Re: IO mappings; verify_area() on SMP

["Arkadi E. Shishlov" <arkadi@it.lv>]

  Thank you for fast response. Now I know, how to deal with memory io.

On Mon, Nov 08, 1999 at 11:25:11AM -0800, Kanoj Sarcar wrote:
> > 
> >   Second question is about verify_area() safety. Many drivers contain
> >   following sequence:
> > 
> >   if ((ret = verify_area(VERIFY_WRITE, buffer, count)))
> > 	    return r;
> >   ...
> >   copy_to_user(buffer, driver_data_buf, count);
> > 
> >   Even protected by cli()/sti() pairs, why multithreaded program on
> >   SMP machine can't unmap this verified buffer between calls to
> >   verify_area() and copy_to_user()? Of course it can't be true, but
> >   maybe somebody can write two-three words about reason that prevent
> >   this situation.
> 
> In most cases, the address spaces' mmap_sem is held, which prevents
> unmap's from happening until the caller of verify_area/copy_to_user
> releases it. This is if copy_to_user takes a page fault. If there
> is no page fault, the caller probably holds the kernel_lock 
> monitor, which excludes anyone else from doing a lot of things 
> inside the kernel, including unmaps.

  Hmm... Your explanation is somewhat different from Andi Kleen wrote.
  I don't see use of mmap_sem in conjunction with drivers (only char/mem).
  If I mistaken - sorry, I will dig into kernel and investigate this.


arkadi.
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