On 11/11/19 3:59 PM, Mikulas Patocka wrote:
> Snapshot doesn't work with realtime kernels since the commit f79ae415b64c.
> hlist_bl is implemented as a raw spinlock and the code takes two non-raw
> spinlocks while holding hlist_bl (non-raw spinlocks are blocking mutexes
> in the realtime kernel, so they couldn't be taken inside a raw spinlock).
>
> This patch fixes the problem by using non-raw spinlock
> exception_table_lock instead of the hlist_bl lock.
>
> Signed-off-by: Mikulas Patocka <[email protected]>
> Fixes: f79ae415b64c ("dm snapshot: Make exception tables scalable")
>
Hi Mikulas,
I wasn't aware that hlist_bl is implemented as a raw spinlock in the
real time kernel. I would expect it to be a standard non-raw spinlock,
so everything works as expected. But, after digging further in the real
time tree, I found commit ad7675b15fd87f1 ("list_bl: Make list head
locking RT safe") which suggests that such a conversion would break
other parts of the kernel.
That said,
Reviewed-by: Nikos Tsironis <[email protected]>
> ---
> drivers/md/dm-snap.c | 65
> ++++++++++++++++++++++++++++++++-------------------
> 1 file changed, 42 insertions(+), 23 deletions(-)
>
> Index: linux-2.6/drivers/md/dm-snap.c
> ===================================================================
> --- linux-2.6.orig/drivers/md/dm-snap.c 2019-11-08 15:51:42.000000000
> +0100
> +++ linux-2.6/drivers/md/dm-snap.c 2019-11-08 15:54:58.000000000 +0100
> @@ -141,6 +141,10 @@ struct dm_snapshot {
> * for them to be committed.
> */
> struct bio_list bios_queued_during_merge;
> +
> +#ifdef CONFIG_PREEMPT_RT_BASE
> + spinlock_t exception_table_lock;
> +#endif
> };
>
> /*
> @@ -625,30 +629,42 @@ static uint32_t exception_hash(struct dm
>
> /* Lock to protect access to the completed and pending exception hash
> tables. */
> struct dm_exception_table_lock {
> +#ifndef CONFIG_PREEMPT_RT_BASE
> struct hlist_bl_head *complete_slot;
> struct hlist_bl_head *pending_slot;
> +#endif
> };
>
> static void dm_exception_table_lock_init(struct dm_snapshot *s, chunk_t
> chunk,
> struct dm_exception_table_lock *lock)
> {
> +#ifndef CONFIG_PREEMPT_RT_BASE
> struct dm_exception_table *complete = &s->complete;
> struct dm_exception_table *pending = &s->pending;
>
> lock->complete_slot = &complete->table[exception_hash(complete, chunk)];
> lock->pending_slot = &pending->table[exception_hash(pending, chunk)];
> +#endif
> }
>
> -static void dm_exception_table_lock(struct dm_exception_table_lock *lock)
> +static void dm_exception_table_lock(struct dm_snapshot *s, struct
> dm_exception_table_lock *lock)
> {
> +#ifdef CONFIG_PREEMPT_RT_BASE
> + spin_lock(&s->exception_table_lock);
> +#else
> hlist_bl_lock(lock->complete_slot);
> hlist_bl_lock(lock->pending_slot);
> +#endif
> }
>
> -static void dm_exception_table_unlock(struct dm_exception_table_lock *lock)
> +static void dm_exception_table_unlock(struct dm_snapshot *s, struct
> dm_exception_table_lock *lock)
> {
> +#ifdef CONFIG_PREEMPT_RT_BASE
> + spin_unlock(&s->exception_table_lock);
> +#else
> hlist_bl_unlock(lock->pending_slot);
> hlist_bl_unlock(lock->complete_slot);
> +#endif
> }
>
> static int dm_exception_table_init(struct dm_exception_table *et,
> @@ -835,9 +851,9 @@ static int dm_add_exception(void *contex
> */
> dm_exception_table_lock_init(s, old, &lock);
>
> - dm_exception_table_lock(&lock);
> + dm_exception_table_lock(s, &lock);
> dm_insert_exception(&s->complete, e);
> - dm_exception_table_unlock(&lock);
> + dm_exception_table_unlock(s, &lock);
>
> return 0;
> }
> @@ -1318,6 +1334,9 @@ static int snapshot_ctr(struct dm_target
> s->first_merging_chunk = 0;
> s->num_merging_chunks = 0;
> bio_list_init(&s->bios_queued_during_merge);
> +#ifdef CONFIG_PREEMPT_RT_BASE
> + spin_lock_init(&s->exception_table_lock);
> +#endif
>
> /* Allocate hash table for COW data */
> if (init_hash_tables(s)) {
> @@ -1651,7 +1670,7 @@ static void pending_complete(void *conte
> invalidate_snapshot(s, -EIO);
> error = 1;
>
> - dm_exception_table_lock(&lock);
> + dm_exception_table_lock(s, &lock);
> goto out;
> }
>
> @@ -1660,13 +1679,13 @@ static void pending_complete(void *conte
> invalidate_snapshot(s, -ENOMEM);
> error = 1;
>
> - dm_exception_table_lock(&lock);
> + dm_exception_table_lock(s, &lock);
> goto out;
> }
> *e = pe->e;
>
> down_read(&s->lock);
> - dm_exception_table_lock(&lock);
> + dm_exception_table_lock(s, &lock);
> if (!s->valid) {
> up_read(&s->lock);
> free_completed_exception(e);
> @@ -1687,16 +1706,16 @@ static void pending_complete(void *conte
>
> /* Wait for conflicting reads to drain */
> if (__chunk_is_tracked(s, pe->e.old_chunk)) {
> - dm_exception_table_unlock(&lock);
> + dm_exception_table_unlock(s, &lock);
> __check_for_conflicting_io(s, pe->e.old_chunk);
> - dm_exception_table_lock(&lock);
> + dm_exception_table_lock(s, &lock);
> }
>
> out:
> /* Remove the in-flight exception from the list */
> dm_remove_exception(&pe->e);
>
> - dm_exception_table_unlock(&lock);
> + dm_exception_table_unlock(s, &lock);
>
> snapshot_bios = bio_list_get(&pe->snapshot_bios);
> origin_bios = bio_list_get(&pe->origin_bios);
> @@ -1968,7 +1987,7 @@ static int snapshot_map(struct dm_target
> }
>
> down_read(&s->lock);
> - dm_exception_table_lock(&lock);
> + dm_exception_table_lock(s, &lock);
>
> if (!s->valid || (unlikely(s->snapshot_overflowed) &&
> bio_data_dir(bio) == WRITE)) {
> @@ -1997,7 +2016,7 @@ static int snapshot_map(struct dm_target
> remap_exception(s, e, bio, chunk);
> if (unlikely(bio_op(bio) == REQ_OP_DISCARD) &&
> io_overlaps_chunk(s, bio)) {
> - dm_exception_table_unlock(&lock);
> + dm_exception_table_unlock(s, &lock);
> up_read(&s->lock);
> zero_exception(s, e, bio, chunk);
> r = DM_MAPIO_SUBMITTED; /* discard is not issued */
> @@ -2024,9 +2043,9 @@ static int snapshot_map(struct dm_target
> if (bio_data_dir(bio) == WRITE) {
> pe = __lookup_pending_exception(s, chunk);
> if (!pe) {
> - dm_exception_table_unlock(&lock);
> + dm_exception_table_unlock(s, &lock);
> pe = alloc_pending_exception(s);
> - dm_exception_table_lock(&lock);
> + dm_exception_table_lock(s, &lock);
>
> e = dm_lookup_exception(&s->complete, chunk);
> if (e) {
> @@ -2037,7 +2056,7 @@ static int snapshot_map(struct dm_target
>
> pe = __find_pending_exception(s, pe, chunk);
> if (!pe) {
> - dm_exception_table_unlock(&lock);
> + dm_exception_table_unlock(s, &lock);
> up_read(&s->lock);
>
> down_write(&s->lock);
> @@ -2063,7 +2082,7 @@ static int snapshot_map(struct dm_target
> if (!pe->started && io_overlaps_chunk(s, bio)) {
> pe->started = 1;
>
> - dm_exception_table_unlock(&lock);
> + dm_exception_table_unlock(s, &lock);
> up_read(&s->lock);
>
> start_full_bio(pe, bio);
> @@ -2076,7 +2095,7 @@ static int snapshot_map(struct dm_target
> /* this is protected by the exception table lock */
> pe->started = 1;
>
> - dm_exception_table_unlock(&lock);
> + dm_exception_table_unlock(s, &lock);
> up_read(&s->lock);
>
> start_copy(pe);
> @@ -2088,7 +2107,7 @@ static int snapshot_map(struct dm_target
> }
>
> out_unlock:
> - dm_exception_table_unlock(&lock);
> + dm_exception_table_unlock(s, &lock);
> up_read(&s->lock);
> out:
> return r;
> @@ -2449,7 +2468,7 @@ static int __origin_write(struct list_he
> dm_exception_table_lock_init(snap, chunk, &lock);
>
> down_read(&snap->lock);
> - dm_exception_table_lock(&lock);
> + dm_exception_table_lock(snap, &lock);
>
> /* Only deal with valid and active snapshots */
> if (!snap->valid || !snap->active)
> @@ -2466,9 +2485,9 @@ static int __origin_write(struct list_he
> if (e)
> goto next_snapshot;
>
> - dm_exception_table_unlock(&lock);
> + dm_exception_table_unlock(snap, &lock);
> pe = alloc_pending_exception(snap);
> - dm_exception_table_lock(&lock);
> + dm_exception_table_lock(snap, &lock);
>
> pe2 = __lookup_pending_exception(snap, chunk);
>
> @@ -2481,7 +2500,7 @@ static int __origin_write(struct list_he
>
> pe = __insert_pending_exception(snap, pe,
> chunk);
> if (!pe) {
> - dm_exception_table_unlock(&lock);
> + dm_exception_table_unlock(snap, &lock);
> up_read(&snap->lock);
>
> invalidate_snapshot(snap, -ENOMEM);
> @@ -2516,7 +2535,7 @@ static int __origin_write(struct list_he
> }
>
> next_snapshot:
> - dm_exception_table_unlock(&lock);
> + dm_exception_table_unlock(snap, &lock);
> up_read(&snap->lock);
>
> if (pe_to_start_now) {
>
--
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