linux-kernel - Re: [patch-rt] drivers/zram: fix zcomp_stream_get() smp_processor

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Date:   Thu, 31 Aug 2017 21:26:38 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Mike Galbraith <efault@....de>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [patch-rt] drivers/zram: fix zcomp_stream_get()
 smp_processor_id() use in preemptible code

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@....de>
> > ---
> >  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.

> Thanks,
> 
> 	tglx

Sebastian