On Mon, Nov 11 2019 at 8:59am -0500,
Mikulas Patocka <mpatocka@xxxxxxxxxx> 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 <mpatocka@xxxxxxxxxx>
> Fixes: f79ae415b64c ("dm snapshot: Make exception tables scalable")
>
> ---
> 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
> };
Why not put the spinlock_t in 'struct dm_exception_table_lock' with the
member name 'lock'?
> 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;
> }
That way you don't need the extra 'struct dm_snapshot' arg to all the
various dm_exception_table_{lock,unlock} calls.
> @@ -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)) {
And this spin_lock_init() would go in dm_exception_table_lock_init()
in appropriate #ifdef with spin_lock_init(&lock->lock)
Doing it that way would seriously reduce the size of this patch.
Unless I'm missing something, please submit a v2 and cc linux-rt-user
mailing list and the other direct CCs suggested by others in reply to
patch 2/2.
Thanks,
Mike
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