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

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Message-ID: <alpine.DEB.2.00.0903211235390.13615@gandalf.stny.rr.com>
Date:	Sat, 21 Mar 2009 12:39:08 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	"K.Prasad" <prasad@...ux.vnet.ibm.com>
cc:	Frederic Weisbecker <fweisbec@...il.com>,
	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>
Subject: Re: [Patch 11/11] ftrace plugin for kernel symbol tracing using HW
 Breakpoint interfaces - v2


On Sat, 21 Mar 2009, K.Prasad wrote:
> > > 
> 
> > > +
> > > +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?

Can that lock ever be taken in an interrupt? If not, document that (and 
perhaps add a WARN_ON(in_interrupt()); ). Otherwise you have a possible:

	spin_lock(&ksym_stat_lock);

		===> take interrupt ...

			(from interrupt)
			spin_lock(&ksym_stat_lock); <== deadlock.


-- Steve

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