Messages in this thread | | | Date | Thu, 24 Mar 2016 09:58:02 +0000 | From | Will Deacon <> | Subject | Re: [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
| |