linux-kernel - Re: [PATCH 3/3] early_printk: Add simple serialization to early

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Message-ID: <20171004130849.av42ermer6ju3q5c@hirez.programming.kicks-ass.net>
Date:   Wed, 4 Oct 2017 15:08:49 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     pmladek@...e.com, sergey.senozhatsky@...il.com,
        linux-kernel@...r.kernel.org, mingo@...nel.org, tglx@...utronix.de,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH 3/3] early_printk: Add simple serialization to
 early_vprintk()

On Wed, Oct 04, 2017 at 09:04:01AM -0400, Steven Rostedt wrote:

> > > If old != -1 and old != cpu, is it possible that the CPU could have
> > > fetched an old value, and never try to fetch it again?  
> > 
> > What? If old != -1 and old != cpu, we'll hit the cpu_relax() and do the
> > READ_ONCE() again. The READ_ONCE() guarantees we'll do the load again,
> > as does the barrier() implied by cpu_relax().
> 
> I'm more worried about other architectures that don't have as strong of
> a cache coherency.

Linux mandates cache-coherency, there's no weak or strong there. Memory
ordering can be weak or strong, but coherency not.

If this patch is broken, lots of code would be broken.