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?
Anyway, I have pulled this, but I really would like fewer of these
random PREEMPT_RT turds around, and more "this code makes sense" code.
Linus
