generic_make_request not locked

[remaper <yp.fangdong@gmail.com>]

/*
 * We only want one ->make_request_fn to be active at a time,
 * else stack usage with stacked devices could be a problem.
 * So use current->bio_list to keep a list of requests
 * submited by a make_request_fn function.
 * current->bio_list is also used as a flag to say if
 * generic_make_request is currently active in this task or not.
 * If it is NULL, then no make_request is active.  If it is non-NULL,
 * then a make_request is active, and new requests should be added
 * at the tail
 */
void generic_make_request(struct bio *bio)
{
...
    if (current->bio_list) {
        /* make_request is active */
        bio_list_add(current->bio_list, bio);
        return;
    }
...
}

current->bio_list is a global variable, and generic_make_request would run on multithread environment, so, i don't understand why here( bio_list_add ) not lock?
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