https://issues.dlang.org/show_bug.cgi?id=20270

          Issue ID: 20270
           Summary: [REG2.087] Deadlock in garbage collection when running
                    processes in parallel
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Severity: regression
          Priority: P1
         Component: druntime
          Assignee: nobody@puremagic.com
          Reporter: dlang-bugzilla@thecybershadow.net
                CC: r.sagitario@gmx.de

///////////// test.d /////////////
import std.parallelism;
import std.process;
import std.range;

void main()
{
    foreach (i; 200.iota.parallel)
        execute(["true"]);
}
//////////////////////////////////

This program has a roughly 60% chance to deadlock and never finish executing on my machine.

Inspecting the program's state with a debugger shows that the threads are generally in one of these states:

Thread 11 (Thread 0x7f2a80ff9700 (LWP 424924)):
#0  0x00007f2a89b82b12 in sigsuspend () from /usr/lib/libc.so.6
#1  0x0000563bd079bb08 in core.thread.thread_suspendHandler(int).op(void*) ()
#2  0x0000563bd079bb68 in core.thread.callWithStackShell(scope void(void*)
nothrow delegate) ()
#3  0x0000563bd079ba95 in thread_suspendHandler ()
#4  <signal handler called>
#5  0x00007f2a89e8da6a in read () from /usr/lib/libpthread.so.0
...


Thread 10 (Thread 0x7f2a817fa700 (LWP 424923)):
#0  0x00007f2a89b82b12 in sigsuspend () from /usr/lib/libc.so.6
#1  0x0000563bd079bb08 in core.thread.thread_suspendHandler(int).op(void*) ()
#2  0x0000563bd079bb68 in core.thread.callWithStackShell(scope void(void*)
nothrow delegate) ()
#3  0x0000563bd079ba95 in thread_suspendHandler ()
#4  <signal handler called>
#5  0x00007f2a89c11414 in fork () from /usr/lib/libc.so.6
...

Thread 9 (Thread 0x7f2a81ffb700 (LWP 424922)):
#0  0x00007f2a89b82b12 in sigsuspend () from /usr/lib/libc.so.6
#1  0x0000563bd079bb08 in core.thread.thread_suspendHandler(int).op(void*) ()
#2  0x0000563bd079bb68 in core.thread.callWithStackShell(scope void(void*)
nothrow delegate) ()
#3  0x0000563bd079ba95 in thread_suspendHandler ()
#4  <signal handler called>
#5  0x00007f2a89c515c9 in __lll_lock_wait_private () from /usr/lib/libc.so.6
#6  0x00007f2a89c51a88 in __run_fork_handlers () from /usr/lib/libc.so.6
#7  0x00007f2a89c113e9 in fork () from /usr/lib/libc.so.6
...

Thread 8 (Thread 0x7f2a827fc700 (LWP 424921)):
#0  0x00007f2a89b82b12 in sigsuspend () from /usr/lib/libc.so.6
#1  0x0000563bd079bb08 in core.thread.thread_suspendHandler(int).op(void*) ()
#2  0x0000563bd079bb68 in core.thread.callWithStackShell(scope void(void*)
nothrow delegate) ()
#3  0x0000563bd079ba95 in thread_suspendHandler ()
#4  <signal handler called>
#5  0x00007f2a89e8e145 in nanosleep () from /usr/lib/libpthread.so.0
#6  0x0000563bd077370e in _D4core6thread6Thread5sleepFNbNiSQBf4time8DurationZv
()
#7  0x0000563bd07b3e2e in core.internal.spinlock.SpinLock.yield(ulong) shared
()
#8  0x0000563bd07b3dc4 in core.internal.spinlock.SpinLock.lock() shared ()
#9  0x0000563bd07c9307 in
_D2gc4impl12conservativeQw14ConservativeGC__T9runLockedS_DQCeQCeQCcQCnQBs12mallocNoSyncMFNbmkKmxC8TypeInfoZPvS_DQEgQEgQEeQEp10mallocTimelS_DQFiQFiQFgQFr10numMallocslTmTkTmTxQCzZQFcMFNbKmKkKmKxQDsZQDl
()
#10 0x0000563bd07c1456 in
_D2gc4impl12conservativeQw14ConservativeGC6qallocMFNbmkxC8TypeInfoZS4core6memory8BlkInfo_
()
#11 0x0000563bd0787fe7 in gc_qalloc ()
...

Thread 7 (Thread 0x7f2a82ffd700 (LWP 424920)):
#0  0x00007f2a89c515cb in __lll_lock_wait_private () from /usr/lib/libc.so.6
#1  0x00007f2a89bd06b3 in calloc () from /usr/lib/libc.so.6
#2  0x0000563bd07c61ad in
_D2gc4impl12conservativeQw3Gcx16startScanThreadsMFNbZv ()
#3  0x0000563bd07c5f44 in _D2gc4impl12conservativeQw3Gcx12markParallelMFNbbZv
()
#4  0x0000563bd07c5862 in _D2gc4impl12conservativeQw3Gcx11fullcollectMFNbbZm ()
#5  0x0000563bd07c4050 in
_D2gc4impl12conservativeQw3Gcx8bigAllocMFNbmKmkxC8TypeInfoZPv ()
#6  0x0000563bd07c935a in
_D2gc4impl12conservativeQw14ConservativeGC__T9runLockedS_DQCeQCeQCcQCnQBs12mallocNoSyncMFNbmkKmxC8TypeInfoZPvS_DQEgQEgQEeQEp10mallocTimelS_DQFiQFiQFgQFr10numMallocslTmTkTmTxQCzZQFcMFNbKmKkKmKxQDsZQDl
()
#7  0x0000563bd07c1456 in
_D2gc4impl12conservativeQw14ConservativeGC6qallocMFNbmkxC8TypeInfoZS4core6memory8BlkInfo_
()
#8  0x0000563bd0787fe7 in gc_qalloc ()
...

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