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



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