Messages in this thread | | | Date | Mon, 3 Apr 2023 11:19:48 +0100 | From | Mark Rutland <> | Subject | Re: [PATCH 01/10] locking/atomic: Add missing cast to try_cmpxchg() fallbacks |
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On Sun, Mar 26, 2023 at 09:28:38PM +0200, Uros Bizjak wrote: > On Fri, Mar 24, 2023 at 5:33 PM Mark Rutland <mark.rutland@arm.com> 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@arm.com> 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.
Whether or not there are fallbacks is immaterial.
In those cases, architectures can just as easily write C wrappers, e.g.
long local_cmpxchg(local_t *l, long old, long new) { return cmpxchg_local(&l->a.counter, old, new); }
long local_try_cmpxchg(local_t *l, long *old, long new) { return try_cmpxchg_local(&l->a.counter, old, new); }
> 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;
Yes, which is why I used atomic_long() above.
> 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.
I understand the problem of arch code differing from generic code, and that we want to have *a* consistent behaviour for hte API.
What I'm saying is that the behaviour we should aim for is where the 'old' pointer has a specific type (long), and we always require that, as we do for the various atomic_*() APIs of which local_*() is a cousin.
> Also, I don't think that "fixing" callsites is the right thing to do.
Why? What's wrong with doing that?
The documentation in Documentation/core-api/local_ops.rst says:
The ``local_t`` type is defined as an opaque ``signed long``
So the obvious and least surprising thing is for the local_*() functions to use 'long' for values and 'long *' for pointers to values.
Requiring a cast in a few places is not the end of the world.
> The generic code should follow x86 and cast the "old" pointer to the > type of "ptr" inside the fallback.
Why?
I disagree, and think it's far better to be strict by default. That way, accidental usage of the wrong type will be caught by the compiler, and if someone *really* wants to use a differently type then can use a cast in the callsite, which makes it really obvious when that is happening.
I appreciate that may require some preparatory cleanup, but I think that's a small price to pay for having this in a clearer and more maintainable state.
> > 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.
FWIW, I agree that we *should* make try_cmpxchg() check that ptr and old pointer are the same type.
However, I don't think that's a prerequisite for doing so for local_try_cmpxchg().
Plese make local_try_cmpxchg() have a proper type-safe C prototype, as we do with the atomic*_try_cmpxchg() APIs.
Thanks, Mark,
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