linux-kernel - Re: [RFC PATCH v2 2/2] locking/percpu-rwsem: Mark rwsem as non-spinnable in percpu_rwsem

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Date:   Tue, 15 May 2018 14:45:32 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Waiman Long <longman@...hat.com>, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Davidlohr Bueso <dave@...olabs.net>,
        "Theodore Y. Ts'o" <tytso@....edu>,
        Amir Goldstein <amir73il@...il.com>, Jan Kara <jack@...e.cz>
Subject: Re: [RFC PATCH v2 2/2] locking/percpu-rwsem: Mark rwsem as
 non-spinnable in percpu_rwsem_release()

On 05/15, Peter Zijlstra wrote:
>
> > > Afaict the whole .owner=NULL thing in release already stops the spinners
> >
> > Not really, the new writer will spin in this case, afaics.
> >
> > But this is another problem and probably we do not care. The new writer is
> > almost impossible in this particular case, another freeze_super() should
> > notice frozen != SB_UNFROZEN and return EBUSY.
>
> rwsem_spin_on_owner() checks rwsem_owner_is_writer(), which does owner
> && owner != RWSEM_READER_OWNED, which will fail for !owner.

Yep. So rwsem_spin_on_owner() goes to "out:" and returns
!rwsem_owner_is_reader() == T.

IOW, afaics owner == NULL means "spin unconditionally", I guess this is for
the case when the new writer is going to do rwsem_set_owner() or up_write()
has already called rwsem_clear_owner() but didn't do up_write() yet.

Probably makes sense, but the code is not very clean,

> Or am I completely confused again?

Or me, I am not sure.

Oleg.