Re: [PATCH v2 3/7] regulator/core: regulator_lock_nested: simplify nested locking

[Michał Mirosław]

On Wed, Aug 30, 2023 at 01:54:21PM -0700, Doug Anderson wrote:
> On Wed, Aug 30, 2023 at 10:35 AM Michał Mirosław
> <mirq-linux@xxxxxxxxxxxx> wrote:
> >
> > Simplify regulator locking by removing locking around locking.
> > rdev->ref check when unlocking is moved inside the critical section.
> >
> > This patch depends on commit 12235da8c80a ("kernel/locking: Add context
> > to ww_mutex_trylock()").
> >
> > Note: return -EALREADY is removed as no caller depends on it and in that
> > case the lock count is incremented anyway.
> >
> > Reviewed-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> > Signed-off-by: Michał Mirosław <mirq-linux@xxxxxxxxxxxx>
> > ---
> >  drivers/regulator/core.c | 23 ++++++-----------------
> >  1 file changed, 6 insertions(+), 17 deletions(-)
> 
> Note that I didn't actually provide a Reviewed-by on this patch in v1.
> I was hoping for something in the commit message that explained why
> commit 12235da8c80a ("kernel/locking: Add context to
> ww_mutex_trylock()") meant that we didn't need the extra lock. You
> responded to the v1, but didn't add anything to the commit message
> about it.
> 
> Looking at your response to v1, I'm not sure it helps enlighten me on
> why adding the context removed the need for the extra lock. Can you
> add more words? Pretend I don't know anything about ww_mutex, which is
> not far from the truth since every time I look at ww_mutex I have to
> re-learn how it works. :-P Specifically, what would actually have been

Thanks for all your (and Stephen's) questions and comments! I had a bit
more of thinking and reading time about the W/W mutex and how it works.
It turns out I can remove some noise from this commit.

The commit 12235da8c80a dependency is due to text changes: the original
code would need a bit of reordering, all not much different than the
two previous patches.

If ww_mutex_lock() was able to return -EALREADY with NULL ww_ctx,
regulator_lock_nested() could be made even simpler.

Best Regards,
Michał Mirosław