optimize attribute timeouts for "noac" and "actimeo=0"

Hi.

I've been looking at a bugzilla which describes a problem where
a customer was advised to use either the "noac" or "actimeo=0"
mount options to solve a consistency problem that they were
seeing in the file attributes.  It turned out that this solution
did not work reliably for them because sometimes, the local
attribute cache was believed to be valid and not timed out.
(With an attribute cache timeout of 0, the cache should always
appear to be timed out.)

In looking at this situation, it appears to me that the problem
is that the attribute cache timeout code has an off-by-one
error in it.  It is assuming that the cache is valid in the
region, [read_cache_jiffies, read_cache_jiffies + attrtimeo].  The
cache should be considered valid only in the region,
[read_cache_jiffies, read_cache_jiffies + attrtimeo).  With this
change, the options, "noac" and "actimeo=0", work as originally
expected.

This problem was previously addressed by special casing the
attrtimeo == 0 case.  However, since the problem is only an off-
by-one error, the cleaner solution is address the off-by-one
error and thus, not require the special case.

    Thanx...

        ps

Signed-off-by: Peter Staubach <staubach@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

2 files changed, 14 insertions, 1 deletions

diff --git a/include/linux/jiffies.h b/include/linux/jiffies.h
index abb6ac639e8e..1a9cf78bfce5 100644
--- a/include/linux/jiffies.h
+++ b/include/linux/jiffies.h
@@ -115,10 +115,20 @@ static inline u64 get_jiffies_64(void)
          ((long)(a) - (long)(b) >= 0))
 #define time_before_eq(a,b)     time_after_eq(b,a)
 
+/*
+ * Calculate whether a is in the range of [b, c].
+ */
 #define time_in_range(a,b,c) \
         (time_after_eq(a,b) && \
          time_before_eq(a,c))
 
+/*
+ * Calculate whether a is in the range of [b, c).
+ */
+#define time_in_range_open(a,b,c) \
+        (time_after_eq(a,b) && \
+         time_before(a,c))
+
 /* Same as above, but does so with platform independent 64bit types.
  * These must be used when utilizing jiffies_64 (i.e. return value of
  * get_jiffies_64() */
diff --git a/include/linux/nfs_fs.h b/include/linux/nfs_fs.h
index b8d9c6dd4f63..db867b04ac3c 100644
--- a/include/linux/nfs_fs.h
+++ b/include/linux/nfs_fs.h
@@ -130,7 +130,10 @@ struct nfs_inode {
          *
          * We need to revalidate the cached attrs for this inode if
          *
-         *      jiffies - read_cache_jiffies > attrtimeo
+         *      jiffies - read_cache_jiffies >= attrtimeo
+         *
+         * Please note the comparison is greater than or equal
+         * so that zero timeout values can be specified.
          */
         unsigned long           read_cache_jiffies;
         unsigned long           attrtimeo;