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Date: Tue, 3 Oct 2017 12:31:05 -0700
From: Tejun Heo <tj@kernel.org>
To: Ming Lei <ming.lei@redhat.com>
Cc: Jens Axboe <axboe@fb.com>, linux-block@vger.kernel.org,
        Christoph Hellwig <hch@infradead.org>, linux-scsi@vger.kernel.org,
        "Martin K . Petersen" <martin.petersen@oracle.com>,
        "James E . J . Bottomley" <jejb@linux.vnet.ibm.com>,
        Bart Van Assche <bart.vanassche@sandisk.com>,
        Oleksandr Natalenko <oleksandr@natalenko.name>,
        Johannes Thumshirn <jthumshirn@suse.de>,
        Cathy Avery <cavery@redhat.com>,
        Martin Steigerwald <martin@lichtvoll.de>, linux-kernel@vger.kernel.org,
        Hannes Reinecke <hare@suse.com>,
        Bart Van Assche <Bart.VanAssche@wdc.com>
Subject: Re: [PATCH V8 5/8] percpu-refcount: introduce
 __percpu_ref_tryget_live
Message-ID: <20171003193105.GL3301751@devbig577.frc2.facebook.com>
References: <20171003140406.26060-1-ming.lei@redhat.com>
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 <20171003141459.GI3301751@devbig577.frc2.facebook.com>
 <20171003192039.GB5362@ming.t460p>
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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