linux-kernel - Re: [PATCH v2] locking/rwsem: Add reader-owned state to the owner field

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Date:	Fri, 20 May 2016 18:56:53 -0400
From:	Waiman Long <waiman.long@....com>
To:	Dave Chinner <david@...morbit.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, <linux-kernel@...r.kernel.org>,
	Davidlohr Bueso <dave@...olabs.net>,
	Jason Low <jason.low2@...com>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>
Subject: Re: [PATCH v2] locking/rwsem: Add reader-owned state to the owner
 field

On 05/18/2016 09:37 PM, Dave Chinner wrote:
> On Fri, May 06, 2016 at 08:20:24PM -0400, Waiman Long wrote:
>> Currently, it is not possible to determine for sure if a reader
>> owns a rwsem by looking at the content of the rwsem data structure.
>> This patch adds a new state RWSEM_READER_OWNED to the owner field
>> to indicate that readers currently own the lock.
> Oh, yes please. This will enable us to get rid of the remaining
> mrlock rwsem abstraction we've carried since the days of Irix in
> XFS. The only reason the abstraction still exists is that we track
> write locks for the purposes of checking for correct inode locking
> contexts via ASSERT(xfs_isilocked()) calls....

I think we could add code to export this read/write ownership 
information. However, that will only work when both 
CONFIG_RWSEM_XCHGADD_ALGORITHM and CONFIG_RWSEM_SPIN_ON_OWNER are set. 
So this functionality may not be available in some architectures that 
use CONFIG_RWSEM_GENERIC_SPINLOCK.

Cheers,
Longman