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Date: Wed, 27 Jan 2010 17:11:51 +0100
From: Frederic Weisbecker <fweisbec@gmail.com>
To: "K.Prasad" <prasad@linux.vnet.ibm.com>
Cc: LKML <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@elte.hu>,
       Alan Stern <stern@rowland.harvard.edu>
Subject: Re: [RFC Patch 2/2][Bugfix][x86][hw-breakpoint] Fix return-code to
	notifier chain in hw_breakpoint_handler
Message-ID: <20100127161146.GC22447@nowhere>
References: <20091226175533.149765731@linux.vnet.ibm.com> <20091226182833.GC9494@in.ibm.com> <20100125221101.GE5087@nowhere> <20100127102826.GA3460@in.ibm.com>
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On Wed, Jan 27, 2010 at 03:58:26PM +0530, K.Prasad wrote:
> On Mon, Jan 25, 2010 at 11:11:04PM +0100, Frederic Weisbecker wrote:
> > Is that < TASK_SIZE an accurate check? We want support for
> > userspace breakpoints on perf tools later, and those don't want
> > signals.
> > 
> 
> Well, signal generation for user-space breakpoints happened
> unconditionally for 'historical' reasons (guess that Alan Stern's
> original patch had it that way).
> 
> We could change that into a 'ptrace-only' signal generation now.


Yeah, now that we can have multiple-purpose concurrent breakpoints,
this is necessary.


> > We do this cleanup in the beginning of the breakpoint handler:
> > 
> > 	current->thread.debugreg6 &= ~DR_TRAP_BITS;
> > 
> > And from ptrace.c:ptrace_triggered():
> > 
> > 	thread->debugreg6 |= (DR_TRAP0 << i);
> > 
> > This is called on perf_bp_event().
> > Instead of checking if this is a userspace thread, we should actually
> > check if this is a ptrace breakpoint by looking at this
> > in the end of hw_breakpoint_handler().
> > 
> > 	current->thread.debugreg6 & DR_TRAP_BITS
> > 
> > Only ptrace breakpoints require signals.
> >
> 
> Yes, this does look like a clean way to limit signals to those requests
> that are interested (I was looking at round-about ways like doing a
> lookup based on callback functions).
> 
> I will send the next version of the patch with the above changes.


Thanks.

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