[dm-devel] [PATCH 1/2] dm-snapshot: fix crash with the realtime kernel
Mike Snitzer
snitzer at redhat.com
Tue Nov 12 16:06:16 UTC 2019
On Tue, Nov 12 2019 at 10:57am -0500,
Mikulas Patocka <mpatocka at redhat.com> wrote:
>
>
> On Tue, 12 Nov 2019, Mike Snitzer wrote:
>
> > On Mon, Nov 11 2019 at 8:59am -0500,
> > Mikulas Patocka <mpatocka at redhat.com> 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 at redhat.com>
> > > 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.
OK, I should've looked at the dm-snap.c code with more context, thanks
for clarifying.
> > > 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.
Sounds good.
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