Re: sunrpc/cache.c: races while updating cache entries

[NeilBrown <neilb@suse.de>]

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On 03 Jun 2013 16:27:06 +0200 Bodo Stroesser <bstroesser@ts.fujitsu.com>
wrote:

> On Fri, Apr 19, 2013 at 06:56:00PM +0200, Bodo Stroesser wrote:
> > 
> > We started the test of the -SP2 (and mainline) series on Tue, 9th, but had no
> > success.
> > We did _not_ find a problem with the patches, but under -SP2 our test scenario
> > has less than 40% of the throughput we saw under -SP1. With that low
> > performance, we had a 4 day run without any dropped RPC request. But we don't
> > know the error rate without the patches under these conditions. So we can't
> > give an o.k. for the patches yet.
> > 
> > Currently we try to find the reason for the different behavior of SP1 and SP2
> > 
> 
> Hi,
> 
> sorry for the delay. Meanwhile we found the reason for the small throughput
> with -SP2. The problem resulted from a change in our own software.
> 
> Thus I could fix this and started a test on last Tuesday. I stopped the test
> today after 6 days without any lost RPC. Without the patches I saw the first
> dropped RPC after 3 hours. Thus, I think the patches for -SP2 are fine. 
> 
> @Neil: would patch 0006 of the -SP1 patchset be a good additional change for
> mainline?
> 
> Bodo

Thanks for all the testing.

Bruce: where are you at with these?  Are you holding one to some that I sent
previously, or should I resend them all?


Bodo: no, I don't think that patch is appropriate for mainline.  It causes
sunrpc_cache_pipe_upcall to abort if ->expiry_time is zero.  There is
certainly no point in doing an upcall in that case, but the code in mainline
is quite different to the code in -SP1 against which that patch made sense.

For mainline an equivalent optimisation which probably makes the interesting
case more obvious would be:

diff --git a/net/sunrpc/cache.c b/net/sunrpc/cache.c
index d01eb07..291cc47 100644
--- a/net/sunrpc/cache.c
+++ b/net/sunrpc/cache.c
@@ -262,7 +262,8 @@ int cache_check(struct cache_detail *detail,
 	if (rqstp == NULL) {
 		if (rv == -EAGAIN)
 			rv = -ENOENT;
-	} else if (rv == -EAGAIN || age > refresh_age/2) {
+	} else if (rv == -EAGAIN ||
+		   (refresh_age > 0 && age > refresh_age/2)) {
 		dprintk("RPC:       Want update, refage=%ld, age=%ld\n",
 				refresh_age, age);
 		if (!test_and_set_bit(CACHE_PENDING, &h->flags)) {


i.e. trap that case in cache_check.

NeilBrown

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