[zfs-code] newbie question about a dbuf-arc eviction race

[Bhaskar Sarkar]

Hi,

Can a dbuf be in DB_CACHED state, db_holds == 0,
b_efunc != NULL while its db_buf is put on the
eviction list ?  From an ASSERT in dbuf_do_evict(),
it appears that it can. If it can, I am wondering what
is preventing the following race

dbuf_hold_impl()
     db = dbuf_find(dn, level, blkid);

     ...
     if (db->db_buf && refcount_is_zero(&db->db_holds)) {
                 arc_buf_add_ref(db->db_buf, db);
                 /*
                  * The above just returns as db_buf is on the
                  * eviction list. Now, suppose arc_do_user_evicts()
                  * selects this buf and calls dbuf_do_evict().
                  * Nothing really stops this function.
                  *
                  * Now there is a race between dbuf_do_evict() and
                  * the following code
                  */
                 if (db->db_buf->b_data == NULL) {
                         ....
     ....
     }

Sorry if I overlooked something that is very obvious.

Thanks
Bhaskar