[PATCH] padata: Use RCU when fetching pd from do_serial

[Herbert Xu]

On Tue, Jul 16, 2019 at 01:14:10PM +0200, Steffen Klassert wrote:
>
> Maybe we can fix it if we call padata_free_pd() from
> padata_serial_worker() when it sent out the last object.

How about using RCU?

We still need to fix up the refcnt if it's supposed to limit the
overall number of outstanding requests.

---8<---
The function padata_do_serial uses parallel_data without obeying
the RCU rules around its life-cycle.  This means that a concurrent
padata_replace call can result in a crash.

This patch fixes it by using RCU just as we do in padata_do_parallel.

Fixes: 16295bec6398 ("padata: Generic parallelization/...")
Signed-off-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>

diff --git a/include/linux/padata.h b/include/linux/padata.h
index 5d13d25da2c8..952f6514dd72 100644
--- a/include/linux/padata.h
+++ b/include/linux/padata.h
@@ -35,7 +35,7 @@
  * struct padata_priv -  Embedded to the users data structure.
  *
  * @list: List entry, to attach to the padata lists.
- * @pd: Pointer to the internal control structure.
+ * @inst: Pointer to the overall control structure.
  * @cb_cpu: Callback cpu for serializatioon.
  * @cpu: Cpu for parallelization.
  * @seq_nr: Sequence number of the parallelized data object.
@@ -45,7 +45,7 @@
  */
 struct padata_priv {
 	struct list_head	list;
-	struct parallel_data	*pd;
+	struct padata_instance	*inst;
 	int			cb_cpu;
 	int			cpu;
 	int			info;
diff --git a/kernel/padata.c b/kernel/padata.c
index 2d2fddbb7a4c..fb5dd1210d2b 100644
--- a/kernel/padata.c
+++ b/kernel/padata.c
@@ -128,7 +128,7 @@ int padata_do_parallel(struct padata_instance *pinst,
 
 	err = 0;
 	atomic_inc(&pd->refcnt);
-	padata->pd = pd;
+	padata->inst = pinst;
 	padata->cb_cpu = cb_cpu;
 
 	target_cpu = padata_cpu_hash(pd);
@@ -367,7 +368,7 @@ void padata_do_serial(struct padata_priv *padata)
 	struct parallel_data *pd;
 	int reorder_via_wq = 0;
 
-	pd = padata->pd;
+	pd = rcu_dereference_bh(padata->inst->pd);
 
 	cpu = get_cpu();
 
-- 
Email: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt