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