On Fri, 8 Apr 2022 11:50:05 +0800 Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx> > > > > 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"?
