Re: [PATCH] perf/core: fast breakpoint modification via _IOC_MODIFY_BREAKPOINT

[Jiri Olsa <jolsa@redhat.com>]

On Thu, Nov 09, 2017 at 08:46:58AM +0100, Jiri Olsa wrote:

SNIP

> > Jirka,
> > 
> > I carefully looked at bp_cpuinfo[] and nr_slots[] data structures.
> > nr_slots[] is an array of length two (one slot of TYPE_INST and
> > another for TYPE_DATA).
> > The accounting "thinks" that there is one limit on the number of
> > instruction breakpoints and another limit on the number of data
> > breakpoints.
> > The assumption is clearly broken; for example, on x86 there exists a
> > limit on the *total* number of all breakpoints disregarding their kind
> > and the code has failed to capture this aspect.
> 
> there's the CONFIG_HAVE_MIXED_BREAKPOINTS_REGS that puts DATA and INST
> under one count on x86.. but that seems to be the enabled only for:
> 
> 	arch/sh/Kconfig:        select HAVE_MIXED_BREAKPOINTS_REGS
> 	arch/x86/Kconfig:       select HAVE_MIXED_BREAKPOINTS_REGS
> 
> > 
> > As such, modify_user_hw_breakpoint() makes no attempt to keep the
> > counts correct. Instead, it simply tries to change and install a new
> > breakpoint and fails if the hardware disallows.
> > This can lead to a situation where, say on x86, someone creates 4
> > TYPE_DATA breakpoints, then changes one of them to TYPE_INS via
> > modify_user_hw_breakpoint() and then releases the TYPE_INS breakpoint.
> > Since the accounting still thinks that there are four TYPE_DATA
> > breakpoints, it will disallow creating a new TYPE_DATA breakpoint,
> > although there is place for one TYPE_DATA breakpoint.
> > 
> > This convinces me that the problem and the solution are outside of
> > this current patch.
> > Do you agree?
> 
> I'll leave this decision to maintainer ;-) but seems better to fix
> the interface before we add any new dependent function calls

how about something like below (untested)

looks like there's no irq caller for modify_user_hw_breakpoint,
so we should be fine with locking nr_bp_mutex

jirka


---
diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
index 3f8cb1e14588..f062b68399ea 100644
--- a/kernel/events/hw_breakpoint.c
+++ b/kernel/events/hw_breakpoint.c
@@ -448,6 +448,8 @@ int modify_user_hw_breakpoint(struct perf_event *bp, struct perf_event_attr *att
 	else
 		perf_event_disable(bp);
 
+	release_bp_slot(bp);
+
 	bp->attr.bp_addr = attr->bp_addr;
 	bp->attr.bp_type = attr->bp_type;
 	bp->attr.bp_len = attr->bp_len;
@@ -455,9 +457,9 @@ int modify_user_hw_breakpoint(struct perf_event *bp, struct perf_event_attr *att
 	if (attr->disabled)
 		goto end;
 
-	err = validate_hw_breakpoint(bp);
+	err = reserve_bp_slot(bp);
 	if (!err)
-		perf_event_enable(bp);
+		err = validate_hw_breakpoint(bp);
 
 	if (err) {
 		bp->attr.bp_addr = old_addr;
@@ -469,6 +471,7 @@ int modify_user_hw_breakpoint(struct perf_event *bp, struct perf_event_attr *att
 		return err;
 	}
 
+	perf_event_enable(bp);
 end:
 	bp->attr.disabled = attr->disabled;