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Date: Sat, 9 Apr 2022 08:40:00 +0800
From: Qi Zheng <zhengqi.arch@...edance.com>
To: Tejun Heo <tj@...nel.org>
Cc: dennis@...nel.org, cl@...ux.com, akpm@...ux-foundation.org,
linux-mm@...ck.org, linux-kernel@...r.kernel.org,
zhouchengming@...edance.com, songmuchun@...edance.com
Subject: Re: [PATCH] percpu_ref: call wake_up_all() after percpu_ref_put()
completes
On 2022/4/9 1:41 AM, Tejun Heo wrote:
> Hello,
>
> On Thu, Apr 07, 2022 at 06:33:35PM +0800, Qi Zheng wrote:
>> In the percpu_ref_call_confirm_rcu(), we call the wake_up_all()
>> before calling percpu_ref_put(), which will cause the value of
>> percpu_ref to be unstable when percpu_ref_switch_to_atomic_sync()
>> returns.
>>
>> CPU0 CPU1
>>
>> percpu_ref_switch_to_atomic_sync(&ref)
>> --> percpu_ref_switch_to_atomic(&ref)
>> --> percpu_ref_get(ref); /* put after confirmation */
>> call_rcu(&ref->data->rcu, percpu_ref_switch_to_atomic_rcu);
>>
>> percpu_ref_switch_to_atomic_rcu
>> --> percpu_ref_call_confirm_rcu
>> --> data->confirm_switch = NULL;
>> wake_up_all(&percpu_ref_switch_waitq);
>>
>> /* here waiting to wake up */
>> wait_event(percpu_ref_switch_waitq, !ref->data->confirm_switch);
>> (A)percpu_ref_put(ref);
>> /* The value of &ref is unstable! */
>> percpu_ref_is_zero(&ref)
>> (B)percpu_ref_put(ref);
>>
>> As shown above, assuming that the counts on each cpu add up to 0 before
>> calling percpu_ref_switch_to_atomic_sync(), we expect that after switching
>> to atomic mode, percpu_ref_is_zero() can return true. But actually it will
>> return different values in the two cases of A and B, which is not what
>> we expected.
>>
>> Maybe the original purpose of percpu_ref_switch_to_atomic_sync() is
>> just to ensure that the conversion to atomic mode is completed, but it
>> should not return with an extra reference count.
>>
>> Calling wake_up_all() after percpu_ref_put() ensures that the value of
>> percpu_ref is stable after percpu_ref_switch_to_atomic_sync() returns.
>> So just do it.
>>
>> Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
>> ---
>> lib/percpu-refcount.c | 3 ++-
>> 1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/lib/percpu-refcount.c b/lib/percpu-refcount.c
>> index af9302141bcf..b11b4152c8cd 100644
>> --- a/lib/percpu-refcount.c
>> +++ b/lib/percpu-refcount.c
>> @@ -154,13 +154,14 @@ static void percpu_ref_call_confirm_rcu(struct rcu_head *rcu)
>>
>> data->confirm_switch(ref);
>> data->confirm_switch = NULL;
>> - wake_up_all(&percpu_ref_switch_waitq);
>>
>> if (!data->allow_reinit)
>> __percpu_ref_exit(ref);
>>
>> /* drop ref from percpu_ref_switch_to_atomic() */
>> percpu_ref_put(ref);
>> +
>> + wake_up_all(&percpu_ref_switch_waitq);
>
> The interface, at least originally, doesn't give any guarantee over whether
> there's gonna be a residual reference on it or not. There's nothing
> necessarily wrong with guaranteeing that but it's rather unusual and given
> that putting the base ref in a percpu_ref is a special "kill" operation and
> a ref in percpu mode always returns %false on is_zero(), I'm not quite sure
> how such semantics would be useful. Do you care to explain the use case with
> concrete examples?
There are currently two users of percpu_ref_switch_to_atomic_sync(), and
both are used in the example, one is mddev->writes_pending in
driver/md/md.c and the other is q->q_usage_counter in block/blk-pm.c.
The former discards the initial reference count after percpu_ref_init(),
and the latter kills the initial reference count(by calling
percpu_ref_kill() in blk_freeze_queue_start()) before
percpu_ref_switch_to_atomic_sync(). Looks like they all expect
percpu_ref to be stable when percpu_ref_switch_to_atomic_sync() returns.
>
> Also, the proposed patch is racy. There's nothing preventing
> percpu_ref_switch_to_atomic_sync() from waking up early between
> confirm_switch clearing and the wake_up_all, so the above change doesn't
> guarantee what it tries to guarantee. For that, you'd have to move
> confirm_switch clearing *after* percpu_ref_put() but then, you'd be
> accessing the ref after its final ref is put which can lead to
> use-after-free.
>
Oh sorry, it is my bad missing.
> In fact, the whole premise seems wrong. The switching needs a reference to
> the percpu_ref because it is accessing it asynchronously. The switching side
> doesn't know when the ref is gonna go away once it puts its reference and
> thus can't signal that they're done after putting their reference.
>
> We *can* make that work by putting the whole thing in its own critical
> section so that we can make confirm_switch clearing atomic with the possibly
> final put, but that's gonna add some complexity and begs the question why
> we'd need such a thing.
How about moving the last percpu_ref_put() outside of the
percpu_ref_switch_to_atomic_rcu() in sync mode like below? But this may
not be elegant.
diff --git a/include/linux/percpu-refcount.h
b/include/linux/percpu-refcount.h
index d73a1c08c3e3..07f92e7e3e19 100644
--- a/include/linux/percpu-refcount.h
+++ b/include/linux/percpu-refcount.h
@@ -98,6 +98,7 @@ struct percpu_ref_data {
percpu_ref_func_t *confirm_switch;
bool force_atomic:1;
bool allow_reinit:1;
+ bool sync;
struct rcu_head rcu;
struct percpu_ref *ref;
};
@@ -123,7 +124,8 @@ int __must_check percpu_ref_init(struct percpu_ref *ref,
gfp_t gfp);
void percpu_ref_exit(struct percpu_ref *ref);
void percpu_ref_switch_to_atomic(struct percpu_ref *ref,
- percpu_ref_func_t *confirm_switch);
+ percpu_ref_func_t *confirm_switch,
+ bool sync);
void percpu_ref_switch_to_atomic_sync(struct percpu_ref *ref);
void percpu_ref_switch_to_percpu(struct percpu_ref *ref);
void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
diff --git a/lib/percpu-refcount.c b/lib/percpu-refcount.c
index af9302141bcf..2a9d777bcf35 100644
--- a/lib/percpu-refcount.c
+++ b/lib/percpu-refcount.c
@@ -99,6 +99,7 @@ int percpu_ref_init(struct percpu_ref *ref,
percpu_ref_func_t *release,
data->release = release;
data->confirm_switch = NULL;
data->ref = ref;
+ data->sync = false;
ref->data = data;
return 0;
}
@@ -146,21 +147,30 @@ void percpu_ref_exit(struct percpu_ref *ref)
}
EXPORT_SYMBOL_GPL(percpu_ref_exit);
+static inline void percpu_ref_switch_to_atomic_post(struct percpu_ref *ref)
+{
+ struct percpu_ref_data *data = ref->data;
+
+ if (!data->allow_reinit)
+ __percpu_ref_exit(ref);
+
+ /* drop ref from percpu_ref_switch_to_atomic() */
+ percpu_ref_put(ref);
+}
+
static void percpu_ref_call_confirm_rcu(struct rcu_head *rcu)
{
struct percpu_ref_data *data = container_of(rcu,
struct percpu_ref_data, rcu);
struct percpu_ref *ref = data->ref;
+ bool need_put = !data->sync;
data->confirm_switch(ref);
data->confirm_switch = NULL;
wake_up_all(&percpu_ref_switch_waitq);
- if (!data->allow_reinit)
- __percpu_ref_exit(ref);
-
- /* drop ref from percpu_ref_switch_to_atomic() */
- percpu_ref_put(ref);
+ if (need_put)
+ percpu_ref_switch_to_atomic_post(ref);
}
static void percpu_ref_switch_to_atomic_rcu(struct rcu_head *rcu)
@@ -302,12 +312,14 @@ static void __percpu_ref_switch_mode(struct
percpu_ref *ref,
* switching to atomic mode, this function can be called from any context.
*/
void percpu_ref_switch_to_atomic(struct percpu_ref *ref,
- percpu_ref_func_t *confirm_switch)
+ percpu_ref_func_t *confirm_switch,
+ bool sync)
{
unsigned long flags;
spin_lock_irqsave(&percpu_ref_switch_lock, flags);
+ ref->data->sync = sync;
ref->data->force_atomic = true;
__percpu_ref_switch_mode(ref, confirm_switch);
@@ -325,8 +337,9 @@ EXPORT_SYMBOL_GPL(percpu_ref_switch_to_atomic);
*/
void percpu_ref_switch_to_atomic_sync(struct percpu_ref *ref)
{
- percpu_ref_switch_to_atomic(ref, NULL);
+ percpu_ref_switch_to_atomic(ref, NULL, true);
wait_event(percpu_ref_switch_waitq, !ref->data->confirm_switch);
+ percpu_ref_switch_to_atomic_post(ref);
}
EXPORT_SYMBOL_GPL(percpu_ref_switch_to_atomic_sync);
>
> Andrew, I don't think the patch as proposed makes much sense. Maybe it'd be
> better to keep it out of the tree for the time being?
>
> Thanks.
>
--
Thanks,
Qi
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