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    Subject[PATCH 08/10] block: make get_request[_wait]() fail if queue is dead
    Date
    Currently get_request[_wait]() allocates request whether queue is dead
    or not. This patch makes get_request[_wait]() return NULL if @q is
    dead. blk_queue_bio() is updated to fail the submitted bio if request
    allocation fails. While at it, add docbook comments for
    get_request[_wait]().

    Note that the current code has rather unclear (there are spurious DEAD
    tests scattered around) assumption that the owner of a queue
    guarantees that no request travels block layer if the queue is dead
    and this patch in itself doesn't change much; however, this will allow
    fixing the broken assumption in the next patch.

    Signed-off-by: Tejun Heo <tj@kernel.org>
    Cc: Jens Axboe <axboe@kernel.dk>
    ---
    block/blk-core.c | 54 ++++++++++++++++++++++++++++++++++++++----------------
    1 files changed, 38 insertions(+), 16 deletions(-)

    diff --git a/block/blk-core.c b/block/blk-core.c
    index 7a90317..fc10f53 100644
    --- a/block/blk-core.c
    +++ b/block/blk-core.c
    @@ -710,10 +710,19 @@ static bool blk_rq_should_init_elevator(struct bio *bio)
    return true;
    }

    -/*
    - * Get a free request, queue_lock must be held.
    - * Returns NULL on failure, with queue_lock held.
    - * Returns !NULL on success, with queue_lock *not held*.
    +/**
    + * get_request - get a free request
    + * @q: request_queue to allocate request from
    + * @rw_flags: RW and SYNC flags
    + * @bio: bio to allocate request for (can be %NULL)
    + * @gfp_mask: allocation mask
    + *
    + * Get a free request from @q. This function may fail under memory
    + * pressure or if @q is dead.
    + *
    + * Must be callled with @q->queue_lock held and,
    + * Returns %NULL on failure, with @q->queue_lock held.
    + * Returns !%NULL on success, with @q->queue_lock *not held*.
    */
    static struct request *get_request(struct request_queue *q, int rw_flags,
    struct bio *bio, gfp_t gfp_mask)
    @@ -724,6 +733,9 @@ static struct request *get_request(struct request_queue *q, int rw_flags,
    const bool is_sync = rw_is_sync(rw_flags) != 0;
    int may_queue;

    + if (unlikely(test_bit(QUEUE_FLAG_DEAD, &q->queue_flags)))
    + return NULL;
    +
    may_queue = elv_may_queue(q, rw_flags);
    if (may_queue == ELV_MQUEUE_NO)
    goto rq_starved;
    @@ -816,11 +828,18 @@ out:
    return rq;
    }

    -/*
    - * No available requests for this queue, wait for some requests to become
    - * available.
    +/**
    + * get_request_wait - get a free request with retry
    + * @q: request_queue to allocate request from
    + * @rw_flags: RW and SYNC flags
    + * @bio: bio to allocate request for (can be %NULL)
    *
    - * Called with q->queue_lock held, and returns with it unlocked.
    + * Get a free request from @q. This function keeps retrying under memory
    + * pressure and fails iff @q is dead.
    + *
    + * Must be callled with @q->queue_lock held and,
    + * Returns %NULL on failure, with @q->queue_lock held.
    + * Returns !%NULL on success, with @q->queue_lock *not held*.
    */
    static struct request *get_request_wait(struct request_queue *q, int rw_flags,
    struct bio *bio)
    @@ -834,6 +853,9 @@ static struct request *get_request_wait(struct request_queue *q, int rw_flags,
    struct io_context *ioc;
    struct request_list *rl = &q->rq;

    + if (unlikely(test_bit(QUEUE_FLAG_DEAD, &q->queue_flags)))
    + return NULL;
    +
    prepare_to_wait_exclusive(&rl->wait[is_sync], &wait,
    TASK_UNINTERRUPTIBLE);

    @@ -864,19 +886,15 @@ struct request *blk_get_request(struct request_queue *q, int rw, gfp_t gfp_mask)
    {
    struct request *rq;

    - if (unlikely(test_bit(QUEUE_FLAG_DEAD, &q->queue_flags)))
    - return NULL;
    -
    BUG_ON(rw != READ && rw != WRITE);

    spin_lock_irq(q->queue_lock);
    - if (gfp_mask & __GFP_WAIT) {
    + if (gfp_mask & __GFP_WAIT)
    rq = get_request_wait(q, rw, NULL);
    - } else {
    + else
    rq = get_request(q, rw, NULL, gfp_mask);
    - if (!rq)
    - spin_unlock_irq(q->queue_lock);
    - }
    + if (!rq)
    + spin_unlock_irq(q->queue_lock);
    /* q->queue_lock is unlocked at this point */

    return rq;
    @@ -1300,6 +1318,10 @@ get_rq:
    * Returns with the queue unlocked.
    */
    req = get_request_wait(q, rw_flags, bio);
    + if (unlikely(!req)) {
    + bio_endio(bio, -ENODEV); /* @q is dead */
    + goto out_unlock;
    + }

    /*
    * After dropping the lock and possibly sleeping here, our request
    --
    1.7.3.1


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