On Wed, Aug 03, 2022 at 11:57:27AM -0700, Linus Torvalds wrote:
> On Wed, Aug 3, 2022 at 11:39 AM Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> >
> > Main part here is making parallel lookups safe for RT - making
> > sure preemption is disabled in start_dir_add()/ end_dir_add() sections (on
> > non-RT it's automatic, on RT it needs to to be done explicitly) and moving
> > wakeups from __d_lookup_done() inside of such to the end of those sections.
>
> Ugh.
>
> I really dislike this pattern:
>
> if (IS_ENABLED(CONFIG_PREEMPT_RT))
> preempt_disable();
> ...
> if (IS_ENABLED(CONFIG_PREEMPT_RT))
> preempt_enable();
>
> and while the new comment explains *why* it exists, it's still very ugly indeed.
>
> We have it in a couple of other places, and we also end up having
> another variation on the theme that is about "migrate_{dis,en}able()",
> except it is written as
>
> if (IS_ENABLED(CONFIG_PREEMPT_RT))
> migrate_disable();
> else
> preempt_disable();
>
> because on non-PREEMPT_RT obviously preempt_disable() is the better
> and simpler thing.
>
> Can we please just introduce helper functions?
>
> At least that
>
> if (IS_ENABLED(CONFIG_PREEMPT_RT))
> preempt_disable();
> ...
>
> pattern could be much more naturally expressed as
>
> preempt_disable_under_spinlock();
> ...
>
> which would make the code really explain what is going on. I would
> still encourage that *comment* about it, but I think we really should
> strive for code that makes sense even without a comment.
>
> The fact that then without PREEMPT_RT, the whole
> "preempt_disable_under_spinlock()" becomes a no-op is then an
> implementation detail - and not so different from how a regular
> preempt_disable() becomes a no-op when on UP (or with PREEMPT_NONE).
>
> And that "preempt_disable_under_spinlock()" really documents what is
> going on, and I feel would make that code easier to understand? The
> fact that PREEMPT_RT has different rules about preemption is not
> something that the dentry code should care about.
>
> The dentry code could just say "I want to disable preemption, and I
> already hold a spinlock, so do what is best".
>
> So then "preempt_disable_under_spinlock()" precisely documents what
> the dentry code really wants.
>
> No?
Fine by me, but I think that this is better dealt with by the rt folks;
I've no objections to replacing that open-coded stuff in dcache.c with
better documented primitives, so when such patches materialize...
