linux-kernel - Re: [PATCH 1/2] seqlock: Add a new blocking reader type

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Date:	Wed, 11 Sep 2013 12:33:35 -0400
From:	Waiman Long <waiman.long@...com>
To:	Al Viro <viro@...IV.linux.org.uk>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>
Subject: Re: [PATCH 1/2] seqlock: Add a new blocking reader type

On 09/11/2013 10:55 AM, Al Viro wrote:
> On Wed, Sep 11, 2013 at 10:28:26AM -0400, Waiman Long wrote:
>> The sequence lock (seqlock) was originally designed for the cases
>> where the readers do not need to block the writers by making the
>> readers retry the read operation when the data change.
>>
>> Since then, the use cases have been expanded to include situations
>> where a thread does not need to change the data (effectively a reader)
>> at all but have to take the writer lock because it can't tolerate
>> changes to the protected structure. Some examples are the d_path()
>> function and the getcwd() syscall in fs/dcache.c where the functions
>> take the writer lock on rename_lock even though they don't need
>> to change anything in the protected data structure at all. This is
>> inefficient as a reader is now blocking other non-blocking readers
>> by pretending to be a writer.
>>
>> This patch tries to eliminate this inefficiency by introducing a new
>> type of blocking reader to the seqlock locking mechanism. This new
>> blocking reader will not block other non-blocking readers, but will
>> block other blocking readers and writers.
> Umm...  That's misleading - it doesn't _block_, it spins.  Moroever,
> seq_readbegin() also spins in presense of writer; the main property
> of this one is that it keeps writers away.

I used "block" in the sense that it will stop a writer from moving 
forward. I will update the commit log to make that more clear.
> Folks, any suggestions on better names?  The semantics we are getting is

I will welcome any better name suggestion and will incorporate that in 
the patch.

-Longman
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