From mboxrd@z Thu Jan 1 00:00:00 1970 From: Waiman Long Subject: Re: [PATCH 1/2] seqlock: Add a new blocking reader type Date: Wed, 11 Sep 2013 12:33:35 -0400 Message-ID: <52309B5F.70406@hp.com> References: <1378909707-3347-1-git-send-email-Waiman.Long@hp.com> <20130911145536.GJ13318@ZenIV.linux.org.uk> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Thomas Gleixner , Linus Torvalds , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, "Chandramouleeswaran, Aswin" , "Norton, Scott J" To: Al Viro Return-path: In-Reply-To: <20130911145536.GJ13318@ZenIV.linux.org.uk> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org 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