Re: [PATCH v4 13/27] lib: add errseq_t type and infrastructure for handling it

[Jeff Layton <jlayton@redhat.com>]

On Wed, 2017-05-10 at 08:03 +1000, NeilBrown wrote:
> On Tue, May 09 2017, Jeff Layton wrote:
> 
> > An errseq_t is a way of recording errors in one place, and allowing any
> > number of "subscribers" to tell whether an error has been set again
> > since a previous time.
> > 
> > It's implemented as an unsigned 32-bit value that is managed with atomic
> > operations. The low order bits are designated to hold an error code
> > (max size of MAX_ERRNO). The upper bits are used as a counter.
> > 
> > The API works with consumers sampling an errseq_t value at a particular
> > point in time. Later, that value can be used to tell whether new errors
> > have been set since that time.
> > 
> > Note that there is a 1 in 512k risk of collisions here if new errors
> > are being recorded frequently, since we have so few bits to use as a
> > counter. To mitigate this, one bit is used as a flag to tell whether the
> > value has been sampled since a new value was recorded. That allows
> > us to avoid bumping the counter if no one has sampled it since it
> > was last bumped.
> > 
> > Later patches will build on this infrastructure to change how writeback
> > errors are tracked in the kernel.
> > 
> > Signed-off-by: Jeff Layton <jlayton@redhat.com>
> 
> I like that this is a separate lib/*.c - nicely structured too.
> 
> Reviewed-by: NeilBrown <neilb@suse.com>
> 
> 

Thanks, yeah...it occurred to me that this scheme is not really specific
to writeback errors. While I can't think of another use-case for
errseq_t's right offhand, I think this makes for cleaner layering and
should make it easy to use in other ways should they arise.

-- 
Jeff Layton <jlayton@redhat.com>