Re: [patch-rt] drivers/zram: fix zcomp_stream_get() smp_processor_id() use in preemptible code

[Thomas Gleixner]

On Thu, 31 Aug 2017, Sebastian Andrzej Siewior wrote:

> On 2017-08-31 21:11:08 [+0200], Thomas Gleixner wrote:
> > On Wed, 23 Aug 2017, Mike Galbraith wrote:
> > 
> > > 
> > > Use get_local_ptr() vs this_cpu_ptr().
> > > 
> > > Signed-off-by: Mike Galbraith <efault@xxxxxx>
> > > ---
> > >  drivers/block/zram/zcomp.c |    3 ++-
> > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > > 
> > > --- a/drivers/block/zram/zcomp.c
> > > +++ b/drivers/block/zram/zcomp.c
> > > @@ -120,7 +120,7 @@ struct zcomp_strm *zcomp_stream_get(stru
> > >  {
> > >  	struct zcomp_strm *zstrm;
> > >  
> > > -	zstrm = *this_cpu_ptr(comp->stream);
> > > +	zstrm = *get_local_ptr(comp->stream);
> > 
> > This looks wrong. On mainline the calling code must have preemption disable
> > somehow, otherwise this_cpu_ptr() would not work.
> 
> This was introduced by Mike in a previous patch. The zstrm is only
> accessed while the spinlock is held.
> 
> > Looking at the call site it is;
> > 
> > 	zram_slot_lock()
> > 	  bit_spin_lock()
> > 
> > which is of course evading lockdep and everything else debugging wise.
> > 
> > Sebastian, do we have migration protection in bitlocked regions? And we
> > shpuld look into converting that into a spinlock on rt.
> 
> zram_lock_table() is bit_spin_lock() on !RT and spin_lock(&table->lock);
> on RT. So this is done.
> !RT has this running in a kmap_atomic() section so they have no
> preemption there.
> zcomp_stream_get() returns a per-CPU object which is protected with a
> spinlock and only accessed locked.

So when we are inside a spinlocked section, why is this_cpu_ptr() not
working? That does not make sense.

Thanks,

	tglx