ref_count.
- T-enter is now ready to do work that requires the monitor to be owned.
- The second ObjectMonitor box is showing the fields at this point and the "2>" markers are showing where each thread is at for that ObjectMonitor box.
- T-enter is doing app work (but it also could have finished and exited the monitor).
- T-deflate resumes, sets calls cmpxchg() to set the ref_count field to -max_jint, and passes the first part of the bailout expression because "prev == 0".
- The third ObjectMonitor box is showing the fields at this point and the "3>" markers are showing where each thread is at for that ObjectMonitor box.
- T-deflate performs the A-B-A check which observes that "owner != DEFLATE_MARKER" and bails out on deflation:
- Depending on when T-deflate resumes after the stall, it will see "owner == T-enter" or "owner == NULL".
- Both of those values will cause deflation to bailout so we have to conditionally undo work:
- restore the owner field to NULL if it is still DEFLATER_MARKER (it's not DEFLATER_MARKER)
- undo setting ref_count to -max_jint by atomically adding max_jint to ref_count which will restore ref_count to its proper value.
- If the T-enter thread has managed to enter but not exit the monitor during the T-deflate stall, then our owner field A-B-A transition is:
NULL → DEFLATE_MARKER → Self/T-enter
so we really have A1-B-A2, but the A-B-A principal still holds.
If the T-enter thread has managed to enter and exit the monitor during the T-deflate stall, then our owner field A-B-A transition is:
NULL → DEFLATE_MARKER → Self/T-enter → NULL
so we really have A-B1-B2-A, but the A-B-A principal still holds.
T-enter finished doing app work and is about to exit the monitor (or it has already exited the monitor).
The fourth ObjectMonitor box is showing the fields at this point and the "4>" markers are showing where each thread is at for that ObjectMonitor box.
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T-save object T-deflate
------------------------------------------- +-------------+ -------------------------------------------
install_displaced_markword_in_object() { | mark=om_ptr | install_displaced_markword_in_object() {
dmw = header() +-------------+ dmw = header()
if (!dmw->is_marked() && if (!dmw->is_marked() &&
dmw->hash() == 0) { dmw->hash() == 0) {
create marked_dmw create marked_dmw
dmw = cmpxchg(marked_dmw, &header, dmw) dmw = cmpxchg(marked_dmw, &header, dmw)
} }
// dmw == marked_dmw here // dmw == original dmw here
if (dmw->is_marked()) if (dmw->is_marked())
unmark dmw unmark dmw
obj = object() obj = object()
obj->cas_set_mark(dmw, this) obj->cas_set_mark(dmw, this)
...
- T-deflate made it past the cmpxchg() of ref_count before T-hash incremented it.
- T-deflate set the ref_count field to -max_jint and is about to make the last of the protocol checks.
- The first ObjectMonitor box is showing the fields at this point and the "1>" markers are showing where each thread is at for that ObjectMonitor box.
- T-deflate sees "prev == 0 && owner == DEFLATER_MARKER" so it knows that it has won the race.
- T-deflate restores obj header (not shown).
- T-hash increments the ref_count.
- T-hash observes "owner == DEFLATER_MARKER && ref_count <= 0" so it restores obj header (not shown) and decrements ref_count.
- The second ObjectMonitor box is showing the fields at this point and the "2>" markers are showing where each thread is at for that ObjectMonitor box.
- T-deflate finishes the deflation work.
- T-hash returns false to cause a retry and when T-hash retries:
- it observes the restored object header (done by T-hash or T-deflate):
- if the object's header does not have a hash, then generate a hash and merge it with the object's header.
- Otherwise, extract the hash from the object's header and return it.
- it observes the restored object header (done by T-hash or T-deflate):
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