Freeze on Cygwin w/ concurrency

"pgbench -i -s 50; pgbench -S -j2 -c16 -T900 -P5" freezes consistently on
Cygwin 2.2.1 and Cygwin 2.6.0. (I suspect most other versions are affected.)
I've pinged[1] the Cygwin bug thread with some additional detail. If a Cygwin
buildfarm member starts using --enable-tap-tests, you may see failures in the
pgbench test suite. (lorikeet used --enable-tap-tests from 2017-03-18 to
2017-03-20, but it failed before reaching the pgbench test suite.) Curious
that "make check" has too little concurrency to see more effects from this.

Frozen backends show a stack trace like this:

#0 0x000000007710139a in ntdll!ZwWriteFile () from /cygdrive/c/Windows/SYSTEM32/ntdll.dll
#1 0x000007fefd851b2b in WriteFile () from /cygdrive/c/Windows/system32/KERNELBASE.dll
#2 0x0000000076fb3576 in WriteFile () from /cygdrive/c/Windows/system32/kernel32.dll
#3 0x0000000180160c6c in transport_layer_pipes::write (this=<optimized out>, buf=<optimized out>, len=<optimized out>)
at /usr/src/debug/cygwin-2.6.0-1/winsup/cygserver/transport_pipes.cc:224
#4 0x000000018015feb6 in client_request::send (this=0xffffa930, conn=0x6000e8290) at /usr/src/debug/cygwin-2.6.0-1/winsup/cygserver/client.cc:134
#5 0x0000000180160591 in client_request::make_request (this=this(at)entry=0xffffa930) at /usr/src/debug/cygwin-2.6.0-1/winsup/cygserver/client.cc:473
#6 0x0000000180114f79 in semop (semid=65540, sops=0xffffaa00, nsops=1) at /usr/src/debug/cygwin-2.6.0-1/winsup/cygwin/sem.cc:125
#7 0x0000000180117a4b in _sigfe () at sigfe.s:35
#8 0x000000010063c81a in PGSemaphoreLock (sema=sema(at)entry=0x6ffffe06a18) at pg_sema.c:387
#9 0x00000001006a962b in LWLockAcquire (lock=lock(at)entry=0x6fff6774d80, mode=mode(at)entry=LW_SHARED) at lwlock.c:1286
#10 0x0000000100687d46 in BufferAlloc (foundPtr=0xffffab0b <incomplete sequence \367\377\006>, strategy=0x0, blockNum=290, forkNum=MAIN_FORKNUM,
relpersistence=112 'p', smgr=0x6000ea588) at bufmgr.c:1012

The postmaster, also frozen, shows a stack trace like this:

#0 0x00000000771018ca in ntdll!ZwWaitForMultipleObjects () from /cygdrive/c/Windows/SYSTEM32/ntdll.dll
#1 0x000007fefd851420 in KERNELBASE!GetCurrentProcess () from /cygdrive/c/Windows/system32/KERNELBASE.dll
#2 0x0000000076fa1220 in WaitForMultipleObjects () from /cygdrive/c/Windows/system32/kernel32.dll
#3 0x0000000180120173 in child_info::sync (this=this(at)entry=0xffffc008, pid=4692, hProcess=(at)0xffffc1b0: 0x4b8, howlong=howlong(at)entry=300000)
at /usr/src/debug/cygwin-2.6.0-1/winsup/cygwin/sigproc.cc:1010
#4 0x00000001800aa163 in frok::parent (this=0xffffc000, stack_here=0xffffbfa0 "") at /usr/src/debug/cygwin-2.6.0-1/winsup/cygwin/fork.cc:501
#5 0x00000001800aaa05 in fork () at /usr/src/debug/cygwin-2.6.0-1/winsup/cygwin/fork.cc:607
#6 0x0000000180117a4b in _sigfe () at sigfe.s:35
#7 0x0000000100641618 in fork_process () at fork_process.c:61
#8 0x000000010063e80a in StartAutoVacWorker () at autovacuum.c:1436

The postmaster log eventually has:

28 [main] postgres 4408 child_info::sync: wait failed, pid 4692, Win32 error 183
292 [main] postgres 4408 fork: child 4692 - died waiting for dll loading, errno 11

[1] https://cygwin.com/ml/cygwin/2017-03/msg00218.html