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SubjectRe: [PATCH RT 1/2 v2] dm-snapshot: fix crash with the realtime kernel
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On 11/12/19 6:09 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).
>
> We can't change hlist_bl to use non-raw spinlocks, this triggers warnings
> in dentry lookup code, because the dentry lookup code uses hlist_bl while
> holding a seqlock.
>
> 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@redhat.com>
> Fixes: f79ae415b64c ("dm snapshot: Make exception tables scalable")
>

Reviewed-by: Nikos Tsironis <ntsironis@arrikto.com>

> ---
> drivers/md/dm-snap.c | 23 +++++++++++++++++++++++
> 1 file changed, 23 insertions(+)
>
> Index: linux-2.6/drivers/md/dm-snap.c
> ===================================================================
> --- linux-2.6.orig/drivers/md/dm-snap.c 2019-11-12 16:44:36.000000000 +0100
> +++ linux-2.6/drivers/md/dm-snap.c 2019-11-12 17:01:46.000000000 +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 +629,46 @@ 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;
> +#else
> + spinlock_t *lock;
> +#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)];
> +#else
> + lock->lock = &s->exception_table_lock;
> +#endif
> }
>
> static void dm_exception_table_lock(struct dm_exception_table_lock *lock)
> {
> +#ifndef CONFIG_PREEMPT_RT_BASE
> hlist_bl_lock(lock->complete_slot);
> hlist_bl_lock(lock->pending_slot);
> +#else
> + spin_lock(lock->lock);
> +#endif
> }
>
> static void dm_exception_table_unlock(struct dm_exception_table_lock *lock)
> {
> +#ifndef CONFIG_PREEMPT_RT_BASE
> hlist_bl_unlock(lock->pending_slot);
> hlist_bl_unlock(lock->complete_slot);
> +#else
> + spin_unlock(lock->lock);
> +#endif
> }
>
> static int dm_exception_table_init(struct dm_exception_table *et,
> @@ -1318,6 +1338,9 @@ 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)) {
>

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