Re: [PATCH bpf-next v9 1/6] locking/local_lock: Introduce localtry_lock_t

[Vlastimil Babka <vbabka@suse.cz>]

On 3/11/25 23:24, Alexei Starovoitov wrote:
> On Tue, Mar 11, 2025 at 9:21 PM Vlastimil Babka <vbabka@suse.cz> wrote:
>>
>> On 3/11/25 17:31, Mateusz Guzik wrote:
>> > On Tue, Mar 11, 2025 at 5:21 PM Sebastian Andrzej Siewior
>> > <bigeasy@linutronix.de> wrote:
>> >>
>> >> On 2025-03-11 16:44:30 [+0100], Mateusz Guzik wrote:
>> >> > On Fri, Feb 21, 2025 at 06:44:22PM -0800, Alexei Starovoitov wrote:
>> >> > > +#define __localtry_lock(lock)                                      \
>> >> > > +   do {                                                    \
>> >> > > +           localtry_lock_t *lt;                            \
>> >> > > +           preempt_disable();                              \
>> >> > > +           lt = this_cpu_ptr(lock);                        \
>> >> > > +           local_lock_acquire(&lt->llock);                 \
>> >> > > +           WRITE_ONCE(lt->acquired, 1);                    \
>> >> > > +   } while (0)
>> >> >
>> >> > I think these need compiler barriers.
>> >> >
>> >> > I checked with gcc docs (https://gcc.gnu.org/onlinedocs/gcc/Volatiles.html)
>> >> > and found this as confirmation:
>> >> > > Accesses to non-volatile objects are not ordered with respect to volatile accesses.
>> >> >
>> >> > Unless the Linux kernel is built with some magic to render this moot(?).
>> >>
>> >> You say we need a barrier() after the WRITE_ONCE()? If so, we need it in
>> >> the whole file…
>> >>
>> >
>> > I see the original local_lock machinery on the stock kernel works fine
>> > as it expands to the preempt pair which has the appropriate fences. If
>> > debug is added, the "locking" remains unaffected, but the debug state
>> > might be bogus when looked at from the "wrong" context and adding the
>> > compiler fences would trivially sort it out. I don't think it's a big
>> > deal for *their* case, but patching that up should not raise any
>> > eyebrows and may prevent eyebrows from going up later.
>> >
>> > The machinery added in this patch does need the addition for
>> > correctness in the base operation though.
>>
>> Yeah my version of this kind of lock in sheaves code had those barrier()'s,
>> IIRC after you or Jann told me. It's needed so that the *compiler* does not
>> e.g. reorder a write to the protected data to happen before the
>> WRITE_ONCE(lt->acquired, 1) (or after the WRITE_ONCE(lt->acquired, 0) in
>> unlock).
> 
> I think you all are missing a fine print in gcc doc:
> "Unless...can be aliased".
> The kernel is compiled with -fno-strict-aliasing.
> No need for barrier()s here.

Note I know next to nothing about these things, but I see here [1]:

"Whether GCC actually performs type-based aliasing analysis depends on the
details of the code. GCC has other ways to determine (in some cases) whether
objects alias, and if it gets a reliable answer that way, it won’t fall back
on type-based heuristics. [...] You can turn off type-based aliasing
analysis by giving GCC the option -fno-strict-aliasing."

I'd read that as -fno-strict-aliasing only disables TBAA, but that does not
necessary mean anything can be assumed to be aliased with anything?
An if we e.g. have a pointer to memcg_stock_pcp through which we access the
stock_lock an the other (protected) fields and that pointer doesn't change
between that, I imagine gcc can reliably determine these can't alias?

[1]
https://www.gnu.org/software/c-intro-and-ref/manual/html_node/Aliasing-Type-Rules.html