On Fri, Jul 08 2016 at 11:04am -0400,
Lars Ellenberg <lars.ellenberg@xxxxxxxxxx> wrote:
> For a long time, generic_make_request() converts recursion into
> iteration by queuing recursive arguments on current->bio_list.
>
> This is convenient for stacking drivers,
> the top-most driver would take the originally submitted bio,
> and re-submit a re-mapped version of it, or one or more clones,
> or one or more new allocated bios to its backend(s). Which
> are then simply processed in turn, and each can again queue
> more "backend-bios" until we reach the bottom of the driver stack,
> and actually dispatch to the real backend device.
>
> Any stacking driver ->make_request_fn() could expect that,
> once it returns, any backend-bios it submitted via recursive calls
> to generic_make_request() would now be processed and dispatched, before
> the current task would call into this driver again.
>
> This is changed by commit
> 54efd50 block: make generic_make_request handle arbitrarily sized bios
>
> Drivers may call blk_queue_split() inside their ->make_request_fn(),
> which may split the current bio into a front-part to be dealt with
> immediately, and a remainder-part, which may need to be split even
> further. That remainder-part will simply also be pushed to
> current->bio_list, and would end up being head-of-queue, in front
> of any backend-bios the current make_request_fn() might submit during
> processing of the fron-part.
>
> Which means the current task would immediately end up back in the same
> make_request_fn() of the same driver again, before any of its backend
> bios have even been processed.
>
> This can lead to resource starvation deadlock.
> Drivers could avoid this by learning to not need blk_queue_split(),
> or by submitting their backend bios in a different context (dedicated
> kernel thread, work_queue context, ...). Or by playing funny re-ordering
> games with entries on current->bio_list.
>
> Instead, I suggest to distinguish between recursive calls to
> generic_make_request(), and pushing back the remainder part in
> blk_queue_split(), by pointing current->bio_lists to a
> struct recursion_to_iteration_bio_lists {
> struct bio_list recursion;
> struct bio_list queue;
> }
>
> By providing each q->make_request_fn() with an empty "recursion"
> bio_list, then merging any recursively submitted bios to the
> head of the "queue" list, we can make the recursion-to-iteration
> logic in generic_make_request() process deepest level bios first,
> and "sibling" bios of the same level in "natural" order.
>
> Signed-off-by: Lars Ellenberg <lars.ellenberg@xxxxxxxxxx>
> Signed-off-by: Roland Kammerer <roland.kammerer@xxxxxxxxxx>
> ---
> block/bio.c | 27 +++++++++++++++++--------
> block/blk-core.c | 50 +++++++++++++++++++++++++----------------------
> block/blk-merge.c | 5 ++++-
> drivers/md/bcache/btree.c | 12 ++++++------
> drivers/md/dm-bufio.c | 2 +-
> drivers/md/md.h | 7 +++++++
> drivers/md/raid1.c | 5 ++---
> drivers/md/raid10.c | 5 ++---
> include/linux/bio.h | 18 +++++++++++++++++
> include/linux/sched.h | 4 ++--
> 10 files changed, 88 insertions(+), 47 deletions(-)
>
> diff --git a/block/bio.c b/block/bio.c
> index 848cd35..1f9fcf0 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -366,12 +366,16 @@ static void punt_bios_to_rescuer(struct bio_set *bs)
> */
>
> bio_list_init(&punt);
> - bio_list_init(&nopunt);
>
> - while ((bio = bio_list_pop(current->bio_list)))
> + bio_list_init(&nopunt);
> + while ((bio = bio_list_pop(¤t->bio_lists->recursion)))
> bio_list_add(bio->bi_pool == bs ? &punt : &nopunt, bio);
> + current->bio_lists->recursion = nopunt;
>
> - *current->bio_list = nopunt;
> + bio_list_init(&nopunt);
> + while ((bio = bio_list_pop(¤t->bio_lists->queue)))
> + bio_list_add(bio->bi_pool == bs ? &punt : &nopunt, bio);
> + current->bio_lists->queue = nopunt;
>
> spin_lock(&bs->rescue_lock);
> bio_list_merge(&bs->rescue_list, &punt);
> @@ -380,6 +384,13 @@ static void punt_bios_to_rescuer(struct bio_set *bs)
> queue_work(bs->rescue_workqueue, &bs->rescue_work);
> }
>
> +static bool current_has_pending_bios(void)
> +{
> + return current->bio_lists &&
> + (!bio_list_empty(¤t->bio_lists->queue) ||
> + !bio_list_empty(¤t->bio_lists->recursion));
> +}
> +
This should be moved to include/linux/bio.h
> diff --git a/block/blk-core.c b/block/blk-core.c
> index 3cfd67d..74bceea 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -2066,35 +2071,34 @@ blk_qc_t generic_make_request(struct bio *bio)
> * Before entering the loop, bio->bi_next is NULL (as all callers
> * ensure that) so we have a list with a single bio.
> * We pretend that we have just taken it off a longer list, so
> - * we assign bio_list to a pointer to the bio_list_on_stack,
> - * thus initialising the bio_list of new bios to be
> - * added. ->make_request() may indeed add some more bios
> - * through a recursive call to generic_make_request. If it
> - * did, we find a non-NULL value in bio_list and re-enter the loop
> - * from the top. In this case we really did just take the bio
> - * of the top of the list (no pretending) and so remove it from
> - * bio_list, and call into ->make_request() again.
> + * we assign bio_list to a pointer to the bio_lists_on_stack,
> + * thus initialising the bio_lists of new bios to be added.
> + * ->make_request() may indeed add some more bios to .recursion
> + * through a recursive call to generic_make_request. If it did,
> + * we find a non-NULL value in .recursion, merge .recursion back to the
> + * head of .queue, and re-enter the loop from the top. In this case we
> + * really did just take the bio of the top of the list (no pretending)
> + * and so remove it from .queue, and call into ->make_request() again.
> */
> BUG_ON(bio->bi_next);
> - bio_list_init(&bio_list_on_stack);
> - current->bio_list = &bio_list_on_stack;
> + bio_list_init(&bio_lists_on_stack.queue);
> + current->bio_lists = &bio_lists_on_stack;
> do {
> struct request_queue *q = bdev_get_queue(bio->bi_bdev);
>
> if (likely(blk_queue_enter(q, false) == 0)) {
> + bio_list_init(&bio_lists_on_stack.recursion);
> ret = q->make_request_fn(q, bio);
> -
> blk_queue_exit(q);
> -
> - bio = bio_list_pop(current->bio_list);
> + bio_list_merge_head(&bio_lists_on_stack.queue,
> + &bio_lists_on_stack.recursion);
> + /* XXX bio_list_init(&bio_lists_on_stack.recursion); */
Please remove this XXX commented code.
> diff --git a/drivers/md/md.h b/drivers/md/md.h
> index b4f3352..b62e65f4 100644
> --- a/drivers/md/md.h
> +++ b/drivers/md/md.h
> @@ -664,6 +664,13 @@ static inline void rdev_dec_pending(struct md_rdev *rdev, struct mddev *mddev)
> }
> }
>
> +static inline bool current_has_pending_bios(void)
> +{
> + return current->bio_lists && (
> + !bio_list_empty(¤t->bio_lists->queue) ||
> + !bio_list_empty(¤t->bio_lists->recursion));
> +}
> +
This can be removed now that include/linux/bio.h exports the same.
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