linux-kernel - Re: [PATCH v4 1/2] locking/rwsem: Add a new RWSEM_ANONYMOUSLY

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Date:   Wed, 16 May 2018 13:59:06 +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>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v4 1/2] locking/rwsem: Add a new RWSEM_ANONYMOUSLY_OWNED
 flag

On Wed, May 16, 2018 at 12:48:30PM +0200, Oleg Nesterov wrote:
> > +static inline bool is_rwsem_owner_spinnable(struct task_struct *owner)
> >  {
> > -	return owner && owner != RWSEM_READER_OWNED;
> > +	return !((unsigned long)owner & RWSEM_ANONYMOUSLY_OWNED);
> >  }
> 
> Perhaps you should add __attribute__(aligned) to struct rw_semaphore then...
> 
> I don't think it is really needed, but see the comment under struct address_space.

Luckily we just dropped CRIS support, but yeah, who knows if some other
dodgy arch also doesn't properly align things.

>From a quick test, m68k is the only odd one, it seems to align pointers
on 2 bytes.