Re: [PATCH] mm/util: fix a data race in __vm_enough_memory()

[Marco Elver <elver@google.com>]

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.

[ Note that the 'data_race()' macro is still only in -next, -tip, and -rcu. ]

Thanks,
-- Marco