svcrpc: ensure cache_check caller sees updated entry
authorJ. Bruce Fields <bfields@redhat.com>
Tue, 4 Jan 2011 19:12:47 +0000 (14:12 -0500)
committerJ. Bruce Fields <bfields@redhat.com>
Tue, 4 Jan 2011 21:49:25 +0000 (16:49 -0500)
commitfdef7aa5d4020fd94ffcbf0078d6bd9e5a111e19
tree270c2b03e9cb5cf5451b0e229c5a328727ef743a
parent6bab93f87ec703bf6650875881b11f9f27d7da56
svcrpc: ensure cache_check caller sees updated entry

Supposes cache_check runs simultaneously with an update on a different
CPU:

cache_check task doing update
^^^^^^^^^^^ ^^^^^^^^^^^^^^^^^

1. test for CACHE_VALID 1'. set entry->data
   & !CACHE_NEGATIVE

2. use entry->data 2'. set CACHE_VALID

If the two memory writes performed in step 1' and 2' appear misordered
with respect to the reads in step 1 and 2, then the caller could get
stale data at step 2 even though it saw CACHE_VALID set on the cache
entry.

Add memory barriers to prevent this.

Reviewed-by: NeilBrown <neilb@suse.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
net/sunrpc/cache.c