GC dead-locking ?

[Marco Leise]

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.

-- 
Marco