Re: [Patch 11/11] ftrace plugin for kernel symbol tracing using HWBreakpoint interfaces - v2

[Steven Rostedt]

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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