[LU-15205] cfs_hash rehashing can race with hash iteration

• Type: Bug
• Resolution: Duplicate
• Priority: Minor
• Fix Version/s: Upstream
• Severity: 3
• Assignee: WC Triage
• Reporter: Alex Zhuravlev

Description

when one thread leaving cfs hash iteration it checks whether it's time to rehash in cfs_hash_for_each_exit():

	if (bits > 0) {
		cfs_hash_rehash(hs, atomic_read(&hs->hs_count) <
				    CFS_HASH_LOOP_HOG);

then cfs_hash_rehash() can start rehashing immediately (not via worker thread):

	if (!do_rehash) {
		/* launch and return */
		queue_work(cfs_rehash_wq, &hs->hs_rehash_work);
		cfs_hash_unlock(hs, 1);
		return;
	}

	/* rehash right now */
	cfs_hash_unlock(hs, 1);

	cfs_hash_rehash_worker(&hs->hs_rehash_work);

if another thread is starting to iterate the same hash it can find it being rehashed and there is no way to wait for rehashing completion, that works for rehashing in a dedicated thread only:

	if (cfs_hash_is_rehashing(hs))
		cfs_hash_rehash_cancel(hs);

so 2nd thread just proceed and find hash under rehashing:

	LASSERT(!cfs_hash_is_rehashing(hs));