Hi Ming On 09/11/2018 12:11 AM, Ming Lei wrote: > Hi Jianchao, > > On Mon, Sep 10, 2018 at 09:40:35AM +0800, jianchao.wang wrote: >> Hi Ming >> >> On 09/09/2018 08:58 PM, Ming Lei wrote: >>> Now percpu_ref_reinit() can only be done on one percpu refcounter >>> when it drops zero. And the limit shouldn't be so strict, and it >>> is quite straightforward that percpu_ref_reinit() can be done when >>> this counter is at atomic mode. >> >> As we know, when the percpu_ref is switched to atomic mode, the values >> of the per cpu will be sumed up to the atomic conter in percpu_ref_switch_to_atomic_rcu. > > Right. > >> >> However, the tricky part is: >> when we switch back to percpu mode, how can we know the exact value of the value of every cpu ? > > The exact value of each CPU is zero at the exact time: > > 1) when percpu mode is switched from atomic mode > > percpu_ref_switch_to_atomic_rcu() is the point where no any percpu inc/dec > can happen any more. And in this function the percpu count is sumed up to > the atomic counter, meantime this patch clears the percpu value. It means > once the refcount is switched to atomic mode, the percpu value is always > zero, doesn't it? > > 2) when the percpu-refcount is initialized at percpu mode > > the percpu value is zero too. What we want to get is to switch the percpu refcounter to percpu mode from atomic mode when the refcounter has _not_ been drained to zero, instead of just to discard the warning, right ? When we have sumed the values of every cpu's refcounter to a global atomic counter, how can we give the values back to the refcounter of every cpu ? Thanks Jianchao > >> >> Draining the percpu refcounter to zero before switch it back to percpu mode should be relatively >> easy to implement. And also, this is the initial intention of percpu refcounter, only switch > > No, I don't think so, we can extend the percpu-refcount implementation to > cover the NVMe timeout case easily. Then no necessary to reinvent a new wheel > to address that issue. > >> to atomic mode when want to drain the refcounter. > > > Thanks, > Ming >
