linux-kernel - Re: [PATCH 8/8] fs/aio: Use rcu_work instead of explicit rcu and work item

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Message-ID: <20180326150459.GE1840639@devbig577.frc2.facebook.com>
Date:   Mon, 26 Mar 2018 08:04:59 -0700
From:   Tejun Heo <tj@...nel.org>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     torvalds@...ux-foundation.org, jannh@...gle.com,
        paulmck@...ux.vnet.ibm.com, bcrl@...ck.org,
        viro@...iv.linux.org.uk, kent.overstreet@...il.com,
        security@...nel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH 8/8] fs/aio: Use rcu_work instead of explicit rcu and
 work item

Hey, Oleg.

On Thu, Mar 22, 2018 at 12:24:12PM +0100, Oleg Nesterov wrote:
> OK. And I agree that the usage of WORK_STRUCT_PENDING_BIT in queue_rcu_work() is
> fine. If nothing else the kernel won't crash if you call queue_rcu_work() twice.
> 
> But why flush_rcu_work() can't simply do flush_work() ?
> 
> If WORK_STRUCT_PENDING_BIT was set by us (rcu_work_rcufn() succeeded) we do not
> need rcu_barrier(). Why should we care about other rcu callbacks?
>
> If rcu_work_rcufn() fails and someone else sets PENDING, how this rcu_barrier()
> can help? We didn't even do call_rcu() in this case.
> 
> In short. Once flush_work() returns we know that rcu callback which queued this
> work is finished. It doesn't matter if it was fired by us or not. And if it was
> not fired by us, then rcu_barrier() doesn't imply a grace period anyway.

flush_*work() guarantees to wait for the completion of the latest
instance of the work item which was visible to the caller.  We can't
guarantee that w/o rcu_barrier().

> > We can try to
> > make it more specialized but then flush_rcu_work()'s behavior would
> > have to different too and it gets confusing quickly.  Unless there are
> > overriding reasons to deviate, I'd like to keep it consistent.
> 
> Perhaps this all is consistent, but I fail to understand this API :/
> 
> And again, at least for fs/aio.c it doesn't offer too much but sub-optimal
> compared to call_rcu() + schedule_work() by hand.

Sure, this isn't about performance.  It's about making code less
painful on the eyes.  If performance matters, we sure can hand-craft
things, which doesn't seem to be the case, right?

Thanks.

-- 
tejun