Re: oprofile can cause an NMI to schedule (was: [RT] scheduling and oprofile)

John Levon
2006-10-24T12:46:49Z

On Mon, Oct 23, 2006 at 10:24:34PM -0400, Steven Rostedt wrote:

> > caller is rt_mutex_slowlock+0x156/0x1dd
> > [<c032051a>] schedule+0x65/0xd2 (8)
> > [<c0321338>] rt_mutex_slowlock+0x156/0x1dd (12)
> > [<c032142a>] rt_mutex_lock+0x24/0x28 (72)
> > [<c0134904>] rt_down_read+0x38/0x3b (20)
> > [<c0322a89>] do_page_fault+0xe3/0x52d (12)
> > [<c03229a6>] do_page_fault+0x0/0x52d (76)
> > [<c01033bb>] error_code+0x4f/0x54 (8)
> > [<c01ce6d0>] __copy_from_user_ll+0x55/0x7c (44)
> > [<f89be7ef>] dump_user_backtrace+0x2e/0x56 [oprofile] (24)
> > [<c0134869>] rt_up_read+0x3e/0x41 (20)
> > [<f89be864>] x86_backtrace+0x4a/0x5a [oprofile] (20)
> > [<f89bd53a>] oprofile_add_sample+0x73/0x89 [oprofile] (20)
> > [<f89beea3>] athlon_check_ctrs+0x22/0x4a [oprofile] (32)
> > [<f89be8c5>] nmi_callback+0x18/0x1b [oprofile] (28)
> > [<c01041ff>] do_nmi+0x24/0x33 (12)
> > [<c0103462>] nmi_stack_correct+0x1d/0x22 (16)
> >
> > It seems strange to me that oprofile would be calling
> > '__copy_from_user_ll' in this context. I can see why the
> > changes made for RT locking expose this. But, doesn't this
> > issue also exist on non-RT (default) kernels? What happens
> > when we generate a page fault in this context on non-RT kernels?
> >
>
> As Mike has pointed out here, oprofile _can_ cause the nmi to schedule.

in_atomic() is supposed to be true in this context, so the test in
do_page_fault() catches it.

john