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Commit44d44aa

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Fix race in parallel hash join batch cleanup, take II.
With unlucky timing and parallel_leader_participation=off (not thedefault), PHJ could attempt to access per-batch shared state just as itwas being freed. There was code intended to prevent that by checkingfor a cleared pointer, but it was racy. Fix, by introducing an extrabarrier phase. The new phase PHJ_BUILD_RUNNING means that it's safe toaccess the per-batch state to find a batch to help with, andPHJ_BUILD_DONE means that it is too late. The last to detach will freethe array of per-batch state as before, but now it will also atomicallyadvance the phase, so that late attachers can avoid the hazard. Thismirrors the way per-batch hash tables are freed (see phasesPHJ_BATCH_PROBING and PHJ_BATCH_DONE).An earlier attempt to fix this (commit3b8981b, later reverted) missedone special case. When the inner side is empty (the "empty inneroptimization), the build barrier would only make it toPHJ_BUILD_HASHING_INNER phase before workers attempted to detach fromthe hashtable. In that case, fast-forward the build barrier toPHJ_BUILD_RUNNING before proceeding, so that our later assertions holdand we can still negotiate who is cleaning up.Revealed by build farm failures, where BarrierAttach() failed a sanitycheck assertion, because the memory had been clobbered by dsa_free().In non-assert builds, the result could be a segmentation fault.Back-patch to all supported releases.Author: Thomas Munro <thomas.munro@gmail.com>Author: Melanie Plageman <melanieplageman@gmail.com>Reported-by: Michael Paquier <michael@paquier.xyz>Reported-by: David Geier <geidav.pg@gmail.com>Tested-by: David Geier <geidav.pg@gmail.com>Discussion:https://postgr.es/m/20200929061142.GA29096%40paquier.xyz
1 parent9286568 commit44d44aa

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3 files changed

+74
-33
lines changed

3 files changed

+74
-33
lines changed

‎src/backend/executor/nodeHash.c‎

Lines changed: 34 additions & 16 deletions
Original file line numberDiff line numberDiff line change
@@ -333,14 +333,21 @@ MultiExecParallelHash(HashState *node)
333333
hashtable->nbuckets=pstate->nbuckets;
334334
hashtable->log2_nbuckets=my_log2(hashtable->nbuckets);
335335
hashtable->totalTuples=pstate->total_tuples;
336-
ExecParallelHashEnsureBatchAccessors(hashtable);
336+
337+
/*
338+
* Unless we're completely done and the batch state has been freed, make
339+
* sure we have accessors.
340+
*/
341+
if (BarrierPhase(build_barrier)<PHJ_BUILD_DONE)
342+
ExecParallelHashEnsureBatchAccessors(hashtable);
337343

338344
/*
339345
* The next synchronization point is in ExecHashJoin's HJ_BUILD_HASHTABLE
340-
* case, which will bring the build phase toPHJ_BUILD_DONE (if it isn't
341-
* there already).
346+
* case, which will bring the build phase toPHJ_BUILD_RUNNING (if it
347+
*isn'tthere already).
342348
*/
343349
Assert(BarrierPhase(build_barrier)==PHJ_BUILD_HASHING_OUTER||
350+
BarrierPhase(build_barrier)==PHJ_BUILD_RUNNING||
344351
BarrierPhase(build_barrier)==PHJ_BUILD_DONE);
345352
}
346353

@@ -624,7 +631,7 @@ ExecHashTableCreate(HashState *state, List *hashOperators, List *hashCollations,
624631
/*
625632
* The next Parallel Hash synchronization point is in
626633
* MultiExecParallelHash(), which will progress it all the way to
627-
*PHJ_BUILD_DONE. The caller must not return control from this
634+
*PHJ_BUILD_RUNNING. The caller must not return control from this
628635
* executor node between now and then.
629636
*/
630637
}
@@ -3019,14 +3026,11 @@ ExecParallelHashEnsureBatchAccessors(HashJoinTable hashtable)
30193026
}
30203027

30213028
/*
3022-
* It's possible for a backend to start up very late so that the whole
3023-
* join is finished and the shm state for tracking batches has already
3024-
* been freed by ExecHashTableDetach(). In that case we'll just leave
3025-
* hashtable->batches as NULL so that ExecParallelHashJoinNewBatch() gives
3026-
* up early.
3029+
* We should never see a state where the batch-tracking array is freed,
3030+
* because we should have given up sooner if we join when the build
3031+
* barrier has reached the PHJ_BUILD_DONE phase.
30273032
*/
3028-
if (!DsaPointerIsValid(pstate->batches))
3029-
return;
3033+
Assert(DsaPointerIsValid(pstate->batches));
30303034

30313035
/* Use hash join memory context. */
30323036
oldcxt=MemoryContextSwitchTo(hashtable->hashCxt);
@@ -3146,9 +3150,18 @@ ExecHashTableDetachBatch(HashJoinTable hashtable)
31463150
void
31473151
ExecHashTableDetach(HashJoinTablehashtable)
31483152
{
3149-
if (hashtable->parallel_state)
3153+
ParallelHashJoinState*pstate=hashtable->parallel_state;
3154+
3155+
/*
3156+
* If we're involved in a parallel query, we must either have gotten all
3157+
* the way to PHJ_BUILD_RUNNING, or joined too late and be in
3158+
* PHJ_BUILD_DONE.
3159+
*/
3160+
Assert(!pstate||
3161+
BarrierPhase(&pstate->build_barrier) >=PHJ_BUILD_RUNNING);
3162+
3163+
if (pstate&&BarrierPhase(&pstate->build_barrier)==PHJ_BUILD_RUNNING)
31503164
{
3151-
ParallelHashJoinState*pstate=hashtable->parallel_state;
31523165
inti;
31533166

31543167
/* Make sure any temporary files are closed. */
@@ -3164,17 +3177,22 @@ ExecHashTableDetach(HashJoinTable hashtable)
31643177
}
31653178

31663179
/* If we're last to detach, clean up shared memory. */
3167-
if (BarrierDetach(&pstate->build_barrier))
3180+
if (BarrierArriveAndDetach(&pstate->build_barrier))
31683181
{
3182+
/*
3183+
* Late joining processes will see this state and give up
3184+
* immediately.
3185+
*/
3186+
Assert(BarrierPhase(&pstate->build_barrier)==PHJ_BUILD_DONE);
3187+
31693188
if (DsaPointerIsValid(pstate->batches))
31703189
{
31713190
dsa_free(hashtable->area,pstate->batches);
31723191
pstate->batches=InvalidDsaPointer;
31733192
}
31743193
}
3175-
3176-
hashtable->parallel_state=NULL;
31773194
}
3195+
hashtable->parallel_state=NULL;
31783196
}
31793197

31803198
/*

‎src/backend/executor/nodeHashjoin.c‎

Lines changed: 38 additions & 16 deletions
Original file line numberDiff line numberDiff line change
@@ -45,7 +45,8 @@
4545
* PHJ_BUILD_ALLOCATING -- one sets up the batches and table 0
4646
* PHJ_BUILD_HASHING_INNER -- all hash the inner rel
4747
* PHJ_BUILD_HASHING_OUTER -- (multi-batch only) all hash the outer
48-
* PHJ_BUILD_DONE -- building done, probing can begin
48+
* PHJ_BUILD_RUNNING -- building done, probing can begin
49+
* PHJ_BUILD_DONE -- all work complete, one frees batches
4950
*
5051
* While in the phase PHJ_BUILD_HASHING_INNER a separate pair of barriers may
5152
* be used repeatedly as required to coordinate expansions in the number of
@@ -73,7 +74,7 @@
7374
* batches whenever it encounters them while scanning and probing, which it
7475
* can do because it processes batches in serial order.
7576
*
76-
* OncePHJ_BUILD_DONE is reached, backends then split up and process
77+
* OncePHJ_BUILD_RUNNING is reached, backends then split up and process
7778
* different batches, or gang up and work together on probing batches if there
7879
* aren't enough to go around. For each batch there is a separate barrier
7980
* with the following phases:
@@ -95,11 +96,16 @@
9596
*
9697
* To avoid deadlocks, we never wait for any barrier unless it is known that
9798
* all other backends attached to it are actively executing the node or have
98-
* already arrived. Practically, that means that we never return a tuple
99-
* while attached to a barrier, unless the barrier has reached its final
100-
* state. In the slightly special case of the per-batch barrier, we return
101-
* tuples while in PHJ_BATCH_PROBING phase, but that's OK because we use
102-
* BarrierArriveAndDetach() to advance it to PHJ_BATCH_DONE without waiting.
99+
* finished. Practically, that means that we never emit a tuple while attached
100+
* to a barrier, unless the barrier has reached a phase that means that no
101+
* process will wait on it again. We emit tuples while attached to the build
102+
* barrier in phase PHJ_BUILD_RUNNING, and to a per-batch barrier in phase
103+
* PHJ_BATCH_PROBING. These are advanced to PHJ_BUILD_DONE and PHJ_BATCH_DONE
104+
* respectively without waiting, using BarrierArriveAndDetach(). The last to
105+
* detach receives a different return value so that it knows that it's safe to
106+
* clean up. Any straggler process that attaches after that phase is reached
107+
* will see that it's too late to participate or access the relevant shared
108+
* memory objects.
103109
*
104110
*-------------------------------------------------------------------------
105111
*/
@@ -296,7 +302,21 @@ ExecHashJoinImpl(PlanState *pstate, bool parallel)
296302
* outer relation.
297303
*/
298304
if (hashtable->totalTuples==0&& !HJ_FILL_OUTER(node))
305+
{
306+
if (parallel)
307+
{
308+
/*
309+
* Advance the build barrier to PHJ_BUILD_RUNNING
310+
* before proceeding so we can negotiate resource
311+
* cleanup.
312+
*/
313+
Barrier*build_barrier=&parallel_state->build_barrier;
314+
315+
while (BarrierPhase(build_barrier)<PHJ_BUILD_RUNNING)
316+
BarrierArriveAndWait(build_barrier,0);
317+
}
299318
returnNULL;
319+
}
300320

301321
/*
302322
* need to remember whether nbatch has increased since we
@@ -317,6 +337,7 @@ ExecHashJoinImpl(PlanState *pstate, bool parallel)
317337

318338
build_barrier=&parallel_state->build_barrier;
319339
Assert(BarrierPhase(build_barrier)==PHJ_BUILD_HASHING_OUTER||
340+
BarrierPhase(build_barrier)==PHJ_BUILD_RUNNING||
320341
BarrierPhase(build_barrier)==PHJ_BUILD_DONE);
321342
if (BarrierPhase(build_barrier)==PHJ_BUILD_HASHING_OUTER)
322343
{
@@ -329,9 +350,18 @@ ExecHashJoinImpl(PlanState *pstate, bool parallel)
329350
BarrierArriveAndWait(build_barrier,
330351
WAIT_EVENT_HASH_BUILD_HASHING_OUTER);
331352
}
332-
Assert(BarrierPhase(build_barrier)==PHJ_BUILD_DONE);
353+
elseif (BarrierPhase(build_barrier)==PHJ_BUILD_DONE)
354+
{
355+
/*
356+
* If we attached so late that the job is finished and
357+
* the batch state has been freed, we can return
358+
* immediately.
359+
*/
360+
returnNULL;
361+
}
333362

334363
/* Each backend should now select a batch to work on. */
364+
Assert(BarrierPhase(build_barrier)==PHJ_BUILD_RUNNING);
335365
hashtable->curbatch=-1;
336366
node->hj_JoinState=HJ_NEED_NEW_BATCH;
337367

@@ -1090,14 +1120,6 @@ ExecParallelHashJoinNewBatch(HashJoinState *hjstate)
10901120
intstart_batchno;
10911121
intbatchno;
10921122

1093-
/*
1094-
* If we started up so late that the batch tracking array has been freed
1095-
* already by ExecHashTableDetach(), then we are finished. See also
1096-
* ExecParallelHashEnsureBatchAccessors().
1097-
*/
1098-
if (hashtable->batches==NULL)
1099-
return false;
1100-
11011123
/*
11021124
* If we were already attached to a batch, remember not to bother checking
11031125
* it again, and detach from it (possibly freeing the hash table if we are

‎src/include/executor/hashjoin.h‎

Lines changed: 2 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -258,7 +258,8 @@ typedef struct ParallelHashJoinState
258258
#definePHJ_BUILD_ALLOCATING1
259259
#definePHJ_BUILD_HASHING_INNER2
260260
#definePHJ_BUILD_HASHING_OUTER3
261-
#definePHJ_BUILD_DONE4
261+
#definePHJ_BUILD_RUNNING4
262+
#definePHJ_BUILD_DONE5
262263

263264
/* The phases for probing each batch, used by for batch_barrier. */
264265
#definePHJ_BATCH_ELECTING0

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