linux-kernel - Re: [Patch 11/11] ftrace plugin for kernel symbol tracing using HW Breakpoint interfaces

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Date:	Sat, 21 Mar 2009 21:54:17 +0530
From:	"K.Prasad" <prasad@...ux.vnet.ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Alan Stern <stern@...land.harvard.edu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Benjamin Herrenschmidt <benh@....ibm.com>,
	Maneesh Soni <maneesh@...ibm.com>,
	Roland McGrath <roland@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [Patch 11/11] ftrace plugin for kernel symbol tracing using HW
	Breakpoint interfaces - v2

On Fri, Mar 20, 2009 at 10:04:52AM +0100, Frederic Weisbecker wrote:
> On Fri, Mar 20, 2009 at 05:20:32AM +0530, K.Prasad wrote:
> > This patch adds an ftrace plugin to detect and profile memory access over
> > kernel variables. It uses HW Breakpoint interfaces to 'watch memory
> > addresses.
> > 
> > Signed-off-by: K.Prasad <prasad@...ux.vnet.ibm.com> 
> > ---
> >  kernel/trace/Kconfig          |   21 +
> >  kernel/trace/Makefile         |    1 
> >  kernel/trace/trace.h          |   25 +
> >  kernel/trace/trace_ksym.c     |  555 ++++++++++++++++++++++++++++++++++++++++++
> >  kernel/trace/trace_selftest.c |   36 ++
> >  5 files changed, 638 insertions(+)
> > 

> > +
> > +void ksym_collect_stats(unsigned long hbkpt_hit_addr)
> > +{
> > +	struct hlist_node *node;
> > +	struct trace_ksym *entry;
> > +
> > +	spin_lock(&ksym_stat_lock);
> 
> 
> I see that can be called from ksym_hbkpt_handler which in turn
> can be called from interrupt context, right?
> You can issue a deadlock if you don't disable interrupts here.
> 
> Thanks,
> Frederic.
> 

ksym_collect_stats<--ksym_hbkpt_handler<--hw_breakpoint_handler<--do_debug
invocation happens with interrupts enabled (IF bit is set). I do find
that a few plugins in kernel/trace enclose the
trace_buffer_lock_reserve()--trace_buffer_unlock_commit() invocation
within interrupt-disabled code. Is that a requirement there?

The potential deadlock scenario you foresee isn't obvious to me. Can you
explain?

Thanks,
K.Prasad

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