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Message-ID: <e5dbcdfe-c011-cf26-07c8-71dd720eb16a@bytedance.com>
Date: Mon, 11 Apr 2022 15:19:23 +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 8:40 AM, Qi Zheng wrote:
>
>
> 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);
Hi all, is this fix ok? I can send the v2 version if it looks good.
Thanks,
Qi
>
>>
>> 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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