'make' stalls on "cc1plus.debug" w/sufficient number of jobs

["John D. Baker" <jdbaker%mylinuxisp.com@localhost>]

When doing a build from scratch (or a non-update build) on my 8-core
amd64 build host, 'make' consistently stalls when building:

[...]
--- cc1plus.debug ---
#    create  cc1plus/cc1plus.debug
(  /r0/build/current/tools/amd64/bin/armv7--netbsdelf-eabihf-objcopy --only-keep-debug cc1plus cc1plus.debug  && /r0/build/current/tools/amd64/bin/armv7--netbsdelf-eabihf-objcopy --strip-debug -p -R .gnu_debuglink  --add-gnu-debuglink=cc1plus.debug cc1plus  ) || (rm -f cc1plus.debug; false)
--- dependall ---
--- dependall ---
--- dependall ---
[stalls here]

The output of 'ps axww' shows:

[...]
 1674 pts/1 I+     0:00.02 sh ./build.sh -U -m evbearmv7hf-el -T /r0/build/current/tools/amd64 -O /r0/build/current/obj/evbearmv7hf-el -D /r0/build/current/DEST/evbearmv7hf-el -R /r0/build/current/REL -X ../xsrc -x -j 9 -V USE_PIGZGZIP=yes -V MKDEBUG=yes -V MKDEBUG_LIB=yes -V MKCROSSGDB=yes tools release iso-image 
 1786 pts/1 S+     0:02.64 /r0/build/current/tools/amd64/bin/nbmake _THISDIR_ BUILD_tools BUILD_lib dependall 
 2527 pts/1 I+     0:00.00 sh 
 6229 pts/1 S+     0:05.94 /r0/build/current/tools/amd64/bin/nbmake _THISDIR_ NOPOSTINSTALL build 
 6754 pts/1 S+     0:01.22 /r0/build/current/tools/amd64/bin/nbmake _THISDIR_ dependall 
 6818 pts/1 S+     0:00.17 /r0/build/current/tools/amd64/bin/nbmake _THISDIR_ dependall 
 7389 pts/1 I+     0:00.00 sh 
 9688 pts/1 S+     0:00.59 /r0/build/current/tools/amd64/bin/nbmake _THISDIR_ dependall 
11335 pts/1 S+     0:00.34 /r0/build/current/tools/amd64/bin/nbmake _THISDIR_ dependall 
12428 pts/1 S+     0:05.77 /r0/build/current/tools/amd64/bin/nbmake _THISDIR_ distribution 
12522 pts/1 S+     0:00.18 /r0/build/current/tools/amd64/bin/nbmake _THISDIR_ dependall 
13412 pts/1 S+     0:00.51 /r0/build/current/tools/amd64/bin/nbmake _THISDIR_ dependall 
14034 pts/1 I+     0:00.00 sh 
14113 pts/1 I+     0:00.00 sh 
14873 pts/1 I+     0:00.00 sh 
15025 pts/1 S+     0:00.33 /r0/build/current/tools/amd64/bin/nbmake _THISDIR_ dependall 
15065 pts/1 Il+    0:00.01 /r0/build/current/tools/amd64/bin/nbpigz -nT -9 -cf 
15929 pts/1 I+     0:00.00 sh 
16434 pts/1 S+     0:01.18 /r0/build/current/tools/amd64/bin/nbmake realall 
17565 pts/1 I+     0:00.00 sh 
17840 pts/1 S+     0:10.02 /r0/build/current/tools/amd64/bin/nbmake -j 9 release 
17930 pts/1 I+     0:00.00 sh 
18281 pts/1 I+     0:00.00 sh 
18979 pts/1 I+     0:00.00 sh 
19236 pts/1 I+     0:00.00 sh 
21268 pts/1 I+     0:00.00 /bin/sh /var/home/sysop/bin/mybuild.sh 
28499 pts/1 I+     0:00.00 sh 
29082 pts/1 S+     0:02.05 /r0/build/current/tools/amd64/bin/nbmake _THISDIR_ do-build 
29758 pts/1 Is     0:00.49 -ksh 
29926 pts/1 I+     0:00.00 sh 
 9379 pts/2 O+     0:00.00 ps -axww 
26580 pts/2 Ss     0:00.01 -ksh 
[...]

It will sit there indefinitely until I stop it with Ctrl-C.  A followup
update build proceeds to completion.  This happens on all target
architectures I build (amd64, i386, sparc, sparc64, prep, mvme68k, amiga,
macppc, mac68k, evbearmv7hf-el).

It seems to only happen on machines with multiple real CPUs/cores.  A
single-processor hyperthreaded machine doesn't stall.

It seems to be a quirk of -current tools.  Building for netbsd-7 from
scratch doesn't stall.

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