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!
