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

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Date:   Wed, 16 May 2018 12:48:30 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Waiman Long <longman@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        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 05/15, Waiman Long wrote:
>
> There are use cases where a rwsem can be acquired by one task, but
> released by another task. In thess cases, optimistic spinning may need
> to be disabled.  One example will be the filesystem freeze/thaw code

You do not read my emails ;)

Let me repeat once again that in this particular case the writer will
never spin because of owner == NULL. freeze_super() checks SB_UNFROZEN
under sb->s_umount and only then calls sb_wait_write(). IOW, sb_wait_write()
can only be called when this rwsem was already released by the previous
writer.

I am not arguing with this change, percpu_rwsem_release/acquire may have
another user sometime, but the changelog is not accurate.

> +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.

Oleg.