> 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/
