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

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

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.