On Thu, 9 Feb 2017, Kent Overstreet wrote: > On Wed, Feb 08, 2017 at 11:34:07AM -0500, Mike Snitzer wrote: > > On Tue, Feb 07 2017 at 11:58pm -0500, > > Kent Overstreet <kent.overstreet@xxxxxxxxx> wrote: > > > > > On Tue, Feb 07, 2017 at 09:39:11PM +0100, Pavel Machek wrote: > > > > On Mon 2017-02-06 17:49:06, Kent Overstreet wrote: > > > > > On Mon, Feb 06, 2017 at 04:47:24PM -0900, Kent Overstreet wrote: > > > > > > On Mon, Feb 06, 2017 at 01:53:09PM +0100, Pavel Machek wrote: > > > > > > > Still there on v4.9, 36 threads on nokia n900 cellphone. > > > > > > > > > > > > > > So.. what needs to be done there? > > > > > > > > > > > But, I just got an idea for how to handle this that might be halfway sane, maybe > > > > > > I'll try and come up with a patch... > > > > > > > > > > Ok, here's such a patch, only lightly tested: > > > > > > > > I guess it would be nice for me to test it... but what it is against? > > > > I tried after v4.10-rc5 and linux-next, but got rejects in both cases. > > > > > > Sorry, I forgot I had a few other patches in my branch that touch > > > mempool/biosets code. > > > > > > Also, after thinking about it more and looking at the relevant code, I'm pretty > > > sure we don't need rescuer threads for block devices that just split bios - i.e. > > > most of them, so I changed my patch to do that. > > > > > > Tested it by ripping out the current->bio_list checks/workarounds from the > > > bcache code, appears to work: > > > > Feedback on this patch below, but first: > > > > There are deeper issues with the current->bio_list and rescue workqueues > > than thread counts. > > > > I cannot help but feel like you (and Jens) are repeatedly ignoring the > > issue that has been raised numerous times, most recently: > > https://www.redhat.com/archives/dm-devel/2017-February/msg00059.html > > > > FYI, this test (albeit ugly) can be used to check if the dm-snapshot > > deadlock is fixed: > > https://www.redhat.com/archives/dm-devel/2017-January/msg00064.html > > > > This situation is the unfortunate pathological worst case for what > > happens when changes are merged and nobody wants to own fixing the > > unforseen implications/regressions. Like everyone else in a position > > of Linux maintenance I've tried to stay away from owning the > > responsibility of a fix -- it isn't working. Ok, I'll stop bitching > > now.. I do bear responsibility for not digging in myself. We're all > > busy and this issue is "hard". > > Mike, it's not my job to debug DM code for you or sift through your bug reports. > I don't read dm-devel, and I don't know why you think I that's my job. > > If there's something you think the block layer should be doing differently, post > patches - or at the very least, explain what you'd like to be done, with words. > Don't get pissy because I'm not sifting through your bug reports. So I post this patch for that bug. Will any of the block device maintainers respond to it? From: Mikulas Patocka <mpatocka@xxxxxxxxxx> Date: Tue, 27 May 2014 11:03:36 -0400 Subject: block: flush queued bios when process blocks to avoid deadlock The block layer uses per-process bio list to avoid recursion in generic_make_request. When generic_make_request is called recursively, the bio is added to current->bio_list and generic_make_request returns immediately. The top-level instance of generic_make_request takes bios from current->bio_list and processes them. The problem is that this bio queuing on current->bio_list creates an artifical locking dependency - a bio further on current->bio_list depends on any locks that preceding bios could take. This could result in a deadlock. Commit df2cb6daa4 ("block: Avoid deadlocks with bio allocation by stacking drivers") created a workqueue for every bio set and code in bio_alloc_bioset() that tries to resolve some low-memory deadlocks by redirecting bios queued on current->bio_list to the workqueue if the system is low on memory. However another deadlock (see below **) may happen, without any low memory condition, because generic_make_request is queuing bios to current->bio_list (rather than submitting them). Fix this deadlock by redirecting any bios on current->bio_list to the bio_set's rescue workqueue on every schedule call. Consequently, when the process blocks on a mutex, the bios queued on current->bio_list are dispatched to independent workqueus and they can complete without waiting for the mutex to be available. Also, now we can remove punt_bios_to_rescuer() and bio_alloc_bioset()'s calls to it because bio_alloc_bioset() will implicitly punt all bios on current->bio_list if it performs a blocking allocation. ** Here is the dm-snapshot deadlock that was observed: 1) Process A sends one-page read bio to the dm-snapshot target. The bio spans snapshot chunk boundary and so it is split to two bios by device mapper. 2) Device mapper creates the first sub-bio and sends it to the snapshot driver. 3) The function snapshot_map calls track_chunk (that allocates a structure dm_snap_tracked_chunk and adds it to tracked_chunk_hash) and then remaps the bio to the underlying device and exits with DM_MAPIO_REMAPPED. 4) The remapped bio is submitted with generic_make_request, but it isn't issued - it is added to current->bio_list instead. 5) Meanwhile, process B (dm's kcopyd) executes pending_complete for the chunk affected be the first remapped bio, it takes down_write(&s->lock) and then loops in __check_for_conflicting_io, waiting for dm_snap_tracked_chunk created in step 3) to be released. 6) Process A continues, it creates a second sub-bio for the rest of the original bio. 7) snapshot_map is called for this new bio, it waits on down_write(&s->lock) that is held by Process B (in step 5). Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1267650 Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx> Signed-off-by: Mike Snitzer <snitzer@xxxxxxxxxx> Depends-on: df2cb6daa4 ("block: Avoid deadlocks with bio allocation by stacking drivers") Cc: stable@xxxxxxxxxxxxxxx --- block/bio.c | 77 +++++++++++++++++++------------------------------ include/linux/blkdev.h | 24 ++++++++++----- kernel/sched/core.c | 7 +--- 3 files changed, 50 insertions(+), 58 deletions(-) Index: linux-4.10-rc2/block/bio.c =================================================================== --- linux-4.10-rc2.orig/block/bio.c +++ linux-4.10-rc2/block/bio.c @@ -357,35 +357,37 @@ static void bio_alloc_rescue(struct work } } -static void punt_bios_to_rescuer(struct bio_set *bs) +/** + * blk_flush_bio_list + * @tsk: task_struct whose bio_list must be flushed + * + * Pop bios queued on @tsk->bio_list and submit each of them to + * their rescue workqueue. + * + * If the bio doesn't have a bio_set, we leave it on @tsk->bio_list. + * If the bio is allocated from fs_bio_set, we must leave it to avoid + * deadlock on loopback block device. + * Stacking bio drivers should use bio_set, so this shouldn't be + * an issue. + */ +void blk_flush_bio_list(struct task_struct *tsk) { - struct bio_list punt, nopunt; struct bio *bio; + struct bio_list list = *tsk->bio_list; + bio_list_init(tsk->bio_list); - /* - * In order to guarantee forward progress we must punt only bios that - * were allocated from this bio_set; otherwise, if there was a bio on - * there for a stacking driver higher up in the stack, processing it - * could require allocating bios from this bio_set, and doing that from - * our own rescuer would be bad. - * - * Since bio lists are singly linked, pop them all instead of trying to - * remove from the middle of the list: - */ - - bio_list_init(&punt); - bio_list_init(&nopunt); - - while ((bio = bio_list_pop(current->bio_list))) - bio_list_add(bio->bi_pool == bs ? &punt : &nopunt, bio); - - *current->bio_list = nopunt; - - spin_lock(&bs->rescue_lock); - bio_list_merge(&bs->rescue_list, &punt); - spin_unlock(&bs->rescue_lock); + while ((bio = bio_list_pop(&list))) { + struct bio_set *bs = bio->bi_pool; + if (unlikely(!bs) || bs == fs_bio_set) { + bio_list_add(tsk->bio_list, bio); + continue; + } - queue_work(bs->rescue_workqueue, &bs->rescue_work); + spin_lock(&bs->rescue_lock); + bio_list_add(&bs->rescue_list, bio); + queue_work(bs->rescue_workqueue, &bs->rescue_work); + spin_unlock(&bs->rescue_lock); + } } /** @@ -425,7 +427,6 @@ static void punt_bios_to_rescuer(struct */ struct bio *bio_alloc_bioset(gfp_t gfp_mask, int nr_iovecs, struct bio_set *bs) { - gfp_t saved_gfp = gfp_mask; unsigned front_pad; unsigned inline_vecs; struct bio_vec *bvl = NULL; @@ -459,23 +460,11 @@ struct bio *bio_alloc_bioset(gfp_t gfp_m * reserve. * * We solve this, and guarantee forward progress, with a rescuer - * workqueue per bio_set. If we go to allocate and there are - * bios on current->bio_list, we first try the allocation - * without __GFP_DIRECT_RECLAIM; if that fails, we punt those - * bios we would be blocking to the rescuer workqueue before - * we retry with the original gfp_flags. + * workqueue per bio_set. If an allocation would block (due to + * __GFP_DIRECT_RECLAIM) the scheduler will first punt all bios + * on current->bio_list to the rescuer workqueue. */ - - if (current->bio_list && !bio_list_empty(current->bio_list)) - gfp_mask &= ~__GFP_DIRECT_RECLAIM; - p = mempool_alloc(bs->bio_pool, gfp_mask); - if (!p && gfp_mask != saved_gfp) { - punt_bios_to_rescuer(bs); - gfp_mask = saved_gfp; - p = mempool_alloc(bs->bio_pool, gfp_mask); - } - front_pad = bs->front_pad; inline_vecs = BIO_INLINE_VECS; } @@ -490,12 +479,6 @@ struct bio *bio_alloc_bioset(gfp_t gfp_m unsigned long idx = 0; bvl = bvec_alloc(gfp_mask, nr_iovecs, &idx, bs->bvec_pool); - if (!bvl && gfp_mask != saved_gfp) { - punt_bios_to_rescuer(bs); - gfp_mask = saved_gfp; - bvl = bvec_alloc(gfp_mask, nr_iovecs, &idx, bs->bvec_pool); - } - if (unlikely(!bvl)) goto err_free; Index: linux-4.10-rc2/include/linux/blkdev.h =================================================================== --- linux-4.10-rc2.orig/include/linux/blkdev.h +++ linux-4.10-rc2/include/linux/blkdev.h @@ -1267,6 +1267,22 @@ static inline bool blk_needs_flush_plug( !list_empty(&plug->cb_list)); } +extern void blk_flush_bio_list(struct task_struct *tsk); + +static inline void blk_flush_queued_io(struct task_struct *tsk) +{ + /* + * Flush any queued bios to corresponding rescue threads. + */ + if (tsk->bio_list && !bio_list_empty(tsk->bio_list)) + blk_flush_bio_list(tsk); + /* + * Flush any plugged IO that is queued. + */ + if (blk_needs_flush_plug(tsk)) + blk_schedule_flush_plug(tsk); +} + /* * tag stuff */ @@ -1921,16 +1937,10 @@ static inline void blk_flush_plug(struct { } -static inline void blk_schedule_flush_plug(struct task_struct *task) +static inline void blk_flush_queued_io(struct task_struct *tsk) { } - -static inline bool blk_needs_flush_plug(struct task_struct *tsk) -{ - return false; -} - static inline int blkdev_issue_flush(struct block_device *bdev, gfp_t gfp_mask, sector_t *error_sector) { Index: linux-4.10-rc2/kernel/sched/core.c =================================================================== --- linux-4.10-rc2.orig/kernel/sched/core.c +++ linux-4.10-rc2/kernel/sched/core.c @@ -3441,11 +3441,10 @@ static inline void sched_submit_work(str if (!tsk->state || tsk_is_pi_blocked(tsk)) return; /* - * If we are going to sleep and we have plugged IO queued, + * If we are going to sleep and we have queued IO, * make sure to submit it to avoid deadlocks. */ - if (blk_needs_flush_plug(tsk)) - blk_schedule_flush_plug(tsk); + blk_flush_queued_io(tsk); } asmlinkage __visible void __sched schedule(void) @@ -5068,7 +5067,7 @@ long __sched io_schedule_timeout(long ti long ret; current->in_iowait = 1; - blk_schedule_flush_plug(current); + blk_flush_queued_io(current); delayacct_blkio_start(); rq = raw_rq(); -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
