linux-kernel - Re: seqcount usage in xt_replace

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Date:   Thu, 10 Jan 2019 21:20:28 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Florian Westphal <fw@...len.de>
Cc:     Anatol Pomozov <anatol.pomozov@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>, paulmck@...ux.ibm.com,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: seqcount usage in xt_replace_table()

On Thu, Jan 10, 2019 at 03:48:12PM +0100, Florian Westphal wrote:
> Peter Zijlstra <peterz@...radead.org> wrote:
> > 	/*
> > 	 * Ensure contents of newinfo are visible before assigning to
> > 	 * private.
> > 	 */
> > 	smp_wmb();
> > 	table->private = newinfo;
> > 
> > we have:
> > 
> > 	smp_store_release(&table->private, newinfo);
> > 
> > But what store does that second smp_wmb() order against? The comment:
> > 
> > 	/* make sure all cpus see new ->private value */
> > 	smp_wmb();
> > 
> > makes no sense what so ever, no smp_*() barrier can provide such
> > guarantees.
> 
> IIRC I added this at the request of a reviewer of an earlier iteration
> of that patch.
> 
> IIRC the concern was that compiler/hw could re-order
> 
> smb_wmb();
> table->private = newinfo;
> /* wait until all cpus are done with old table */
> 
>  into:
> 
> smb_wmb();
> /* wait until all cpus are done with old table */
> ...
> table->private = newinfo;
> 
> and that (obviously) makes the wait-loop useless.

The thing is, the 'wait for all cpus' thing is pure loads, not stores,
so smp_wmb() is a complete NOP there.

If you want to ensure those loads happen after that store (which does
indeed seem like a sensible thing), you're going to have to use
smp_mb().