Re: [PATCH 04/12] sunrpc/cache: recheck cache validity after cache_defer_req

J. Bruce Fields
2009-08-25T21:50:36Z

On Thu, Aug 06, 2009 at 02:57:04PM +1000, Neil Brown wrote:
> On Tuesday August 4, bfields@fieldses.org wrote:
> > On Tue, Aug 04, 2009 at 03:22:38PM +1000, NeilBrown wrote:
> > > If cache_defer_req did not leave the request on a queue, then it could
> > > possibly have waited long enough that the cache became valid. So check the
> > > status after the call.
> > >
> > > Signed-off-by: NeilBrown <neilb@suse.de>
> > > * Returns 0 if the cache_head can be used, or cache_puts it and returns
> > > - * -EAGAIN if upcall is pending,
> > > - * -ETIMEDOUT if upcall failed and should be retried,
> > > + * -EAGAIN if upcall is pending and request has been queued
> > > + * -ETIMEDOUT if upcall failed or request could not be queue or
> >
> > s/queue/queued/
> >
>
> :-)
>
> > > @@ -235,10 +243,14 @@ int cache_check(struct cache_detail *detail,
> > > }
> > > }
> > >
> > > - if (rv == -EAGAIN)
> > > - if (cache_defer_req(rqstp, h) != 0)
> > > - rv = -ETIMEDOUT;
> > > -
> > > + if (rv == -EAGAIN) {
> > > + if (cache_defer_req(rqstp, h) == 0) {
> > > + /* Request is not deferred */
> >
> > The code might be more self-explanatory if we wrote:
> >
> > if (cache_defer_req(rqstp, h) == -ETIMEDOUT) {
> >
> > Well, at least it would be obvious we're handling the "failure" case?
> > (Even if admittedly it's a "failure" that we may be able to handle).
> >
> > It always takes me a little thought whenever I encounter a
> > boolean-returning function whose name doesn't have an obvious truth
> > value (list_empty, cache_is_valid).
>
> I certainly see you point. For consistency in the kernel, if the
> function name doesn't sound like a boolean it should return 0 or
> positive on success and negative for error.
>
> But despite that I changed cache_defer_req to return 0 or 1 rather
> than -ETIMEDOUT or 0...
>
> There are three possibly results of cache_defer_req:
> a/ the request has been stored for later processing
> b/ there was a failure while trying to store the request
> c/ there was no need to store the request because the cache
> item is no longer waiting for a reply.
>
> While 'a' is success and 'b' is an error, 'c' doesn't exactly fit in
> to either. However 'b' and 'c' are treated the same way by
> cache_check.
> So returning '-ETIMEDOUT' for both 'b' and 'c' seemed wrong.
>
> The current return value is a true/false value for the assertion "the
> request was successfully deferred". But choosing a name for
> cache_defer_req which makes that meaning obvious seems clumsy.
>
> Thinks.....
>
> Maybe
> a -> 0 (success, we deferred the request)
> b -> -ENOMEM (failed to find somewhere to store the request)
> c -> -EAGAIN (something happened .. check again).
>
> and in cache_check we write
>
> if (cache_defer_req(rqstp, h) < 0) {
> /* Request is not deferred */
>
> which maybe a bit more self explanatory??

Yes, OK, seems reasonable enough.

So, apologies, I've lost track of the rest of your patches, but I'm
still very much interested. Will you have a chance to rebase and
resend? My for-2.6.32 branch at

git://linux-nfs.org/~bfields/linux.git for-2.6.32

has what I've applied so far, plus a merge of Trond's queued patches
(which includes some changes to the cache code).

--b.

>
> NeilBrown
>
>
> >From c970b6abce98044de573336b3a867b7ed39642e4 Mon Sep 17 00:00:00 2001
> From: NeilBrown <neilb@suse.de>
> Date: Thu, 6 Aug 2009 14:56:13 +1000
> Subject: [PATCH] sunrpc/cache: recheck cache validity after cache_defer_req
>
> If cache_defer_req did not leave the request on a queue, then it could
> possibly have waited long enough that the cache became valid. So check the
> status after the call.
>
> Signed-off-by: NeilBrown <neilb@suse.de>
> ---
> net/sunrpc/cache.c | 51 ++++++++++++++++++++++++++++++++-------------------
> 1 files changed, 32 insertions(+), 19 deletions(-)
>
> diff --git a/net/sunrpc/cache.c b/net/sunrpc/cache.c
> index c1f897c..cec2574 100644
> --- a/net/sunrpc/cache.c
> +++ b/net/sunrpc/cache.c
> @@ -173,6 +173,22 @@ struct cache_head *sunrpc_cache_update(struct cache_detail *detail,
> EXPORT_SYMBOL_GPL(sunrpc_cache_update);
>
> static int cache_make_upcall(struct cache_detail *detail, struct cache_head *h);
> +
> +static inline int cache_is_valid(struct cache_detail *detail, struct cache_head *h)
> +{
> + if (!test_bit(CACHE_VALID, &h->flags) ||
> + h->expiry_time < get_seconds())
> + return -EAGAIN;
> + else if (detail->flush_time > h->last_refresh)
> + return -EAGAIN;
> + else {
> + /* entry is valid */
> + if (test_bit(CACHE_NEGATIVE, &h->flags))
> + return -ENOENT;
> + else
> + return 0;
> + }
> +}
> /*
> * This is the generic cache management routine for all
> * the authentication caches.
> @@ -181,8 +197,10 @@ static int cache_make_upcall(struct cache_detail *detail, struct cache_head *h);
> *
> *
> * Returns 0 if the cache_head can be used, or cache_puts it and returns
> - * -EAGAIN if upcall is pending,
> - * -ETIMEDOUT if upcall failed and should be retried,
> + * -EAGAIN if upcall is pending and request has been queued
> + * -ETIMEDOUT if upcall failed or request could not be queued or
> + * upcall completed but item is still invalid (implying that
> + * the cache item has been replaced with a newer one).
> * -ENOENT if cache entry was negative
> */
> int cache_check(struct cache_detail *detail,
> @@ -192,17 +210,7 @@ int cache_check(struct cache_detail *detail,
> long refresh_age, age;
>
> /* First decide return status as best we can */
> - if (!test_bit(CACHE_VALID, &h->flags) ||
> - h->expiry_time < get_seconds())
> - rv = -EAGAIN;
> - else if (detail->flush_time > h->last_refresh)
> - rv = -EAGAIN;
> - else {
> - /* entry is valid */
> - if (test_bit(CACHE_NEGATIVE, &h->flags))
> - rv = -ENOENT;
> - else rv = 0;
> - }
> + rv = cache_is_valid(detail, h);
>
> /* now see if we want to start an upcall */
> refresh_age = (h->expiry_time - h->last_refresh);
> @@ -235,10 +243,14 @@ int cache_check(struct cache_detail *detail,
> }
> }
>
> - if (rv == -EAGAIN)
> - if (cache_defer_req(rqstp, h) != 0)
> - rv = -ETIMEDOUT;
> -
> + if (rv == -EAGAIN) {
> + if (cache_defer_req(rqstp, h) < 0) {
> + /* Request is not deferred */
> + rv = cache_is_valid(detail, h);
> + if (rv == -EAGAIN)
> + rv = -ETIMEDOUT;
> + }
> + }
> if (rv)
> cache_put(h, detail);
> return rv;
> @@ -557,11 +569,11 @@ static int cache_defer_req(struct cache_req *req, struct cache_head *item)
> * or continue and drop the oldest below
> */
> if (net_random()&1)
> - return -ETIMEDOUT;
> + return -ENOMEM;
> }
> dreq = req->defer(req);
> if (dreq == NULL)
> - return -ETIMEDOUT;
> + return -ENOMEM;
>
> dreq->item = item;
>
> @@ -591,6 +603,7 @@ static int cache_defer_req(struct cache_req *req, struct cache_head *item)
> if (!test_bit(CACHE_PENDING, &item->flags)) {
> /* must have just been validated... */
> cache_revisit_request(item);
> + return -EAGAIN;
> }
> return 0;
> }
> --
> 1.6.3.3
>