Re: GC collecting "too much"..

Thu, 29 Mar 2012 06:54:59 -0700 

On Thu, 29 Mar 2012 14:14:10 +0200, simendsjo <simend...@gmail.com> wrote:
On Mon, 26 Mar 2012 20:15:40 +0200, simendsjo <simend...@gmail.com> wrote:
On Mon, 26 Mar 2012 17:10:34 +0200, Timon Gehr <timon.g...@gmx.ch> wrote: 


On 03/26/2012 11:55 AM, simendsjo wrote:

It seems threads created in the c library is totally unknown to D. How
can I make D aware of these threads when there is no library support for 
it?


You may be looking for this:
http://dlang.org/phobos/core_thread.html#thread_attachThis


Thanks, but I tried that too and couldn't get it to work.

I added the following:

extern(C) handler()
{
synchronized // needed here to avoid the GC to collect while attaching thread? 
     {
         if(!Thread.getThis()) // thread unknown to D
         {
             thread_attachThis();
             assert(Thread.getThis()); // now D knows about it
         }
     }
     GC.collect(); // still segfaults
}
Actually, using attachThis segfaults GC.collect() outside the thread handling code too.
I compiled with a debug build of druntime, and it fails on dereferencing a pointer, pbot in gcx.d:2511: 

void mark(void *pbot, void *ptop, int nRecurse)
        void **p1 = cast(void **)pbot;
(...)
        for (; p1 < p2; p1++)
        {
            auto p = cast(byte *)(*p1); // segfault!
(...)

gdb) print p1
$10 = (void **) 0xf74fc000
(gdb) print *p1
$11 = (void *) 0x0

Some more dbg info:

(gdb) bt
#0 0x08098753 in gc.gcx.Gcx.mark() (this=0x80df030, nRecurse=64, ptop=0xffffd190, pbot=0xf74f4df4) at src/gc/gcx.d:2511 #1 0x080986d6 in gc.gcx.Gcx.mark() (this=0x80df030, ptop=0xffffd190, pbot=0xf74f4df4) at src/gc/gcx.d:2494 #2 0x0809ef95 in core.thread.thread_scanAll() (this=0xf74f4cf8, p2=0xffffd190, p1=0xf74f4df4, type=<incomplete type>) 
    at src/core/thread.d:2743
#3 0x080a6488 in thread_scanAllType (scan=..., curStackTop=0xf74f4df4) at src/core/thread.d:2698 #4 0x0809ef68 in thread_scanAll (scan=..., curStackTop=0xf74f4df4) at src/core/thread.d:2746 #5 0x08098c34 in gc.gcx.Gcx.fullcollect() (this=0x80df030, stackTop=0xf74f4df4) at src/gc/gcx.d:2776 #6 0x080989f9 in gc.gcx.Gcx.fullcollectshell() (this=0x80df030) at src/gc/gcx.d:2662 #7 0x08096da1 in gc.gcx.GC.fullCollect() (this=0x80df018) at src/gc/gcx.d:1372 
#8  0x08086da6 in gc_collect () at src/gc/gc.d:154
#9  0x08085380 in core.memory.GC.collect() () at src/core/memory.d:101
#10 0x08080e2c in c_callback (event=<incomplete type>, conn=0x80ebec0, request_info=0x80ebec0) at main.d:54 
#11 0xf7fd28c1 in handle_request(mg_connection*) () from ./libmongoose.so
#12 0xf7fd4b7d in worker_thread(mg_context*) () from ./libmongoose.so
#13 0xf7f8dd1a in start_thread (arg=0xf74fbb70) at pthread_create.c:304
#14 0xf7ed5e4e in clone () at ../sysdeps/unix/sysv/linux/i386/clone.S:130
Backtrace stopped: Not enough registers or memory available to unwind further 
(gdb) l 2511
2506            uint changes = 0;
2507
2508            //printf("marking range: %p -> %p\n", pbot, ptop);
2509            for (; p1 < p2; p1++)
2510            {
2511                auto p = cast(byte *)(*p1);
2512
2513                //if (log) debug(PRINTF) printf("\tmark %p\n", p);
2514                if (p >= minAddr && p < maxAddr)
2515                {




And if I change the code to:
extern(C) handler()
{
     GC.collect();
}

(snip)
Checking with debug build of druntime, and I see it tries to suspend(thread_getThis()). This gives a null reference error as thread_getThis() returns null when it's not attached.

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