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svcrpc: ensure cache_check caller sees updated entry

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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>
This commit is contained in:
J. Bruce Fields 2011-01-04 14:12:47 -05:00
parent 6bab93f87e
commit fdef7aa5d4
1 changed files with 10 additions and 1 deletions
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11
net/sunrpc/cache.c
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@ -128,6 +128,7 @@ static void cache_fresh_locked(struct cache_head *head, time_t expiry)
{
head->expiry_time = expiry;
head->last_refresh = seconds_since_boot();
smp_wmb(); /* paired with smp_rmb() in cache_is_valid() */
set_bit(CACHE_VALID, &head->flags);
}
@ -208,8 +209,16 @@ static inline int cache_is_valid(struct cache_detail *detail, struct cache_head
/* entry is valid */
if (test_bit(CACHE_NEGATIVE, &h->flags))
return -ENOENT;
else
else {
/*
* In combination with write barrier in
* sunrpc_cache_update, ensures that anyone
* using the cache entry after this sees the
* updated contents:
*/
smp_rmb();
return 0;
}
}
}