Re: [PATCH] powerpc: Fix hcall tracepoint recursion

[Subrata Modak <subrata@linux.vnet.ibm.com>]

On Fri, 2010-10-22 at 15:22 +0800, Li Zefan wrote:
> Anton Blanchard wrote:
> > Hi,
> > 
> >> This is a dead loop:
> >>
> >> trace_hcall_entry() -> trace_clock_global() -> trace_hcall_entry() ..
> >>
> >> And this is a PPC specific bug. Hope some ppc guys will fix it?
> >> Or we kill trace_clock_global() if no one actually uses it..
> > 
> > Nasty! How does the patch below look? I had to disable irqs otherwise
> > we would sometimes drop valid events (if we take an interrupt anywhere
> > in the region where depth is elevated, then the entire interrupt will
> > be blocked from calling hcall tracepoints.
> > 
> 
> Thanks!
> 
> Subrata, could you test the patch below?

Yes, definitely. Givmme some time.

Regards--
Subrata

> 
> > Anton
> > --
> > 
> > Subject: [PATCH] powerpc: Fix hcall tracepoint recursion
> > 
> > Spinlocks on shared processor partitions use H_YIELD to notify the
> > hypervisor we are waiting on another virtual CPU. Unfortunately this means
> > the hcall tracepoints can recurse.
> > 
> > The patch below adds a percpu depth and checks it on both the entry and
> > exit hcall tracepoints.
> > 
> > Signed-off-by: Anton Blanchard <anton@samba.org>
> > ---
> > 
> > Index: powerpc.git/arch/powerpc/platforms/pseries/lpar.c
> > ===================================================================
> > --- powerpc.git.orig/arch/powerpc/platforms/pseries/lpar.c	2010-10-21 17:32:00.980003644 +1100
> > +++ powerpc.git/arch/powerpc/platforms/pseries/lpar.c	2010-10-21 17:34:54.942681273 +1100
> > @@ -701,6 +701,13 @@ EXPORT_SYMBOL(arch_free_page);
> >  /* NB: reg/unreg are called while guarded with the tracepoints_mutex */
> >  extern long hcall_tracepoint_refcount;
> >  
> > +/* 
> > + * Since the tracing code might execute hcalls we need to guard against
> > + * recursion. One example of this are spinlocks calling H_YIELD on
> > + * shared processor partitions.
> > + */
> > +static DEFINE_PER_CPU(unsigned int, hcall_trace_depth);
> > +
> >  void hcall_tracepoint_regfunc(void)
> >  {
> >  	hcall_tracepoint_refcount++;
> > @@ -713,12 +720,42 @@ void hcall_tracepoint_unregfunc(void)
> >  
> >  void __trace_hcall_entry(unsigned long opcode, unsigned long *args)
> >  {
> > +	unsigned long flags;
> > +	unsigned int *depth;
> > +
> > +	local_irq_save(flags);
> > +
> > +	depth = &__get_cpu_var(hcall_trace_depth);
> > +
> > +	if (*depth)
> > +		goto out;
> > +
> > +	(*depth)++;
> >  	trace_hcall_entry(opcode, args);
> > +	(*depth)--;
> > +
> > +out:
> > +	local_irq_restore(flags);
> >  }
> >  
> >  void __trace_hcall_exit(long opcode, unsigned long retval,
> >  			unsigned long *retbuf)
> >  {
> > +	unsigned long flags;
> > +	unsigned int *depth;
> > +
> > +	local_irq_save(flags);
> > +
> > +	depth = &__get_cpu_var(hcall_trace_depth);
> > +
> > +	if (*depth)
> > +		goto out;
> > +
> > +	(*depth)++;
> >  	trace_hcall_exit(opcode, retval, retbuf);
> > +	(*depth)--;
> > +
> > +out:
> > +	local_irq_restore(flags);
> >  }
> >  #endif
> > 
> >