Re: [PATCH v9 11/12] x86, mpx: cleanup unused bound tables

[Thomas Gleixner]

On Tue, 4 Nov 2014, Dave Hansen wrote:
> On 11/03/2014 01:29 PM, Thomas Gleixner wrote:
> > On Mon, 3 Nov 2014, Dave Hansen wrote:
> 
> > That's not really true. You can evaluate that information with
> > mmap_sem held for read as well. Nothing can change the mappings until
> > you drop it. So you could do:
> > 
> >    down_write(mm->bd_sem);
> >    down_read(mm->mmap_sem;
> >    evaluate_size_of_shm_to_unmap();
> >    clear_bounds_directory_entries();
> >    up_read(mm->mmap_sem);
> >    do_the_real_shm_unmap();
> >    up_write(mm->bd_sem);
> > 
> > That should still be covered by the above scheme.
> 
> Yep, that'll work.  It just means rewriting the shmdt()/mremap() code to
> do a "dry run" of sorts.

Right. So either that or we hold bd_sem write locked accross all write
locked mmap_sem sections. Dunno, which solution is prettier :)

> Do you have any concerns about adding another mutex to these paths?

You mean bd_sem? I don't think its an issue. You need to get mmap_sem
for write as well. So 

> munmap() isn't as hot of a path as the allocation side, but it does
> worry me a bit that we're going to perturb some workloads.  We might
> need to find a way to optimize out the bd_sem activity on processes that
> never used MPX.

I think using mm->bd_addr as a conditional for the bd_sem/mpx activity
is good enough. You just need to make sure that you store the result
of the starting conditional and use it for the closing one as well.

   mpx = mpx_pre_unmap(mm);
       {
	  if (!kernel_managing_bounds_tables(mm)
       	     return 0;
	  down_write(mm->bd_sem);
	  ...
	  return 1;
       }

   unmap();

   mxp_post_unmap(mm, mpx);
       {
          if (mpx) {
	     ....
	     up_write(mm->bd_sem);
       }

So this serializes nicely with the bd_sem protected write to
mm->bd_addr. There is a race there, but I don't think it matters. The
worst thing what can happen is a stale bound table.

Thanks,

	tglx
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