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Date:   Tue, 25 Feb 2020 17:29:52 -0800
From:   Matthew Wilcox <willy@infradead.org>
To:     Qian Cai <cai@lca.pw>
Cc:     akpm@linux-foundation.org, elver@google.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm/vmscan: fix data races at kswapd_classzone_idx
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On Tue, Feb 25, 2020 at 11:55:26AM -0500, Qian Cai wrote:
> -	if (pgdat->kswapd_classzone_idx == MAX_NR_ZONES)
> -		pgdat->kswapd_classzone_idx = classzone_idx;
> -	else
> -		pgdat->kswapd_classzone_idx = max(pgdat->kswapd_classzone_idx,
> -						  classzone_idx);
> +	if (READ_ONCE(pgdat->kswapd_classzone_idx) == MAX_NR_ZONES ||
> +	    READ_ONCE(pgdat->kswapd_classzone_idx) < classzone_idx)
> +		WRITE_ONCE(pgdat->kswapd_classzone_idx, classzone_idx);
> +
>  	pgdat->kswapd_order = max(pgdat->kswapd_order, order);

Doesn't this line have the exact same problem you're "solving" above?

Also, why would you do this crazy "f(READ_ONCE(x)) || g(READ_ONCE(x))?
Surely you should be stashing READ_ONCE(x) into a local variable?

And there are a _lot_ of places which access kswapd_classzone_idx
without a lock.  Are you sure this patch is sufficient?