Messages in this thread | | | Date | Fri, 8 Apr 2022 12:06:20 +0800 | Subject | Re: [PATCH] percpu_ref: call wake_up_all() after percpu_ref_put() completes | From | Qi Zheng <> |
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On 2022/4/8 11:54 AM, Andrew Morton wrote: > On Fri, 8 Apr 2022 11:50:05 +0800 Qi Zheng <zhengqi.arch@bytedance.com> wrote: > >> >> >> On 2022/4/8 10:54 AM, Muchun Song wrote: >>> 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@bytedance.com> >>> >>> Are any users affected by this? If so, I think a Fixes tag >>> is necessary. >> >> Looks all current users(blk_pre_runtime_suspend() and set_in_sync()) are >> affected by this. >> >> I see that this patch has been merged into the mm tree, can Andrew help >> me add the following Fixes tag? > > Andrew is helpful ;) > > Do you see reasons why we should backport this into -stable trees? > It's 8 years old, so my uninformed guess is "no"?
Hmm, although the commit 490c79a65708 add wake_up_all(), it is no problem for the usage at that time, maybe the correct Fixes tag is the following:
Fixes: 210f7cdcf088 ("percpu-refcount: support synchronous switch to atomic mode.")
But in fact, there is no problem with it, but all current users expect the refcount is stable after percpu_ref_switch_to_atomic_sync() returns.
I have no idea as which Fixes tag to add.
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-- Thanks, Qi
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