linux-kernel - Re: Is read barrier missed in kfifo?

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Message-ID: <20180511083242.GJ12217@hirez.programming.kicks-ass.net>
Date:   Fri, 11 May 2018 10:32:42 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Xiao Guangrong <guangrong.xiao@...il.com>
Cc:     paulmck@...ux.vnet.ibm.com, rostedt@...dmis.org,
        Lai Jiangshan <jiangshanlai@...il.com>, stefani@...bold.net,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Is read barrier missed in kfifo?

On Fri, May 11, 2018 at 03:25:18PM +0800, Xiao Guangrong wrote:
> 
> Hi,
> 
> Currently, there is no read barrier between reading the index
> (kfifo.in) and fetching the real data from the fifo.
> 
> I am afraid that will cause the vfifo is observed as not empty
> however the data is not actually ready for read. Right?

That code is decidedly dodgy indeed. I can only see smp_wmb() but no
matching barriers at all -- therefore the code is almost certainly as
good as not having any barriers at all.

I would suggest you try and convert the code to smp_store_release() and
smp_load_acquire() while you're at it.