Hi all,
I have thought carefully about the the thread-safe memcached recently,
and found that if the re-balance is running, it may not thread-safety. The
code "do_item_get->do_item_unlink_nolock" may corrupt the hash table.
Whenever it trying to modify the hash table, it should get cache_lock, but
the function do_item_get have not got the cache_lock.
Please tell me if anything i neglected.
/** wrapper around assoc_find which does the lazy expiration logic */
item *do_item_get(const char *key, const size_t nkey, const uint32_t hv) {
//mutex_lock(&cache_lock);
item *it = assoc_find(key, nkey, hv);
if (it != NULL) {
refcount_incr(&it->refcount);
/* Optimization for slab reassignment. prevents popular items from
* jamming in busy wait. Can only do this here to satisfy lock order
* of item_lock, cache_lock, slabs_lock. */
if (slab_rebalance_signal &&
((void *)it >= slab_rebal.slab_start && (void *)it <
slab_rebal.slab_end)) {
do_item_unlink_nolock(it, hv);
-------------------------------------------------------------------> no
lock before unlink.
do_item_remove(it);
it = NULL;
}
}
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