On Fri, Mar 24, 2023 at 5:33 PM Mark Rutland <mark.rutland@xxxxxxx> wrote:
>
> On Fri, Mar 24, 2023 at 04:14:22PM +0000, Mark Rutland wrote:
> > On Fri, Mar 24, 2023 at 04:43:32PM +0100, Uros Bizjak wrote:
> > > On Fri, Mar 24, 2023 at 3:13 PM Mark Rutland <mark.rutland@xxxxxxx> wrote:
> > > >
> > > > On Sun, Mar 05, 2023 at 09:56:19PM +0100, Uros Bizjak wrote:
> > > > > Cast _oldp to the type of _ptr to avoid incompatible-pointer-types warning.
> > > >
> > > > Can you give an example of where we are passing an incompatible pointer?
> > >
> > > An example is patch 10/10 from the series, which will fail without
> > > this fix when fallback code is used. We have:
> > >
> > > - } while (local_cmpxchg(&rb->head, offset, head) != offset);
> > > + } while (!local_try_cmpxchg(&rb->head, &offset, head));
> > >
> > > where rb->head is defined as:
> > >
> > > typedef struct {
> > > atomic_long_t a;
> > > } local_t;
> > >
> > > while offset is defined as 'unsigned long'.
> >
> > Ok, but that's because we're doing the wrong thing to start with.
> >
> > Since local_t is defined in terms of atomic_long_t, we should define the
> > generic local_try_cmpxchg() in terms of atomic_long_try_cmpxchg(). We'll still
> > have a mismatch between 'long *' and 'unsigned long *', but then we can fix
> > that in the callsite:
> >
> > while (!local_try_cmpxchg(&rb->head, &(long *)offset, head))
>
> Sorry, that should be:
>
> while (!local_try_cmpxchg(&rb->head, (long *)&offset, head))
The fallbacks are a bit more complicated than above, and are different
from atomic_try_cmpxchg.
Please note in patch 2/10, the falbacks when arch_try_cmpxchg_local
are not defined call arch_cmpxchg_local. Also in patch 2/10,
try_cmpxchg_local is introduced, where it calls
arch_try_cmpxchg_local. Targets (and generic code) simply define (e.g.
:
#define local_cmpxchg(l, o, n) \
(cmpxchg_local(&((l)->a.counter), (o), (n)))
+#define local_try_cmpxchg(l, po, n) \
+ (try_cmpxchg_local(&((l)->a.counter), (po), (n)))
which is part of the local_t API. Targets should either define all
these #defines, or none. There are no partial fallbacks as is the case
with atomic_t.
The core of the local_h API is in the local.h header. If the target
doesn't define its own local.h header, then asm-generic/local.h is
used that does exactly what you propose above regarding the usage of
atomic functions.
OTOH, when the target defines its own local.h, then the above
target-dependent #define path applies. The target should define its
own arch_try_cmpxchg_local, otherwise a "generic" target-dependent
fallback that calls target arch_cmpxchg_local applies. In the case of
x86, patch 9/10 enables new instruction by defining
arch_try_cmpxchg_local.
FYI, the patch sequence is carefully chosen so that x86 also exercises
fallback code between different patches in the series.
Targets are free to define local_t to whatever they like, but for some
reason they all define it to:
typedef struct {
atomic_long_t a;
} local_t;
so they have to dig the variable out of the struct like:
#define local_cmpxchg(l, o, n) \
(cmpxchg_local(&((l)->a.counter), (o), (n)))
Regarding the mismatch of 'long *' vs 'unsigned long *': x86
target-specific code does for try_cmpxchg:
#define __raw_try_cmpxchg(_ptr, _pold, _new, size, lock) \
({ \
bool success; \
__typeof__(_ptr) _old = (__typeof__(_ptr))(_pold); \
__typeof__(*(_ptr)) __old = *_old; \
__typeof__(*(_ptr)) __new = (_new); \
so, it *does* cast the "old" pointer to the type of "ptr". The generic
code does *not*. This difference is dangerous, since the compilation
of some code involving try_cmpxchg will compile OK for x86 but will
break for other targets that use try_cmpxchg fallback templates (I was
the unlucky one that tripped on this in the past). Please note that
this problem is not specific to the proposed local_try_cmpxchg series,
but affects the existing try_cmpxchg API.
Also, I don't think that "fixing" callsites is the right thing to do.
The generic code should follow x86 and cast the "old" pointer to the
type of "ptr" inside the fallback.
> The fundamenalthing I'm trying to say is that the
> atomic/atomic64/atomic_long/local/local64 APIs should be type-safe, and for
> their try_cmpxchg() implementations, the type signature should be:
>
> ${atomictype}_try_cmpxchg(${atomictype} *ptr, ${inttype} *old, ${inttype} new)
This conversion should be performed also for the cmpxchg family of
functions, if desired at all. try_cmpxchg fallback is just cmpxchg
with some extra code around.
Thanks,
Uros.
