On Tue, 12 Nov 2019, Mike Snitzer wrote:
> 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'?
struct dm_exception_table_lock is allocated temporarily on the stack - we
can't put locks into it, because every user uses different structurer.
However, I can put pointer to to the spinlock to this structure. It
shortens the patch - because then we don't have to pass a pointer to
struct dm_snapshot to dm_exception_table_lock and
dm_exception_table_unlock.
> > 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)
dm_exception_table_lock_init initializes an on-stack structure. It can't
contain locks.
> Doing it that way would seriously reduce the size of this patch.
I reduced the size and I'll send next version.
> 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
Mikulas
--
dm-devel mailing list
dm-devel@xxxxxxxxxx
https://www.redhat.com/mailman/listinfo/dm-devel
