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

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Date:   Thu, 31 Aug 2017 21:11:08 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Mike Galbraith <efault@....de>
cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.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 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.

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