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

[Waiman Long]

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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