On Mon, Jan 4, 2010 at 11:27 AM, Helge Deller <deller@xxxxxx> wrote:
> I think I have an idea what could have happened and why it most of the times (but not always) crashes in the child process...
>
> In ports/sysdeps/unix/sysv/linux/hppa/bits/atomic.h we have:
> #define atomic_compare_and_exchange_val_acq(mem, newval, oldval) \
> ({ \
> volatile int lws_errno; \
> volatile int lws_ret; \
> asm volatile( \
> ...some assembly...
> "stw %%r28, %0 \n\t" \
> "sub %%r0, %%r21, %%r21 \n\t" \
> "stw %%r21, %1 \n\t" \
> : "=m" (lws_ret), "=m" (lws_errno) \
> : "r" (mem), "r" (oldval), "r" (newval) \
> : _LWS_CLOBBER
>
> this means, that lws_errno and lws_ret are located on the stack.
Correct. We could place them in registers if we wanted, they are
registers r28 (lws return) and r21 (lws error).
> With gdb I see this expanded to:
> 0x40705494 <start_thread+1204>: stw ret0,-1b8(sp)
> 0x40705498 <start_thread+1208>: sub r0,r21,r21
> 0x4070549c <start_thread+1212>: stw r21,-1b4(sp)
>
> So, lws_ret/lws_errno are at -1b8/-1b4(sp).
Correct.
> And this LWS code is called from
> ../nptl/sysdeps/pthread/createthread.c:
> static int create_thread (struct pthread *pd, const struct pthread_attr *attr, STACK_VARIABLES_PARMS)
> ...
> int res = do_clone (pd, attr, clone_flags, start_thread,
> STACK_VARIABLES_ARGS, 1);
> if (res == 0)
> {
> ...(line 216):
> /* Enqueue the descriptor. */
> do
> pd->nextevent = __nptl_last_event;
> while (atomic_compare_and_exchange_bool_acq(&__nptl_last_event, pd, pd->nextevent) != 0);
>
>
> And here is what could have happened:
> a) do_clone() creates the child process.
> b) the child process gets a new stack
> c) the child calls atomic_compare_and_exchange_bool_acq() and thus the LWS code above.
> d) the LWS code writes to the stack location at -1b8(sp), which is out of bounds for the child process (the child stack got only ~ 0x40 bytes initial room)
This is wrong. Each thread should have 8MB of stack. If we only get ~
0x40 bytes then npt/nptl-init.c is setting __default_stacksize
incorrectly.
Even PTHREAD_STACK_MIN should be 16kb?
Could you verify that your assertion that only ~ 0x40 bytes of initial
room were allocated?
> e) Thus the child either crashes, overwrites memory of the parent or does other things wrong.
I agree with your analysis, but the error is that more stack should be
allocated.
Cheers,
Carlos.
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