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

[Qian Cai]

> On Jan 30, 2020, at 9:18 PM, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> 
> On Thu, 30 Jan 2020 13:35:18 +0100 Marco Elver <elver@xxxxxxxxxx> wrote:
> 
>> On Thu, 30 Jan 2020 at 12:50, Qian Cai <cai@xxxxxx> wrote:
>>> 
>>>> On Jan 29, 2020, at 11:20 PM, Matthew Wilcox <willy@xxxxxxxxxxxxx> 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!

 I send a patch to use data_race() which should be sufficient,

https://lore.kernel.org/linux-mm/20200130145649.1240-1-cai@xxxxxx/