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Date:   Thu, 30 Jan 2020 18:18:34 -0800
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Marco Elver <elver@google.com>
Cc:     Qian Cai <cai@lca.pw>, Matthew Wilcox <willy@infradead.org>,
        dennis@kernel.org, tj@kernel.org, Christoph Lameter <cl@linux.com>,
        Linux Memory Management List <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] mm/util: fix a data race in __vm_enough_memory()
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On Thu, 30 Jan 2020 13:35:18 +0100 Marco Elver <elver@google.com> wrote:

> On Thu, 30 Jan 2020 at 12:50, Qian Cai <cai@lca.pw> wrote:
> >
> > > On Jan 29, 2020, at 11:20 PM, Matthew Wilcox <willy@infradead.org> wrote:
> > >
> > > I'm really not a fan of exposing the internals of a percpu_counter outside
> > > the percpu_counter.h file.  Why shouldn't this be fixed by putting the
> > > READ_ONCE() inside percpu_counter_read()?
> >
> > It is because not all places suffer from a data race. For example, in __wb_update_bandwidth(), it was protected by a lock. I was a bit worry about blindly adding READ_ONCE() inside percpu_counter_read() might has unexpected side-effect. For example, it is unnecessary to have READ_ONCE() for a volatile variable. So, I thought just to keep the change minimal with a trade off by exposing a bit internal details as you mentioned.
> >
> > However, I had also copied the percpu maintainers to see if they have any preferences?
> 
> I would not add READ_ONCE to percpu_counter_read(), given the writes
> (increments) are not atomic either, so not much is gained.
> 
> Notice that this is inside a WARN_ONCE, so you may argue that a data
> race here doesn't matter to the correct behaviour of the system
> (except if you have panic_on_warn on).
> 
> For the warning to trigger, vm_committed_as must decrease. Assume that
> a data race (assuming bad compiler optimizations) can somehow
> accomplish this, then the load or write must cause a transient value
> to somehow be less than a stable value. My hypothesis is this is very
> unlikely.
> 
> Given the fact this is a WARN_ONCE, and the fact that a transient
> decrease in the value is unlikely, you may consider
> 'VM_WARN_ONCE(data_race(percpu_counter_read(&vm_committed_as)) <
> ...)'. That way you won't modify percpu_counter_read and still catch
> unintended races elsewhere.
> 

That, or add an alternative version of per_cpu_counter_read() to the
percpu API.  A very carefully commented version!