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 14:58:30 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Oleg Nesterov <oleg@...hat.com>
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 Tue, May 15, 2018 at 02:45:32PM +0200, Oleg Nesterov wrote:
> 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,

Arrgh, you're right... I hate this rwsem code.

Some day I'll finish the atomic_long_t version, which similar to mutex,
merges the owner and 'count' fields.