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SubjectRe: [PATCH 2/5] perf core: Set event's default overflow_handler
On Wed, Mar 23, 2016 at 08:29:38PM +0100, Peter Zijlstra wrote:
> On Wed, Mar 23, 2016 at 06:13:49PM +0000, Will Deacon wrote:
> > 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.
>
> Would it, perhaps, make sense to invert this test and check for
> ->overflow_handler == ptrace_hbptriggered instead? That way nobody gets
> surprise live-locks, endlessly triggering the same trap.

Not sure... I can imagine kgdb, for example, wanting to handle the stepping
itself. You also need to play clever tricks if you want to step through
LL/SC atomics, which the code here doesn't even try to handle (because
it involves disassembling the instructions and applying a bunch of
heuristics), so I imagine most debuggers wanting to take care of the step
themselves.

> But yes, this kinda blows.

Yup.

Will

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