powerpc: fork && stepping (Was: [RFC, PATCH 0/14] utrace/ptrace)

[Oleg Nesterov]

On 11/27, Paul Mackerras wrote:
>
> Oleg Nesterov writes:
>
> > 	0xfeacd24
> > 	0xfeacd28
> > 	0xfeacd2c
> > 	0xfeacd30
> > 	0xfeacd34
> > 	...
> >
> > and so on forever,
> ...
> > beg->	0x0feacd24 <__GI__IO_list_lock+68>:	lwarx   r0,0,r31
> > 	0x0feacd28 <__GI__IO_list_lock+72>:	cmpw    r0,r11
> > 	0x0feacd2c <__GI__IO_list_lock+76>:	bne-    0xfeacd38 <__GI__IO_list_lock+88>
> > 	0x0feacd30 <__GI__IO_list_lock+80>:	stwcx.  r9,0,r31
> > end->	0x0feacd34 <__GI__IO_list_lock+84>:	bne+    0xfeacd24 <__GI__IO_list_lock+68>
> >
> > I don't even know whether this is user-space bug or kernel bug,
> > the asm above is the black magic for me.
>
> The lwarx and stwcx. work together to do an atomic update to the word
> whose address is in r31.  They are like LL (load-linked) and SC
> (store-conditional) on other architectures such as alpha.  Basically
> the lwarx creates an internal "reservation" on the word pointed to by
> r31 and loads its value into r0.  The stwcx. stores into that word but
> only if the reservation still exists.  The reservation gets cleared
> (in hardware) if any other cpu writes to that word in the meantime.
> If the reservation did get cleared, the bne (branch if not equal)
> instruction will be taken and we loop around to try again.
>
> There is a difficulty when single-stepping through such a sequence
> because the process of taking the single-step exception and returning
> will clear the reservation.  Thus if you single-step through that
> sequence it will never succeed.  I believe gdb has code to recognize
> this kind of sequence and run through it without stopping until after
> the bne, precisely to avoid this problem.

Thanks! This explains everything, I think.

Could you look at this

	ptrace-copy_process-should-disable-stepping.patch
	http://marc.info/?l=linux-mm-commits&m=125789789322573

patch? It is not clear to me how we can modify the test-case to
verify it fixes the original problem for powerpc.

At least, do you think this patch is good for powerpc ?

Oleg.