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Subject[v2 PATCH] padata: Use RCU when fetching pd from do_serial
On Tue, Jul 16, 2019 at 09:09:28PM +0800, Herbert Xu wrote:
>
> Hmm, it doesn't work because the refcnt is attached to the old
> pd. That shouldn't be a problem though as we could simply ignore
> the refcnt in padata_flush_queue.

This should fix it:

---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.

As the refcnt may now span two parallel_data structures, this patch
moves it to padata_instance instead. FWIW the refcnt is used to
limit the number of outstanding requests (albeit a soft limit as
we don't do a proper atomic inc and test).

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

diff --git a/include/linux/padata.h b/include/linux/padata.h
index 5d13d25da2c8..ce51555cb86c 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;
@@ -120,7 +120,6 @@ struct padata_cpumask {
* @pqueue: percpu padata queues used for parallelization.
* @squeue: percpu padata queues used for serialuzation.
* @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.
* @cpumask: The cpumasks in use for parallel and serial workers.
* @lock: Reorder lock.
@@ -132,7 +131,6 @@ struct parallel_data {
struct padata_parallel_queue __percpu *pqueue;
struct padata_serial_queue __percpu *squeue;
atomic_t reorder_objects;
- atomic_t refcnt;
atomic_t seq_nr;
struct padata_cpumask cpumask;
spinlock_t lock ____cacheline_aligned;
@@ -152,6 +150,7 @@ struct parallel_data {
* or both cpumasks change.
* @kobj: padata instance kernel object.
* @lock: padata instance lock.
+ * @refcnt: Number of objects holding a reference on this padata_instance.
* @flags: padata flags.
*/
struct padata_instance {
@@ -162,6 +161,7 @@ struct padata_instance {
struct blocking_notifier_head cpumask_change_notifier;
struct kobject kobj;
struct mutex lock;
+ atomic_t refcnt;
u8 flags;
#define PADATA_INIT 1
#define PADATA_RESET 2
diff --git a/kernel/padata.c b/kernel/padata.c
index 2d2fddbb7a4c..c86333fa3cec 100644
--- a/kernel/padata.c
+++ b/kernel/padata.c
@@ -123,12 +123,12 @@ int padata_do_parallel(struct padata_instance *pinst,
if ((pinst->flags & PADATA_RESET))
goto out;

- if (atomic_read(&pd->refcnt) >= MAX_OBJ_NUM)
+ if (atomic_read(&pinst->refcnt) >= MAX_OBJ_NUM)
goto out;

err = 0;
- atomic_inc(&pd->refcnt);
- padata->pd = pd;
+ atomic_inc(&pinst->refcnt);
+ padata->inst = pinst;
padata->cb_cpu = cb_cpu;

target_cpu = padata_cpu_hash(pd);
@@ -347,7 +347,7 @@ static void padata_serial_worker(struct work_struct *serial_work)
list_del_init(&padata->list);

padata->serial(padata);
- atomic_dec(&pd->refcnt);
+ atomic_dec(&pd->pinst->refcnt);
}
local_bh_enable();
}
@@ -367,7 +367,7 @@ 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();

@@ -489,7 +489,6 @@ static struct parallel_data *padata_alloc_pd(struct padata_instance *pinst,
timer_setup(&pd->timer, padata_reorder_timer, 0);
atomic_set(&pd->seq_nr, -1);
atomic_set(&pd->reorder_objects, 0);
- atomic_set(&pd->refcnt, 0);
pd->pinst = pinst;
spin_lock_init(&pd->lock);

@@ -535,8 +534,6 @@ static void padata_flush_queues(struct parallel_data *pd)
squeue = per_cpu_ptr(pd->squeue, cpu);
flush_work(&squeue->work);
}
-
- BUG_ON(atomic_read(&pd->refcnt) != 0);
}

static void __padata_start(struct padata_instance *pinst)
--
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
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