generic_make_request not locked

From: remaper
Date: Tue Apr 30 2013 - 01:39:49 EST

* 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);

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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