The patch below does not apply to the 4.9-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From b21555786f18cd77f2311ad89074533109ae3ffa Mon Sep 17 00:00:00 2001

From: Mikulas Patocka mpatocka@redhat.com Date: Wed, 2 Oct 2019 06:15:53 -0400 Subject: [PATCH] dm snapshot: rework COW throttling to fix deadlock

Commit 721b1d98fb517a ("dm snapshot: Fix excessive memory usage and workqueue stalls") introduced a semaphore to limit the maximum number of in-flight kcopyd (COW) jobs.

The implementation of this throttling mechanism is prone to a deadlock:

1. One or more threads write to the origin device causing COW, which is performed by kcopyd.

2. At some point some of these threads might reach the s->cow_count semaphore limit and block in down(&s->cow_count), holding a read lock on _origins_lock.

3. Someone tries to acquire a write lock on _origins_lock, e.g., snapshot_ctr(), which blocks because the threads at step (2) already hold a read lock on it.

4. A COW operation completes and kcopyd runs dm-snapshot's completion callback, which ends up calling pending_complete(). pending_complete() tries to resubmit any deferred origin bios. This requires acquiring a read lock on _origins_lock, which blocks.

This happens because the read-write semaphore implementation gives priority to writers, meaning that as soon as a writer tries to enter the critical section, no readers will be allowed in, until all writers have completed their work.

So, pending_complete() waits for the writer at step (3) to acquire and release the lock. This writer waits for the readers at step (2) to release the read lock and those readers wait for pending_complete() (the kcopyd thread) to signal the s->cow_count semaphore: DEADLOCK.

The above was thoroughly analyzed and documented by Nikos Tsironis as part of his initial proposal for fixing this deadlock, see: https://www.redhat.com/archives/dm-devel/2019-October/msg00001.html

Fix this deadlock by reworking COW throttling so that it waits without holding any locks. Add a variable 'in_progress' that counts how many kcopyd jobs are running. A function wait_for_in_progress() will sleep if 'in_progress' is over the limit. It drops _origins_lock in order to avoid the deadlock.

Reported-by: Guruswamy Basavaiah guru2018@gmail.com Reported-by: Nikos Tsironis ntsironis@arrikto.com Reviewed-by: Nikos Tsironis ntsironis@arrikto.com Tested-by: Nikos Tsironis ntsironis@arrikto.com Fixes: 721b1d98fb51 ("dm snapshot: Fix excessive memory usage and workqueue stalls") Cc: stable@vger.kernel.org # v5.0+ Depends-on: 4a3f111a73a8c ("dm snapshot: introduce account_start_copy() and account_end_copy()") Signed-off-by: Mikulas Patocka mpatocka@redhat.com Signed-off-by: Mike Snitzer snitzer@redhat.com

diff --git a/drivers/md/dm-snap.c b/drivers/md/dm-snap.c index da3bd1794ee0..4fb1a40e68a0 100644 --- a/drivers/md/dm-snap.c +++ b/drivers/md/dm-snap.c @@ -18,7 +18,6 @@ #include <linux/vmalloc.h> #include <linux/log2.h> #include <linux/dm-kcopyd.h> -#include <linux/semaphore.h>

#include "dm.h"

@@ -107,8 +106,8 @@ struct dm_snapshot { /* The on disk metadata handler */ struct dm_exception_store *store;

- /* Maximum number of in-flight COW jobs. */ - struct semaphore cow_count; + unsigned in_progress; + struct wait_queue_head in_progress_wait;

struct dm_kcopyd_client *kcopyd_client;

@@ -162,8 +161,8 @@ struct dm_snapshot { */ #define DEFAULT_COW_THRESHOLD 2048

-static int cow_threshold = DEFAULT_COW_THRESHOLD; -module_param_named(snapshot_cow_threshold, cow_threshold, int, 0644); +static unsigned cow_threshold = DEFAULT_COW_THRESHOLD; +module_param_named(snapshot_cow_threshold, cow_threshold, uint, 0644); MODULE_PARM_DESC(snapshot_cow_threshold, "Maximum number of chunks being copied on write");

DECLARE_DM_KCOPYD_THROTTLE_WITH_MODULE_PARM(snapshot_copy_throttle, @@ -1327,7 +1326,7 @@ static int snapshot_ctr(struct dm_target *ti, unsigned int argc, char **argv) goto bad_hash_tables; }

- sema_init(&s->cow_count, (cow_threshold > 0) ? cow_threshold : INT_MAX); + init_waitqueue_head(&s->in_progress_wait);

s->kcopyd_client = dm_kcopyd_client_create(&dm_kcopyd_throttle); if (IS_ERR(s->kcopyd_client)) { @@ -1509,17 +1508,54 @@ static void snapshot_dtr(struct dm_target *ti)

dm_put_device(ti, s->origin);

+ WARN_ON(s->in_progress); + kfree(s); }

static void account_start_copy(struct dm_snapshot *s) { - down(&s->cow_count); + spin_lock(&s->in_progress_wait.lock); + s->in_progress++; + spin_unlock(&s->in_progress_wait.lock); }

static void account_end_copy(struct dm_snapshot *s) { - up(&s->cow_count); + spin_lock(&s->in_progress_wait.lock); + BUG_ON(!s->in_progress); + s->in_progress--; + if (likely(s->in_progress <= cow_threshold) && + unlikely(waitqueue_active(&s->in_progress_wait))) + wake_up_locked(&s->in_progress_wait); + spin_unlock(&s->in_progress_wait.lock); +} + +static bool wait_for_in_progress(struct dm_snapshot *s, bool unlock_origins) +{ + if (unlikely(s->in_progress > cow_threshold)) { + spin_lock(&s->in_progress_wait.lock); + if (likely(s->in_progress > cow_threshold)) { + /* + * NOTE: this throttle doesn't account for whether + * the caller is servicing an IO that will trigger a COW + * so excess throttling may result for chunks not required + * to be COW'd. But if cow_threshold was reached, extra + * throttling is unlikely to negatively impact performance. + */ + DECLARE_WAITQUEUE(wait, current); + __add_wait_queue(&s->in_progress_wait, &wait); + __set_current_state(TASK_UNINTERRUPTIBLE); + spin_unlock(&s->in_progress_wait.lock); + if (unlock_origins) + up_read(&_origins_lock); + io_schedule(); + remove_wait_queue(&s->in_progress_wait, &wait); + return false; + } + spin_unlock(&s->in_progress_wait.lock); + } + return true; }

/* @@ -1537,7 +1573,7 @@ static void flush_bios(struct bio *bio) } }

-static int do_origin(struct dm_dev *origin, struct bio *bio); +static int do_origin(struct dm_dev *origin, struct bio *bio, bool limit);

/* * Flush a list of buffers. @@ -1550,7 +1586,7 @@ static void retry_origin_bios(struct dm_snapshot *s, struct bio *bio) while (bio) { n = bio->bi_next; bio->bi_next = NULL; - r = do_origin(s->origin, bio); + r = do_origin(s->origin, bio, false); if (r == DM_MAPIO_REMAPPED) generic_make_request(bio); bio = n; @@ -1926,6 +1962,11 @@ static int snapshot_map(struct dm_target *ti, struct bio *bio) if (!s->valid) return DM_MAPIO_KILL;

+ if (bio_data_dir(bio) == WRITE) { + while (unlikely(!wait_for_in_progress(s, false))) + ; /* wait_for_in_progress() has slept */ + } + down_read(&s->lock); dm_exception_table_lock(&lock);

@@ -2122,7 +2163,7 @@ static int snapshot_merge_map(struct dm_target *ti, struct bio *bio)

if (bio_data_dir(bio) == WRITE) { up_write(&s->lock); - return do_origin(s->origin, bio); + return do_origin(s->origin, bio, false); }

out_unlock: @@ -2497,15 +2538,24 @@ static int __origin_write(struct list_head *snapshots, sector_t sector, /* * Called on a write from the origin driver. */ -static int do_origin(struct dm_dev *origin, struct bio *bio) +static int do_origin(struct dm_dev *origin, struct bio *bio, bool limit) { struct origin *o; int r = DM_MAPIO_REMAPPED;

+again: down_read(&_origins_lock); o = __lookup_origin(origin->bdev); - if (o) + if (o) { + if (limit) { + struct dm_snapshot *s; + list_for_each_entry(s, &o->snapshots, list) + if (unlikely(!wait_for_in_progress(s, true))) + goto again; + } + r = __origin_write(&o->snapshots, bio->bi_iter.bi_sector, bio); + } up_read(&_origins_lock);

return r; @@ -2618,7 +2668,7 @@ static int origin_map(struct dm_target *ti, struct bio *bio) dm_accept_partial_bio(bio, available_sectors);

/* Only tell snapshots if this is a write */ - return do_origin(o->dev, bio); + return do_origin(o->dev, bio, true); }

/*