Re: [PATCH] net: xfrm: use preempt-safe this_cpu_read() in ipcomp_alloc_tfms()

[Eric Dumazet]

On 03/07/2018 11:33 PM, Herbert Xu wrote:
> On Wed, Mar 07, 2018 at 11:24:16AM -0800, Greg Hackmann wrote:
> > f7c83bcbfaf5 ("net: xfrm: use __this_cpu_read per-cpu helper") added a
> > __this_cpu_read() call inside ipcomp_alloc_tfms().  Since this call was
> > introduced, the rules around per-cpu accessors have been tightened and
> > __this_cpu_read() cannot be used in a preemptible context.
> >
> > syzkaller reported this leading to the following kernel BUG while
> > fuzzing sendmsg:
>
> How about reverting f7c83bcbfaf5 instead?
>
> Thanks,

Because that would not be nice.

this_cpu_read() is faster than going through raw_smp_processor_id() and 
per_cpu_ptr()