Re: [PATCH] percpu-refcount: relax limit on percpu_ref_reinit()

[Ming Lei <ming.lei@redhat.com>]

Hi Tejun,

On Tue, Sep 18, 2018 at 05:49:09AM -0700, Tejun Heo wrote:
> Hello, Ming.
> 
> Sorry about the delay.
> 
> On Thu, Sep 13, 2018 at 06:11:40AM +0800, Ming Lei wrote:
> > > Yeah but what guards ->release() starting to run and then the ref
> > > being switched to percpu mode?  Or maybe that doesn't matter?
> > 
> > OK, we may add synchronize_rcu() just after clearing the DEAD flag in
> > the new introduced helper to avoid the race.
> 
> That doesn't make sense to me.  How is synchronize_rcu() gonna change
> anything there?

As you saw in the new post, synchronize_rcu() isn't used for avoiding
the race. Instead, it is done by grabbing one extra ref on atomic part.

> 
> > > > 4) after the queue is recovered(or the controller is reset successfully), it
> > > > isn't necessary to wait until the refcount drops zero, since it is fine to
> > > > reinit it by clearing DEAD and switching back to percpu mode from atomic mode.
> > > > And waiting for the refcount dropping to zero in the reset handler may trigger
> > > > IO hang if IO timeout happens again during reset.
> > > 
> > > Does the recovery need the in-flight commands actually drained or does
> > > it just need to block new issues for a while.  If latter, why is
> > 
> > The recovery needn't to drain the in-flight commands actually.
> 
> Is it just waiting till confirm_kill is called?  So that new ref is
> not given away?  If synchronization like that is gonna work, the
> percpu ref operations on the reader side must be wrapped in a larger
> critical region, which brings up two issues.
> 
> 1. Callers of percpu_ref must not depend on what internal
>    synchronization construct percpu_ref uses.  Again, percpu_ref
>    doesn't even use regular RCU.
> 
> 2. If there is already an outer RCU protection around ref operation,
>    that RCU critical section can and should be used for
>    synchronization, not percpu_ref.

I guess the above doesn't apply any more because there isn't new 
synchronize_rcu() introduced in my new post.

> 
> > > percpu_ref even being used?
> > 
> > Just for avoiding to invent a new wheel, especially .q_usage_counter
> > has served for this purpose for long time.
> 
> It sounds like this was more of an abuse.  So, basically what you want
> is sth like the following.
> 
> READER
> 
>  rcu_read_lock();
>  if (can_issue_new_commands)
> 	issue;
>  else
> 	abort;
>  rcu_read_unlock();
> 
> WRITER
> 
>  can_issue_new_commands = false;
>  synchronize_rcu();
>  // no new command will be issued anymore
> 
> Right?  There isn't much wheel to reinvent here and using percpu_ref
> for the above is likely already incorrect due to the different RCU
> type being used.

No RCU story any more, :-)

It might work, but still a reinvented wheel since perpcu-refcount does
provide same function. Not mention the inter-action between the two
mechanism may have to be considered.

Also there is still cost introduced in WRITER side, and the
synchronize_rcu() often takes a bit long, especially there might be lots
of namespaces, each need to run one synchronize_rcu(). We have learned
lessons in converting to blk-mq for scsi, in which synchronize_rcu()
introduces long delay in booting.


Thanks,
Ming