Missed condition-variable wakeups on FreeBSD

About once a month over the last six months, my buildfarm animal
florican has gotten stuck while running the core regression tests.
The symptoms have looked very much the same each time: there is
a backend with two parallel worker processes that are just sitting
and not consuming any CPU time.  Each time I've attached to these
processes with gdb to check their stack traces, and seen pretty
much the same story every time (traces below).  What is really
interesting is that after I detach from the second worker, the
processes resume running and finish out the test successfully!
I don't know much about how gdb interacts with kernel calls on
FreeBSD, but I speculate that the poll(2) call returns with EINTR
after gdb releases the process, and then things resume fine,
suggesting that we lost an interrupt somewhere.

I have observed this three times in the REL_11 branch, once
in REL_12, and a couple of times last summer before it occurred
to me to start keeping notes.  Over that time the machine has
been running various patchlevels of FreeBSD 13.0.

Here's the stack trace from the leader process in the most
recent event (REL_11 as of yesterday).  It's not always the
same query that gets stuck, but it's always a parallel hash join:

(gdb) p debug_query_string
$1 = 0x21873090 "select count(*) from simple r join simple s using (id);"
(gdb) bt
#0  _poll () at _poll.S:4
#1  0x21701361 in __thr_poll (fds=0x219dc170, nfds=2, timeout=-1) at /usr/src/lib/libthr/thread/thr_syscalls.c:338
#2  0x215eaf3f in poll (pfd=0x219dc170, nfds=2, timeout=-1) at /usr/src/lib/libc/sys/poll.c:47
#3  0x0097b0fd in WaitEventSetWaitBlock (set=<optimized out>, cur_timeout=-1, occurred_events=<optimized out>,
nevents=<optimizedout>) at latch.c:1171 
#4  WaitEventSetWait (set=0x219dc138, timeout=-1, occurred_events=<optimized out>, nevents=1,
wait_event_info=134217738)at latch.c:1000 
#5  0x0099842b in ConditionVariableSleep (cv=0x2bf0b230, wait_event_info=134217738) at condition_variable.c:157
#6  0x00977e12 in BarrierArriveAndWait (barrier=0x2bf0b218, wait_event_info=134217738) at barrier.c:191
#7  0x00873441 in ExecHashJoinImpl (pstate=<optimized out>, parallel=true) at nodeHashjoin.c:328
#8  ExecParallelHashJoin (pstate=0x2a887740) at nodeHashjoin.c:600
#9  0x0086a509 in ExecProcNode (node=0x2a887740) at ../../../src/include/executor/executor.h:248
#10 fetch_input_tuple (aggstate=aggstate@entry=0x2a887500) at nodeAgg.c:407
#11 0x00869646 in agg_retrieve_direct (aggstate=<optimized out>) at nodeAgg.c:1746
#12 ExecAgg (pstate=0x2a887500) at nodeAgg.c:1561
#13 0x0086e181 in ExecProcNode (node=0x2a887500) at ../../../src/include/executor/executor.h:248
#14 gather_getnext (gatherstate=0x2a887414) at nodeGather.c:276
#15 ExecGather (pstate=0x2a887414) at nodeGather.c:207
#16 0x0086a509 in ExecProcNode (node=0x2a887414) at ../../../src/include/executor/executor.h:248
#17 fetch_input_tuple (aggstate=aggstate@entry=0x2a8871b8) at nodeAgg.c:407
#18 0x00869646 in agg_retrieve_direct (aggstate=<optimized out>) at nodeAgg.c:1746
#19 ExecAgg (pstate=0x2a8871b8) at nodeAgg.c:1561
#20 0x0085956b in ExecProcNode (node=0x2a8871b8) at ../../../src/include/executor/executor.h:248
#21 ExecutePlan (estate=0x2a887090, planstate=0x2a8871b8, use_parallel_mode=<optimized out>, operation=CMD_SELECT,
sendTuples=<optimizedout>,  
    numberTuples=<optimized out>, direction=<optimized out>, dest=0x2ab629b4, execute_once=<optimized out>) at
execMain.c:1712
#22 standard_ExecutorRun (queryDesc=0x218a1490, direction=ForwardScanDirection, count=0, execute_once=<optimized out>)
atexecMain.c:353 
#23 0x00859445 in ExecutorRun (queryDesc=0x218a1490, direction=ForwardScanDirection, count=0, execute_once=<optimized
out>)at execMain.c:296 
#24 0x009a3d57 in PortalRunSelect (portal=portal@entry=0x2197f090, forward=<optimized out>, count=0, dest=0x2ab629b4)
atpquery.c:941 
#25 0x009a39d6 in PortalRun (portal=0x2197f090, count=2147483647, isTopLevel=<optimized out>, run_once=<optimized out>,
dest=0x2ab629b4,altdest=0x2ab629b4,  
    completionTag=0xffbfdc70 "") at pquery.c:782
#26 0x009a2b53 in exec_simple_query (query_string=query_string@entry=0x21873090 "select count(*) from simple r join
simples using (id);") at postgres.c:1145 
#27 0x009a04ec in PostgresMain (argc=1, argv=0x21954ed4, dbname=<optimized out>, username=0x21954d38 "buildfarm") at
postgres.c:4052

Worker 1 looks like this:

(gdb) bt
#0  _poll () at _poll.S:4
#1  0x21701361 in __thr_poll (fds=0x2187cb48, nfds=2, timeout=-1) at /usr/src/lib/libthr/thread/thr_syscalls.c:338
#2  0x215eaf3f in poll (pfd=0x2187cb48, nfds=2, timeout=-1) at /usr/src/lib/libc/sys/poll.c:47
#3  0x0097b0fd in WaitEventSetWaitBlock (set=<optimized out>, cur_timeout=-1, occurred_events=<optimized out>,
nevents=<optimizedout>) at latch.c:1171 
#4  WaitEventSetWait (set=0x2187cb10, timeout=-1, occurred_events=<optimized out>, nevents=1,
wait_event_info=134217738)at latch.c:1000 
#5  0x0099842b in ConditionVariableSleep (cv=0x2196b230, wait_event_info=134217738) at condition_variable.c:157
#6  0x00977e12 in BarrierArriveAndWait (barrier=0x2196b218, wait_event_info=134217738) at barrier.c:191
#7  0x00873441 in ExecHashJoinImpl (pstate=<optimized out>, parallel=true) at nodeHashjoin.c:328
#8  ExecParallelHashJoin (pstate=0x2a8378d8) at nodeHashjoin.c:600
#9  0x0086a509 in ExecProcNode (node=0x2a8378d8) at ../../../src/include/executor/executor.h:248
#10 fetch_input_tuple (aggstate=aggstate@entry=0x2a8376c8) at nodeAgg.c:407
#11 0x00869646 in agg_retrieve_direct (aggstate=<optimized out>) at nodeAgg.c:1746
#12 ExecAgg (pstate=0x2a8376c8) at nodeAgg.c:1561
#13 0x0085956b in ExecProcNode (node=0x2a8376c8) at ../../../src/include/executor/executor.h:248
#14 ExecutePlan (estate=0x2a837090, planstate=0x2a8376c8, use_parallel_mode=<optimized out>, operation=CMD_SELECT,
sendTuples=<optimizedout>,  
    numberTuples=<optimized out>, direction=<optimized out>, dest=0x219ac780, execute_once=<optimized out>) at
execMain.c:1712
#15 standard_ExecutorRun (queryDesc=0x219ad3e0, direction=ForwardScanDirection, count=0, execute_once=<optimized out>)
atexecMain.c:353 
#16 0x00859445 in ExecutorRun (queryDesc=0x219ad3e0, direction=ForwardScanDirection, count=0, execute_once=<optimized
out>)at execMain.c:296 
#17 0x0085d6de in ParallelQueryMain (seg=0x2187c89c, toc=0x2196b000) at execParallel.c:1402
#18 0x00750ee0 in ParallelWorkerMain (main_arg=1903254317) at parallel.c:1433

Worker 2 looks like this:

(gdb) bt
#0  _poll () at _poll.S:4
#1  0x21701361 in __thr_poll (fds=0x2187cb48, nfds=2, timeout=-1) at /usr/src/lib/libthr/thread/thr_syscalls.c:338
#2  0x215eaf3f in poll (pfd=0x2187cb48, nfds=2, timeout=-1) at /usr/src/lib/libc/sys/poll.c:47
#3  0x0097b0fd in WaitEventSetWaitBlock (set=<optimized out>, cur_timeout=-1, occurred_events=<optimized out>,
nevents=<optimizedout>) at latch.c:1171 
#4  WaitEventSetWait (set=0x2187cb10, timeout=-1, occurred_events=<optimized out>, nevents=1,
wait_event_info=134217737)at latch.c:1000 
#5  0x0099842b in ConditionVariableSleep (cv=0x2196b230, wait_event_info=134217737) at condition_variable.c:157
#6  0x00977e12 in BarrierArriveAndWait (barrier=0x2196b218, wait_event_info=134217737) at barrier.c:191
#7  0x0086ea44 in MultiExecParallelHash (node=<optimized out>) at nodeHash.c:312
#8  MultiExecHash (node=0x2a837e88) at nodeHash.c:112
#9  0x0085fa03 in MultiExecProcNode (node=0x2a837e88) at execProcnode.c:501
#10 0x00872f80 in ExecHashJoinImpl (pstate=<optimized out>, parallel=true) at nodeHashjoin.c:290
#11 ExecParallelHashJoin (pstate=0x2a8378d8) at nodeHashjoin.c:600
#12 0x0086a509 in ExecProcNode (node=0x2a8378d8) at ../../../src/include/executor/executor.h:248
#13 fetch_input_tuple (aggstate=aggstate@entry=0x2a8376c8) at nodeAgg.c:407
#14 0x00869646 in agg_retrieve_direct (aggstate=<optimized out>) at nodeAgg.c:1746
#15 ExecAgg (pstate=0x2a8376c8) at nodeAgg.c:1561
#16 0x0085956b in ExecProcNode (node=0x2a8376c8) at ../../../src/include/executor/executor.h:248
#17 ExecutePlan (estate=0x2a837090, planstate=0x2a8376c8, use_parallel_mode=<optimized out>, operation=CMD_SELECT,
sendTuples=<optimizedout>,  
    numberTuples=<optimized out>, direction=<optimized out>, dest=0x219ac780, execute_once=<optimized out>) at
execMain.c:1712
#18 standard_ExecutorRun (queryDesc=0x219ad3e0, direction=ForwardScanDirection, count=0, execute_once=<optimized out>)
atexecMain.c:353 
#19 0x00859445 in ExecutorRun (queryDesc=0x219ad3e0, direction=ForwardScanDirection, count=0, execute_once=<optimized
out>)at execMain.c:296 
#20 0x0085d6de in ParallelQueryMain (seg=0x2187c89c, toc=0x2196b000) at execParallel.c:1402
#21 0x00750ee0 in ParallelWorkerMain (main_arg=1903254317) at parallel.c:1433
#22 0x009193d3 in StartBackgroundWorker () at bgworker.c:911

In each case I attached to the process, ran the gdb commands shown
above, and detached.  As I've grown used to seeing, the processes
resumed running after I detached from the second worker.  I've not
tried attaching in something other than PID order, but I probably
will next time.

Thoughts?  Ideas on debugging this?

            regards, tom lane