On Mon, 6 May 2019, Josh Poimboeuf wrote:
> On Tue, Apr 30, 2019 at 11:10:48AM +0200, Petr Mladek wrote:
> > WARN_ON_ONCE() could not be called safely under rq lock because
> > of console deadlock issues. Fortunately, there is another check
> > for the reliable stacktrace support in klp_enable_patch().
> >
> > Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
> > ---
> > kernel/livepatch/transition.c | 9 ++++++++-
> > 1 file changed, 8 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c
> > index 9c89ae8b337a..8e0274075e75 100644
> > --- a/kernel/livepatch/transition.c
> > +++ b/kernel/livepatch/transition.c
> > @@ -263,8 +263,15 @@ static int klp_check_stack(struct task_struct *task, char *err_buf)
> > trace.nr_entries = 0;
> > trace.max_entries = MAX_STACK_ENTRIES;
> > trace.entries = entries;
> > +
> > ret = save_stack_trace_tsk_reliable(task, &trace);
> > - WARN_ON_ONCE(ret == -ENOSYS);
> > + /*
> > + * pr_warn() under task rq lock might cause a deadlock.
> > + * Fortunately, missing reliable stacktrace support has
> > + * already been handled when the livepatch was enabled.
> > + */
> > + if (ret == -ENOSYS)
> > + return ret;
>
> I find the comment to be a bit wordy and confusing (and vague).
>
> Also this check is effectively the same as the klp_have_reliable_stack()
> check which is done in kernel/livepatch/core.c. So I think it would be
> clearer and more consistent if the same check is done here:
>
> if (!klp_have_reliable_stack())
> return -ENOSYS;
We removed it in 1d98a69e5cef ("livepatch: Remove reliable stacktrace
check in klp_try_switch_task()") and I do think it does not belong here.
We can check for the facility right at the beginning in klp_enable_patch()
and it is not necessary to do it every time klp_check_stack() is called.
But it is nothing I could not live with, so if you decide it is better, I
will not object.
Miroslav
