linux-kernel - Re: [GIT PULL] tracing: use raw spinlocks for trace

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Date:	Wed, 11 Mar 2009 15:04:54 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [GIT PULL] tracing: use raw spinlocks for trace_vprintk

On Wed, Mar 11, 2009 at 10:00:51AM -0400, Steven Rostedt wrote:
> 
> On Wed, 11 Mar 2009, Frederic Weisbecker wrote:
> 
> > On Wed, Mar 11, 2009 at 07:59:24AM +0100, Peter Zijlstra wrote:
> > > On Tue, 2009-03-10 at 21:26 -0400, Steven Rostedt wrote:
> > > 
> > > > commit 80370cb758e7ca2692cd9fb5e413d970b1f4b2b2
> > > > Author: Steven Rostedt <srostedt@...hat.com>
> > > > Date:   Tue Mar 10 17:16:35 2009 -0400
> > > > 
> > > >     tracing: use raw spinlocks for trace_vprintk
> > > >     
> > > >     Impact: prevent locking up by lockdep tracer
> > > >     
> > > >     The lockdep tracer uses trace_vprintk and thus trace_vprintk can not
> > > >     call back into lockdep without locking up.
> > > 
> > > Hmm, I did this when I posted the lockdep tracepoints, so someone then
> > > did a bad copy/paste job when renaming ftrace_printk or something?
> > > 
> > > See efed792d6738964f399a508ef9e831cd60fa4657
> > 
> > 
> > 
> > Must be my bad :-s
> > I think I lost this modification that was done on the old trace_vprintf
> > between two iterations of the bprintk patchset.
> > 
> > BTW, Ingo reported one or two monthes ago that ftrace_printk was not NMI safe
> > because of this spinlock.
> > 
> > He suggested to drop the spinlock and then make trace_buf per_cpu.
> > 
> > By disabling the irq we prevent from race with maskable irqs. And in
> > case of racy accesses to trace_buf because of an nmi, then the buffer
> > might be mixed up but it must be harmless compared to a hardlockup that
> > can occur now. On the worst case, the trace will be weird and that's it.
> 
> But the lock is only used in this function, and the function can not 
> recurse. It is NMI safe. see below.
> 
> > 
> > Frederic.
> > 
> > 
> > > >     Signed-off-by: Steven Rostedt <srostedt@...hat.com>
> > > > 
> > > > diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> > > > index 8c6a902..4c97947 100644
> > > > --- a/kernel/trace/trace.c
> > > > +++ b/kernel/trace/trace.c
> > > > @@ -1176,7 +1176,8 @@ void trace_graph_return(struct ftrace_graph_ret *trace)
> > > >   */
> > > >  int trace_vprintk(unsigned long ip, int depth, const char *fmt, va_list args)
> > > >  {
> > > > -	static DEFINE_SPINLOCK(trace_buf_lock);
> > > > +	static raw_spinlock_t trace_buf_lock =
> > > > +		(raw_spinlock_t)__RAW_SPIN_LOCK_UNLOCKED;
> > > >  	static u32 trace_buf[TRACE_BUF_SIZE];
> > > >  
> > > >  	struct ring_buffer_event *event;
> > > > @@ -1201,7 +1202,9 @@ int trace_vprintk(unsigned long ip, int depth, const char *fmt, va_list args)
> > > >  	if (unlikely(atomic_read(&data->disabled)))
> > > >  		goto out;
> 
> The above disable is exactly for NMIs. We should have preemption disabled 
> here, and we disable this per cpu. If an NMI comes in after this point, it 
> will exit the function without taking the lock. If it runs on another CPU, 
> we really don't care. That's what NMIs are for ;-)


Aaah, ok :-)

 
> -- Steve
> 
> 
> 
> > > >  
> > > > -	spin_lock_irqsave(&trace_buf_lock, flags);
> > > > +	/* Lockdep uses trace_printk for lock tracing */
> > > > +	local_irq_save(flags);
> > > 
> > > Shouldn't you also use raw_local_irq_save() and friends?
> > > 
> > > > +	__raw_spin_lock(&trace_buf_lock);
> > > >  	len = vbin_printf(trace_buf, TRACE_BUF_SIZE, fmt, args);
> > > >  
> > > >  	if (len > TRACE_BUF_SIZE || len < 0)
> > > > @@ -1220,7 +1223,8 @@ int trace_vprintk(unsigned long ip, int depth, const char *fmt, va_list args)
> > > >  	ring_buffer_unlock_commit(tr->buffer, event);
> > > >  
> > > >  out_unlock:
> > > > -	spin_unlock_irqrestore(&trace_buf_lock, flags);
> > > > +	__raw_spin_unlock(&trace_buf_lock);
> > > > +	local_irq_restore(flags);
> > > >  
> > > >  out:
> > > >  	ftrace_preempt_enable(resched);
> > > > 
> > > --
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> > > Please read the FAQ at  http://www.tux.org/lkml/
> > 
> > 

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