Re: [PATCH 2/6] uprobes: introduce is_swbp_at_addr_fast()

[Oleg Nesterov <oleg@redhat.com>]

On 04/16, Peter Zijlstra wrote:
>
> On Fri, 2012-04-06 at 00:21 +0200, Oleg Nesterov wrote:
> > +int __weak is_swbp_at_addr_fast(unsigned long vaddr)
> > +{
> > +       uprobe_opcode_t opcode;
> > +       int fault;
> > +
> > +       pagefault_disable();
> > +       fault = __copy_from_user_inatomic(&opcode, (void __user*)vaddr,
> > +                                                       sizeof(opcode));
> > +       pagefault_enable();
> > +
> > +       if (unlikely(fault)) {
> > +               /*
> > +                * XXX: read_opcode() lacks FOLL_FORCE, it can fail if
> > +                * we race with another thread which does mprotect(NONE)
> > +                * after we hit bp.
> > +                */
> > +               if (read_opcode(current->mm, vaddr, &opcode))
> > +                       return -EFAULT;
> > +       }
> > +
> > +       return is_swbp_insn(&opcode);
> > +}
>
> Why bother with the pagefault_disable() and unlikely fault case and not
> simply do copy_from_user() and have it deal with the fault if its needed
> anyway?

But we can't do this under down_read(mmap_sem) ?

If another thread waits for down_write() then do_page_fault() can't take
this lock, right?

Oleg.