linux-kernel - Re: [PATCH V8 5/8] percpu-refcount: introduce __percpu_ref_tryget

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Message-ID: <20171003193105.GL3301751@devbig577.frc2.facebook.com>
Date:   Tue, 3 Oct 2017 12:31:05 -0700
From:   Tejun Heo <tj@...nel.org>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     Jens Axboe <axboe@...com>, linux-block@...r.kernel.org,
        Christoph Hellwig <hch@...radead.org>,
        linux-scsi@...r.kernel.org,
        "Martin K . Petersen" <martin.petersen@...cle.com>,
        "James E . J . Bottomley" <jejb@...ux.vnet.ibm.com>,
        Bart Van Assche <bart.vanassche@...disk.com>,
        Oleksandr Natalenko <oleksandr@...alenko.name>,
        Johannes Thumshirn <jthumshirn@...e.de>,
        Cathy Avery <cavery@...hat.com>,
        Martin Steigerwald <martin@...htvoll.de>,
        linux-kernel@...r.kernel.org, Hannes Reinecke <hare@...e.com>,
        Bart Van Assche <Bart.VanAssche@....com>
Subject: Re: [PATCH V8 5/8] percpu-refcount: introduce
 __percpu_ref_tryget_live

Hello,

On Wed, Oct 04, 2017 at 03:20:40AM +0800, Ming Lei wrote:
> On Tue, Oct 03, 2017 at 07:14:59AM -0700, Tejun Heo wrote:
> > Hello,
> > 
> > On Tue, Oct 03, 2017 at 10:04:03PM +0800, Ming Lei wrote:
> > > Block layer need to call this function after holding
> > > rcu lock in a real hot path, so introduce this helper.
> > 
> > The patch description is too anemic.  It doesn't even describe what
> > changes are being made, let alone justifying them.
> 
> Sorry for Cc you, and this change is needed by the following
> patch:
> 
> 	https://marc.info/?l=linux-scsi&m=150703956929333&w=2
> 
> Block layer need to call percpu_ref_tryget_live()
> with holding RCU read lock, given it is a hot I/O
> path, that is why I make this patch so that we can
> avoid nested RCU read lock and save a bit cost.

I'm not necessarily against it but please name it sth like
percpu_ref_tryget_live_locked(), add lockdep assertions in the inner
function and document properly.

Thanks.

-- 
tejun