GC dead-locking ?

[Sean Kelly]

On Jun 18, 2013, at 7:01 AM, Marco Leise <Marco.Leise at gmx.de> wrote:

> Am Mon, 17 Jun 2013 10:46:19 -0700
> schrieb Sean Kelly <sean at invisibleduck.org>:
> 
>> On Jun 13, 2013, at 2:22 AM, Marco Leise <Marco.Leise at gmx.de> wrote:
>> 
>>> Here is an excerpt from a stack trace I got while profiling
>>> with OProfile:
>>> 
>>> #0  sem_wait () from /lib64/libpthread.so.0
>>> #1  thread_suspendAll () at core/thread.d:2471
>>> #2  gc.gcx.Gcx.fullcollect() (this=...) at gc/gcx.d:2427
>>> #3  gc.gcx.Gcx.bigAlloc() (this=..., size=16401, poolPtr=0x7fc3d4bfe3c8, alloc_size=0x7fc3d4bfe418) at gc/gcx.d:2099
>>> #4  gc.gcx.GC.mallocNoSync (alloc_size=0x7fc3d4bfe418, bits=10, size=16401, this=...) gc/gcx.d:503
>>> #5  gc.gcx.GC.malloc() (this=..., size=16401, bits=10, alloc_size=0x7fc3d4bfe418) gc/gcx.d:421
>>> #6  gc.gc.gc_qalloc (ba=10, sz=<optimized out>) gc/gc.d:203
>>> #7  gc_qalloc (sz=<optimized out>, ba=10) gc/gc.d:198
>>> #8  _d_newarrayT (ti=..., length=4096) rt/lifetime.d:807
>>> #9  sequencer.algorithm.gzip.HuffmanTree.__T6__ctorTG32hZ.__ctor() (this=..., bitLengths=...) sequencer/algorithm/gzip.d:444
>>> 
>>> Two more threads are alive, but waiting on a condition
>>> variable (i.e.: in pthread_cond_wait(), but from my own and
>>> not from druntime code. Is there some obvious way I could have
>>> dead-locked the GC ? Or is there a bug ?
>> 
>> I assume you're running on Linux, which uses signals (SIGUSR1, specifically) to suspend threads for a collection.  So I imagine what's happening is that your thread is trying to suspend all the other threads so it can collect, and those threads are ignoring the signal for some reason.  I would expect pthread_cond_wait to be interrupted if a signal arrives though.  Have you overridden the signal handler for SIGUSR1?
> 
> No, I have not overridden the signal handler. I'm aware of the
> fact that signals make pthread_cond_wait() return early and
> put them in a while loop as one would expect, that is all.

Hrm... Can you trap this in a debugger and post the stack traces of all threads?  That stack above is a thread waiting for others to say they're suspended so it can collect.