padata: Fix race on sequence number wrap

When padata_do_parallel() is called from multiple cpus for the same
padata instance, we can get object reordering on sequence number wrap
because testing for sequence number wrap and reseting the sequence
number must happen atomically but is implemented with two atomic
operations. This patch fixes this by converting the sequence number
from atomic_t to an unsigned int and protect the access with a
spin_lock. As a side effect, we get rid of the sequence number wrap
handling because the seqence number wraps back to null now without
the need to do anything.

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

1 files changed, 2 insertions, 4 deletions

diff --git a/include/linux/padata.h b/include/linux/padata.h
index 4633b2f726b6..86292beebfe2 100644
--- a/include/linux/padata.h
+++ b/include/linux/padata.h
@@ -46,7 +46,6 @@ struct padata_priv {
         struct list_head        list;
         struct parallel_data    *pd;
         int                     cb_cpu;
-        int                     seq_nr;
         int                     info;
         void                    (*parallel)(struct padata_priv *padata);
         void                    (*serial)(struct padata_priv *padata);
@@ -116,7 +115,6 @@ struct padata_cpumask {
  * @pinst: padata instance.
  * @pqueue: percpu padata queues used for parallelization.
  * @squeue: percpu padata queues used for serialuzation.
- * @seq_nr: The sequence number that will be attached to the next object.
  * @reorder_objects: Number of objects waiting in the reorder queues.
  * @refcnt: Number of objects holding a reference on this parallel_data.
  * @max_seq_nr:  Maximal used sequence number.
@@ -129,12 +127,12 @@ struct parallel_data {
         struct padata_instance          *pinst;
         struct padata_parallel_queue    __percpu *pqueue;
         struct padata_serial_queue      __percpu *squeue;
-        atomic_t                        seq_nr;
         atomic_t                        reorder_objects;
         atomic_t                        refcnt;
-        unsigned int                    max_seq_nr;
         struct padata_cpumask           cpumask;
         spinlock_t                      lock ____cacheline_aligned;
+        spinlock_t                      seq_lock;
+        unsigned int                    seq_nr;
         unsigned int                    processed;
         struct timer_list               timer;
 };