Messages in this thread Patch in this message | | | Subject | Re: hw_breakpoint: Fix Oops at destroying hw_breakpoint event on powerpc | From | Michael Ellerman <> | Date | Thu, 03 Mar 2016 20:23:38 +1100 |
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On Wed, 2016-03-02 at 12:59 +0100, Peter Zijlstra wrote:
> On Wed, Mar 02, 2016 at 10:53:24PM +1100, Michael Ellerman wrote:
> > Peterz, acme, do you guys want to take this? Or should I? > > I'm not too happy its touching event->ctx at all. It really should not > be doing that.
Hmm OK.
It's been using ctx->task since it was merged in 2010. In fact that commit also added arch_unregister_hw_breakpoint(), and we're still the only user of that.
The prima facie reason it's using ctx is to get at task->thread to clear last_hit_ubp.
It looks like other arches avoid needing to do something similar by storing the break point in a per-cpu array. Which I guess is what you meant in your other mail ("Why do you keep per task state anyway?").
I can't think of a reason why we can't also store it per-cpu, but I could be wrong, I don't know the code well and I haven't thought about it for very long.
Do you mind if I merge the following fix for now as a band-aid, and we'll try and fix it up properly in the next few weeks (but maybe not in time for 4.5 final).
cheers
diff --git a/arch/powerpc/kernel/hw_breakpoint.c b/arch/powerpc/kernel/hw_breakpoint.c index 05e804cdecaa..aec9a1b1d25b 100644 --- a/arch/powerpc/kernel/hw_breakpoint.c +++ b/arch/powerpc/kernel/hw_breakpoint.c @@ -109,8 +109,9 @@ void arch_unregister_hw_breakpoint(struct perf_event *bp) * If the breakpoint is unregistered between a hw_breakpoint_handler() * and the single_step_dabr_instruction(), then cleanup the breakpoint * restoration variables to prevent dangling pointers. + * FIXME, this should not be using bp->ctx at all! Sayeth peterz. */ - if (bp->ctx && bp->ctx->task) + if (bp->ctx && bp->ctx->task && bp->ctx->task != ((void *)-1L)) bp->ctx->task->thread.last_hit_ubp = NULL; }
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