On 08/15/2016 07:35 AM, Holger Brunck wrote:
> I tried this but unfortunately the error only occurs while remote debugging.
> Locally with gdb everything works fine. BTW we double-checked with a 85xx ppc
> target which is also 32-bit and it ends up with the same behaviour.
>
> I was also investigating where I have to move the line in the struct
> task_struct
> and it turns out to be like this (diff to 4.7 kernel):
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 253538f..4868874 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1655,7 +1655,9 @@ struct task_struct {
> struct signal_struct *signal;
> struct sighand_struct *sighand;
>
> + // struct thread_struct thread; // until here everything is fine
> sigset_t blocked, real_blocked;
> + struct thread_struct thread; // from here it's broken
> sigset_t saved_sigmask; /* restored if set_restore_sigmask() was used
> */
> struct sigpending pending;
Wow, thanks for all the debugging here!
So, we know it has to do with signals, thread_info, and probably only
affects 32-bit powerpc. Seems awfully weird. Have you checked with any
of the 64-bit powerpc guys to see if they have any ideas?
I went grepping around for a bit.
Where is the task_struct stored? Is it on-stack on ppc32 or something?
The thread_info is, I assume, but I see some THREAD_INFO vs. THREAD
(thread struct) math happening in here, which confuses me:
.globl ret_from_debug_exc
ret_from_debug_exc:
mfspr r9,SPRN_SPRG_THREAD
lwz r10,SAVED_KSP_LIMIT(r1)
stw r10,KSP_LIMIT(r9)
lwz r9,THREAD_INFO-THREAD(r9)
CURRENT_THREAD_INFO(r10, r1)
lwz r10,TI_PREEMPT(r10)
stw r10,TI_PREEMPT(r9)
RESTORE_xSRR(SRR0,SRR1);
RESTORE_xSRR(CSRR0,CSRR1);
RESTORE_MMU_REGS;
RET_FROM_EXC_LEVEL(SPRN_DSRR0, SPRN_DSRR1, PPC_RFDI)
But, I'm really at a loss to explain this. It still seems like a deeply
ppc-specific issue. We can obviously work around it with an #ifdef for
your platform, but that's awfully hackish and hides the real bug,
whatever it is.
My suspicion is that there's a bug in the 32-bit ppc assembly somewhere.
I don't see any references to 'blocked' or 'real_blocked' in assembly
though. You could add a bunch of padding instead of moving the
thread_struct and see if that does anything, but that's really a stab in
the dark.
