Re: [PATCH 2/5] perf core: Set event's default overflow_handler

[Will Deacon]

On Wed, Mar 23, 2016 at 06:50:21PM +0100, Peter Zijlstra wrote:
> On Mon, Mar 14, 2016 at 09:59:42AM +0000, Wang Nan wrote:
> > +++ b/arch/arm/kernel/hw_breakpoint.c
> > @@ -631,7 +631,7 @@ int arch_validate_hwbkpt_settings(struct perf_event *bp)
> >  	info->address &= ~alignment_mask;
> >  	info->ctrl.len <<= offset;
> >  
> > -	if (!bp->overflow_handler) {
> > +	if (is_default_overflow_handler(bp)) {
> >  		/*
> >  		 * Mismatch breakpoints are required for single-stepping
> >  		 * breakpoints.
> > @@ -754,7 +754,7 @@ static void watchpoint_handler(unsigned long addr, unsigned int fsr,
> >  		 * mismatch breakpoint so we can single-step over the
> >  		 * watchpoint trigger.
> >  		 */
> > -		if (!wp->overflow_handler)
> > +		if (is_default_overflow_handler(wp))
> >  			enable_single_step(wp, instruction_pointer(regs));
> >  
> >  unlock:
> > diff --git a/arch/arm64/kernel/hw_breakpoint.c b/arch/arm64/kernel/hw_breakpoint.c
> > index b45c95d..4ef5373 100644
> > --- a/arch/arm64/kernel/hw_breakpoint.c
> > +++ b/arch/arm64/kernel/hw_breakpoint.c
> > @@ -616,7 +616,7 @@ static int breakpoint_handler(unsigned long unused, unsigned int esr,
> >  		perf_bp_event(bp, regs);
> >  
> >  		/* Do we need to handle the stepping? */
> > -		if (!bp->overflow_handler)
> > +		if (is_default_overflow_handler(bp))
> >  			step = 1;
> >  unlock:
> >  		rcu_read_unlock();
> > @@ -712,7 +712,7 @@ static int watchpoint_handler(unsigned long addr, unsigned int esr,
> >  		perf_bp_event(wp, regs);
> >  
> >  		/* Do we need to handle the stepping? */
> > -		if (!wp->overflow_handler)
> > +		if (is_default_overflow_handler(wp))
> >  			step = 1;
> >  
> >  unlock:
> 
> Will, why does it matter what the overflow handler is for this stuff?

Because ptrace registers an overflow handler for raising a SIGTRAP and
ptrace users (e.g. GDB) expect to handle the single-stepping themselves.
Perf, on the other hand, will livelock if the kernel doesn't do the
stepping.

FWIW, I hate this whole thing. The only users of the perf side just seem
to be people running whacky test cases and then pointing out the places
where we're not identical to x86 :(

Will