linux-kernel - Re: [PATCH v3 2/2] locking/percpu-rwsem: Annotate rwsem ownership transfer by setting RWSEM_OWNER

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Message-ID: <65b6a078-a063-dc9b-d2de-ec7d37831cba@redhat.com>
Date:   Tue, 15 May 2018 17:30:32 -0400
From:   Waiman Long <longman@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        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>,
        Oleg Nesterov <oleg@...hat.com>,
        Amir Goldstein <amir73il@...il.com>, Jan Kara <jack@...e.cz>
Subject: Re: [PATCH v3 2/2] locking/percpu-rwsem: Annotate rwsem ownership
 transfer by setting RWSEM_OWNER_UNKNOWN

On 05/15/2018 05:21 PM, Matthew Wilcox wrote:
> On Tue, May 15, 2018 at 02:45:12PM -0400, Waiman Long wrote:
>> On 05/15/2018 02:02 PM, Matthew Wilcox wrote:
>>> On Tue, May 15, 2018 at 07:58:05PM +0200, Peter Zijlstra wrote:
>>>> On Tue, May 15, 2018 at 01:38:04PM -0400, Waiman Long wrote:
>>>>> +/*
>>>>> + * Owner value to indicate the rwsem's owner is not currently known.
>>>>> + */
>>>>> +#define RWSEM_OWNER_UNKNOWN	((struct task_struct *)-1)
>>>> It might be nice to comment that this works and relies on having that
>>>> ANON_OWNER bit set.
>>> I'd rather change the definition to be ((struct task_struct *)2)
>>> otherwise this is both reader-owned and anonymously-owned which doesn't
>>> make much sense.
>> Thinking about it a bit more. I can actually just use one special bit
>> (bit 0) to designate an unknown owner. So for a reader-owned lock, it is
>> just owner == 1 as the owners are unknown for a reader owned lock. For a
>> lock owned by an unknown writer, it is (owner & 1) && (owner != 1). That
>> will justify the use of -1L and save bit 1 for future extension.
> To quote from your patch:
>
> - * In essence, the owner field now has the following 3 states:
> + * In essence, the owner field now has the following 4 states:
>   *  1) 0
>   *     - lock is free or the owner hasn't set the field yet
>   *  2) RWSEM_READER_OWNED
>   *     - lock is currently or previously owned by readers (lock is free
>   *	  or not set by owner yet)
> - *  3) Other non-zero value
> - *     - a writer owns the lock
> + *  3) RWSEM_ANONYMOUSLY_OWNED
> + *     - lock is owned by an anonymous writer, so spinning on the lock
> + *	  owner should be disabled.
> + *  4) Other non-zero value
> + *     - a writer owns the lock and other writers can spin on the lock owner.
>
> I'd leave these as 0, 1, 2, other.  It's not really worth messing with
> testing bits.
>
> Actually, if you change them to all be values -- s/NULL/RWSEM_NO_OWNER/
>
> then you could define them as:
>
> RWSEM_READER_OWNED	0
> RWSEM_ANON_OWNED	1
> RWSEM_NO_OWNER		2
>
> and rwsem_should_spin() is just sem->owner > 1.

I would like to have owner equal to NULL if it is not locked. If it is
locked, the owner can be used to get information about the owner. So I
am not sure your scheme will work.

Cheers,
Longman