lkml.org 
[lkml]   [2018]   [May]   [15]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH v3 2/2] locking/percpu-rwsem: Annotate rwsem ownership transfer by setting RWSEM_OWNER_UNKNOWN
From
Date
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

\
 
 \ /
  Last update: 2018-05-15 23:31    [W:0.093 / U:0.900 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site