On 10/08/21 20:49, Boqun Feng wrote:
> On Sun, Aug 08, 2021 at 05:15:20PM +0100, Valentin Schneider wrote:
>> On 07/08/21 03:42, Mike Galbraith wrote:
>> > On Sat, 2021-08-07 at 01:58 +0100, Valentin Schneider wrote:
>> >>
>> >> +static inline bool is_pcpu_safe(void)
>> >
>> > Nit: seems odd to avoid spelling it out to save two characters, percpu
>> > is word like, rolls off the ole tongue better than p-c-p-u.
>> >
>> > -Mike
>>
>> True. A quick grep says both versions are used, though "percpu" wins by
>> about a factor of 2. I'll tweak that for a v3.
>
> I wonder why is_percpu_safe() is the correct name. The safety of
> accesses to percpu variables means two things to me:
>
> a) The thread cannot migrate to other CPU in the middle of
> accessing a percpu variable, in other words, the following
> cannot happen:
>
> { percpu variable X is 0 on CPU 0 and 2 on CPU 1
> CPU 0 CPU 1
> ======== =========
> <in thread A>
> __this_cpu_inc(X);
> tmp = X; // tmp is 0
> <preempted>
> <migrate to CPU 1>
> // continue __this_cpu_inc(X);
> X = tmp + 1; // CPU 0 miss this
> // increment (this
> // may be OK), and
> // CPU 1's X got
> // corrupted.
>
> b) The accesses to a percpu variable are exclusive, i.e. no
> interrupt or preemption can happen in the middle of accessing,
> in other words, the following cannot happen:
>
> { percpu variable X is 0 on CPU 0 }
> CPU 0
> ========
> <in thread A>
> __this_cpu_inc(X);
> tmp = X; // tmp is 0
> <preempted>
> <in other thread>
> this_cpu_inc(X); // X is 1 afterwards.
> <back to thread A>
> X = tmp + 1; // X is 1, and we have a race condition.
>
> And the is_p{er}cpu_safe() only detects the first, and it doesn't mean
> totally safe for percpu accesses.
>
Right. I do briefly point this out in the changelog (the bit about
"acquiring a sleepable lock if relevant"), but that doesn't do much to
clarify the helper name itself.
> Maybe we can implement a migratable()? Although not sure it's a English
> word.
>
Funnily enough that is exactly how I named the thing in my initial draft,
but then I somehow convinced myself that tailoring the name to per-CPU
accesses would make its intent clearer.
I think you're right that "migratable()" is less confusing at the end of
the day. Oh well, so much for overthinking the naming problem :-)
> Regards,
> Boqun
