Jan Kiszka wrote:
Philippe Gerum wrote:

Jan Kiszka wrote:

Philippe Gerum wrote:


Hi Jan,

Based on your previous work, here is a set of patches coupling KGDB and
the I-pipe. Basically, I've attempted to shrink the extra patches needed
against the original KGDB + I-pipe ones to the bare minimum. This has
been obtained by having the I-pipe provide ipipe_current_safe(), and
drastically reduce the amount of fiddling with smp_processor_id().

The key difference with the former implementation is that a domain (e.g.
Xenomai) is now expected to tell the I-pipe when it's switching to a
non-Linux stack, and the I-pipe makes good use of this information to
return the proper "current" value when asked to through
ipipe_safe_current() from the KGDB code. The issue of swapping


This looks nice.



smp_processor_id() with ipipe_processor_id() has been addressed the hard
way: smp_processor_id() is simply defined as ipipe_processor_id() when
CONFIG_IPIPE and CONFIG_KGDB are both enabled in include/linux/smp.h.
This approach was actually used during the old Adeos times when pipeline
domain had their own separate stack. I take for granted that the CPU
penalty taken in doing this is perfectly acceptable, since well, we are
debugging after all.


There is only one drawback: we will not be able to debug
smp_processor_id-related bugs in ipipe/Xenomai anymore...


Good point. Here is an updated patch.



Nope that's not the point I meant. I was referring to bugs like the
missing smp_processor_id patch in asm-i386/mmu_context.h. Your way makes
such problems disappear once you switch on the debugger. Remember that
we spotted the mmu_context issue via kgdb.


Got it now. Indeed, making such kind of bugs disappear would be a very undesirable side-effect of enabling KGDB.

The attached kgdb-ipipe patch (which also updates to kgdb-CVS head)
addresses maintenance and smp-safeness by redefining the involved
services only per source file, not kernel-wide. What's do you think
about this approach?


Mixed feelings. On one hand, it locally fixes the issue without masking any bugs and that's good. On the other hand, I've been bitten using this kind of tricks in earlier Adeos releases for patching printk() the same way, substituting it with spin_lock_irqsave_hw. At some point, spin_lock_irqsave became local_irq_save + spin_lock in the vanilla code, making the substitution of the former pointless, and opening a preemption hole in the code. This said, no spinlock macros are involved in the substitution you are suggesting, so there is no such risk, and in any case, the risk is confined to one file and not spread all over the place like with the raw_smp_processor_id() substitution. IOW, let's go your way.

While we are at it,
#define current ipipe_safe_current() /* ? */

I'm also posting two fixes for ipipe itself to 1) make kgdb work over
SMP and 2) fix a compiler warning.

This version works fine for me, at least in qemu. I tried to check SMP
as well, but qemu obviously lacks support for NMIs raised via IPI, and
this is used by kgdb to sync all CPUs on debugging events (vanilla kgdb
suffers too).


--

Philippe.

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