Re: [PATCH 1/6] system/cpus: rename qemu_mutex_lock_iothread() to qemu_bql_lock()

[Peter Xu]

On Thu, Nov 30, 2023 at 03:43:25PM -0500, Stefan Hajnoczi wrote:
> On Thu, Nov 30, 2023 at 03:08:49PM -0500, Peter Xu wrote:
> > On Wed, Nov 29, 2023 at 04:26:20PM -0500, Stefan Hajnoczi wrote:
> > > The Big QEMU Lock (BQL) has many names and they are confusing. The
> > > actual QemuMutex variable is called qemu_global_mutex but it's commonly
> > > referred to as the BQL in discussions and some code comments. The
> > > locking APIs, however, are called qemu_mutex_lock_iothread() and
> > > qemu_mutex_unlock_iothread().
> > > 
> > > The "iothread" name is historic and comes from when the main thread was
> > > split into into KVM vcpu threads and the "iothread" (now called the main
> > > loop thread). I have contributed to the confusion myself by introducing
> > > a separate --object iothread, a separate concept unrelated to the BQL.
> > > 
> > > The "iothread" name is no longer appropriate for the BQL. Rename the
> > > locking APIs to:
> > > - void qemu_bql_lock(void)
> > > - void qemu_bql_unlock(void)
> > > - bool qemu_bql_locked(void)
> > > 
> > > There are more APIs with "iothread" in their names. Subsequent patches
> > > will rename them. There are also comments and documentation that will be
> > > updated in later patches.
> > > 
> > > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> > 
> > Acked-by: Peter Xu <peterx@redhat.com>
> > 
> > Two nickpicks:
> > 
> >   - BQL contains "QEMU" as the 2nd character, so maybe easier to further
> >     rename qemu_bql into bql_?
> 
> Philippe wondered whether the variable name should end with _mutex (or
> _lock is common too), so an alternative might be big_qemu_lock. That's

IMHO mutex isn't important in this context, but an implementation detail of
the "lock" as an abstract concept.

For example, we won't need to rename it again then if the impl changes,
e.g. using pure futex or a rwlock replacement.  When that happens we don't
need to change all call sites again.

(never really meant to change the lock impl, just an example.. :)

KVM actually has that example of KVM_MMU_LOCK() macro taking as the rwlock
write lock when the spinlock is replaced with rwlock, while it'll keep to
be the spinlock "lock()" when !KVM_HAVE_MMU_RWLOCK.

> imperfect because it doesn't start with the usual qemu_ prefix.
> qemu_big_lock is better in that regard but inconsistent with our BQL
> abbreviation.
> 
> I don't like putting an underscore at the end. It's unusual and would
> make me wonder what that means.

Ah, I meant replacing the "qemu_bql_" prefix with "bql_", as that contains
QEMU already, rather than making "_" at the end.  So they'll be bql_lock(),
bql_unlock(), bql_locked().

> 
> Naming is hard, but please discuss and I'm open to change to BQL
> variable's name to whatever we all agree on.

I'm pretty okay with qemu_bql_lock(), etc. too.  I prefer a tiny little bit
on bql_ over qemu_bql_ in this regard, but frankly they're all names good
enough to me.  The "qemu_" prefix can still be a good thing saying "this is
a qemu global function", even if contained inside "bql" itself.

> 
> > 
> >   - Could we keep the full spell of BQL at some places, so people can still
> >     reference it if not familiar?  IIUC most of the BQL helpers will root
> >     back to the major three functions (_lock, _unlock, _locked), perhaps
> >     add a comment of "BQL stands for..." over these three functions as
> >     comment?
> 
> Yes, I'll update the doc comments to say "Big QEMU Lock (BQL)" for each
> of these functions.

Thanks!

-- 
Peter Xu