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

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  Re: [patch-rt] drivers/zram: fix zcomp_stream_get() smp_processor_id()  use in preemptible code Thomas Gleixner <tglx@linutronix.de> - 2017-08-31 21:20 +0200
    Re: [patch-rt] drivers/zram: fix zcomp_stream_get()  smp_processor_id() use in preemptible code Sebastian Andrzej Siewior <bigeasy@linutronix.de> - 2017-08-31 21:30 +0200
      Re: [patch-rt] drivers/zram: fix zcomp_stream_get() smp_processor_id()  use in preemptible code Thomas Gleixner <tglx@linutronix.de> - 2017-08-31 21:40 +0200
        Re: [patch-rt] drivers/zram: fix zcomp_stream_get() smp_processor_id()  use in preemptible code Thomas Gleixner <tglx@linutronix.de> - 2017-09-01 09:50 +0200
        Re: [patch-rt] drivers/zram: fix zcomp_stream_get()  smp_processor_id() use in preemptible code Sebastian Andrzej Siewior <bigeasy@linutronix.de> - 2017-09-01 09:50 +0200

#1724508 — Re: [patch-rt] drivers/zram: fix zcomp_stream_get() smp_processor_id() use in preemptible code

On Wed, 23 Aug 2017, Mike Galbraith wrote:

> 
> Use get_local_ptr() vs this_cpu_ptr().
> 
> Signed-off-by: Mike Galbraith <efault@gmx.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.

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.

Thanks,

	tglx

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#1724519 — 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@gmx.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

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#1724520

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@gmx.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.

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

Thanks,

	tglx

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#1724841

On Fri, 1 Sep 2017, Sebastian Andrzej Siewior wrote:

> On 2017-08-31 21:32:30 [+0200], Thomas Gleixner wrote:
> > > 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.
> 
> zram_decompress_page() invokes zcomp_stream_get() within a spin_lock
> section (zram_lock_table()). This is fine.
> zram_bvec_write() does not invoke zcomp_stream_get() within a spin_lock
> section.

Fair enough.

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#1724844 — Re: [patch-rt] drivers/zram: fix zcomp_stream_get() smp_processor_id() use in preemptible code

On 2017-08-31 21:32:30 [+0200], Thomas Gleixner wrote:
> > 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.

zram_decompress_page() invokes zcomp_stream_get() within a spin_lock
section (zram_lock_table()). This is fine.
zram_bvec_write() does not invoke zcomp_stream_get() within a spin_lock
section.

> Thanks,
> 
> 	tglx

Sebastian

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