linux-kernel - Why don't we always grab bfqd->lock for bio

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Message-ID: <CACGdZYJ0_8t1JV2CqW-5B92n2YFNkf6jzi7oVkpsBfNmBGd_VA@mail.gmail.com>
Date:   Mon, 31 Oct 2022 21:37:20 -0700
From:   Khazhy Kumykov <khazhy@...omium.org>
To:     Paolo Valente <paolo.valente@...aro.org>,
        Jens Axboe <axboe@...nel.dk>
Cc:     linux-block@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Why don't we always grab bfqd->lock for bio_bfqq?

I'm investigating a NULL deref crash in bfq_add_bfqq_busy(), wherein
bfqq->woken_list_node is hashed, but bfqq->waker_bfqq is NULL - which
seems inconsistent per my reading of the code.

Wherein I see bfq_allow_bio_merge() both accesses and modifies
accesses bfqd->bio_bfqq without bfqd->lock, which strikes me as odd.
The call there though to bfq_setup_cooperator and bfq_merge_bfqqs()
seem wrong to me. In particular, the call to bfq_merge_bfqqs() I am
suspecting can cause the inconsistency seen above, since it's the only
place I've found that modifies bfqq->waker_bfqq without bfqd->lock.

But I'm curious in general - what's special about bio_bfqq? Should we
grab bfqd->lock when touching it? e.g. bfq_request_merge() also
accesses bio_bfqq without grabbing the lock, where-in we traverse
bfqq->sort_list - that strikes me as odd as well, but I'm not fully
familiar with the locking conventions here. But it feels like,
especially since we can merge bfqqs, so bio_bfqq is shared - this
lockless access seems wrong.