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Commitb1ffe3f

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Fix bugs in MultiXact truncation
1. TruncateMultiXact() performs the SLRU truncations in a criticalsection. Deleting the SLRU segments calls ForwardSyncRequest(), whichwill try to compact the request queue if it's full(CompactCheckpointerRequestQueue()). That in turn allocates memory,which is not allowed in a critical section. Backtrace: TRAP: failed Assert("CritSectionCount == 0 || (context)->allowInCritSection"), File: "../src/backend/utils/mmgr/mcxt.c", Line: 1353, PID: 920981 postgres: autovacuum worker template0(ExceptionalCondition+0x6e)[0x560a501e866e] postgres: autovacuum worker template0(+0x5dce3d)[0x560a50217e3d] postgres: autovacuum worker template0(ForwardSyncRequest+0x8e)[0x560a4ffec95e] postgres: autovacuum worker template0(RegisterSyncRequest+0x2b)[0x560a50091eeb] postgres: autovacuum worker template0(+0x187b0a)[0x560a4fdc2b0a] postgres: autovacuum worker template0(SlruDeleteSegment+0x101)[0x560a4fdc2ab1] postgres: autovacuum worker template0(TruncateMultiXact+0x2fb)[0x560a4fdbde1b] postgres: autovacuum worker template0(vac_update_datfrozenxid+0x4b3)[0x560a4febd2f3] postgres: autovacuum worker template0(+0x3adf66)[0x560a4ffe8f66] postgres: autovacuum worker template0(AutoVacWorkerMain+0x3ed)[0x560a4ffe7c2d] postgres: autovacuum worker template0(+0x3b1ead)[0x560a4ffecead] postgres: autovacuum worker template0(+0x3b620e)[0x560a4fff120e] postgres: autovacuum worker template0(+0x3b3fbb)[0x560a4ffeefbb] postgres: autovacuum worker template0(+0x2f724e)[0x560a4ff3224e] /lib/x86_64-linux-gnu/libc.so.6(+0x27c8a)[0x7f62cc642c8a] /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0x85)[0x7f62cc642d45] postgres: autovacuum worker template0(_start+0x21)[0x560a4fd16f31]To fix, bail out in CompactCheckpointerRequestQueue() without doinganything, if it's called in a critical section. That covers the abovecall path, as well as any other similar cases whereRegisterSyncRequest might be called in a critical section.2. After fixing that, another problem became apparent: Autovacuumprocess doing that truncation can deadlock with the checkpointerprocess. TruncateMultiXact() sets "MyProc->delayChkptFlags |=DELAY_CHKPT_START". If the sync request queue is full and cannot becompacted, the process will repeatedly sleep and retry, until there isroom in the queue. However, if the checkpointer is trying to start acheckpoint at the same time, and is waiting for the DELAY_CHKPT_STARTprocesses to finish, the queue will never shrink.More concretely, the autovacuum process is stuck here: #0 0x00007fc934926dc3 in epoll_wait () from /lib/x86_64-linux-gnu/libc.so.6#1 0x000056220b24348b in WaitEventSetWaitBlock (set=0x56220c2e4b50, occurred_events=0x7ffe7856d040, nevents=1, cur_timeout=<optimized out>) at ../src/backend/storage/ipc/latch.c:1570#2 WaitEventSetWait (set=0x56220c2e4b50, timeout=timeout@entry=10, occurred_events=<optimized out>, occurred_events@entry=0x7ffe7856d040, nevents=nevents@entry=1, wait_event_info=wait_event_info@entry=150994949) at ../src/backend/storage/ipc/latch.c:1516#3 0x000056220b243224 in WaitLatch (latch=<optimized out>, latch@entry=0x0, wakeEvents=wakeEvents@entry=40, timeout=timeout@entry=10, wait_event_info=wait_event_info@entry=150994949) at ../src/backend/storage/ipc/latch.c:538#4 0x000056220b26cf46 in RegisterSyncRequest (ftag=ftag@entry=0x7ffe7856d0a0, type=type@entry=SYNC_FORGET_REQUEST, retryOnError=true) at ../src/backend/storage/sync/sync.c:614#5 0x000056220af9db0a in SlruInternalDeleteSegment (ctl=ctl@entry=0x56220b7beb60 <MultiXactMemberCtlData>, segno=segno@entry=11350) at ../src/backend/access/transam/slru.c:1495postgres#6 0x000056220af9dab1 in SlruDeleteSegment (ctl=ctl@entry=0x56220b7beb60 <MultiXactMemberCtlData>, segno=segno@entry=11350) at ../src/backend/access/transam/slru.c:1566postgres#7 0x000056220af98e1b in PerformMembersTruncation (oldestOffset=<optimized out>, newOldestOffset=<optimized out>) at ../src/backend/access/transam/multixact.c:3006postgres#8 TruncateMultiXact (newOldestMulti=newOldestMulti@entry=3221225472, newOldestMultiDB=newOldestMultiDB@entry=4) at ../src/backend/access/transam/multixact.c:3201postgres#9 0x000056220b098303 in vac_truncate_clog (frozenXID=749, minMulti=<optimized out>, lastSaneFrozenXid=749, lastSaneMinMulti=3221225472) at ../src/backend/commands/vacuum.c:1917postgres#10 vac_update_datfrozenxid () at ../src/backend/commands/vacuum.c:1760postgres#11 0x000056220b1c3f76 in do_autovacuum () at ../src/backend/postmaster/autovacuum.c:2550postgres#12 0x000056220b1c2c3d in AutoVacWorkerMain (startup_data=<optimized out>, startup_data_len=<optimized out>) at ../src/backend/postmaster/autovacuum.c:1569and the checkpointer is stuck here: #0 0x00007fc9348ebf93 in clock_nanosleep () from /lib/x86_64-linux-gnu/libc.so.6#1 0x00007fc9348fe353 in nanosleep () from /lib/x86_64-linux-gnu/libc.so.6#2 0x000056220b40ecb4 in pg_usleep (microsec=microsec@entry=10000) at ../src/port/pgsleep.c:50#3 0x000056220afb43c3 in CreateCheckPoint (flags=flags@entry=108) at ../src/backend/access/transam/xlog.c:7098#4 0x000056220b1c6e86 in CheckpointerMain (startup_data=<optimized out>, startup_data_len=<optimized out>) at ../src/backend/postmaster/checkpointer.c:464To fix, add AbsorbSyncRequests() to the loops where the checkpointerwaits for DELAY_CHKPT_START or DELAY_CHKPT_COMPLETE operations tofinish.Backpatch to v14. Before that, SLRU deletion didn't callRegisterSyncRequest, which avoided this failure. I'm not sure if thereare other similar scenarios on older versions, but we haven't hadany such reports.Discussion:https://www.postgresql.org/message-id/ccc66933-31c1-4f6a-bf4b-45fef0d4f22e@iki.fi
1 parentf92fd18 commitb1ffe3f

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‎src/backend/access/transam/xlog.c

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@@ -7094,6 +7094,13 @@ CreateCheckPoint(int flags)
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{
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do
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{
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/*
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* Keep absorbing fsync requests while we wait. There could even
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* be a deadlock if we don't, if the process that prevents the
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* checkpoint is trying to add a request to the queue.
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*/
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AbsorbSyncRequests();
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pgstat_report_wait_start(WAIT_EVENT_CHECKPOINT_DELAY_START);
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pg_usleep(10000L);/* wait for 10 msec */
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pgstat_report_wait_end();
@@ -7109,6 +7116,8 @@ CreateCheckPoint(int flags)
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{
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do
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{
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AbsorbSyncRequests();
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pgstat_report_wait_start(WAIT_EVENT_CHECKPOINT_DELAY_COMPLETE);
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pg_usleep(10000L);/* wait for 10 msec */
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pgstat_report_wait_end();

‎src/backend/postmaster/checkpointer.c

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@@ -1169,6 +1169,10 @@ CompactCheckpointerRequestQueue(void)
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/* must hold CheckpointerCommLock in exclusive mode */
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Assert(LWLockHeldByMe(CheckpointerCommLock));
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/* Avoid memory allocations in a critical section. */
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if (CritSectionCount>0)
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return false;
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/* Initialize skip_slot array */
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skip_slot=palloc0(sizeof(bool)*CheckpointerShmem->num_requests);
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