Messages in this thread Patch in this message | | | Date | Tue, 26 Nov 2019 17:37:43 +0100 | From | Luc Van Oostenryck <> | Subject | Re: [PATCH] percpu: add __percpu to SHIFT_PERCPU_PTR |
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On Mon, Nov 25, 2019 at 05:41:19PM -0500, Dennis Zhou wrote: > On Thu, Oct 17, 2019 at 02:13:01PM -0400, Dennis Zhou wrote: > > On Tue, Oct 15, 2019 at 11:26:15AM +0100, Ben Dooks wrote: > > > The SHIFT_PERCPU_PTR() returns a pointer used by a number > > > of functions that expect the pointer to be __percpu annotated > > > (sparse address space 3). Adding __percpu to this makes the > > > following sparse warnings go away. > > > > > > Note, this then creates the problem the __percup is marked > > > as noderef, which may need removing for some of the internal > > > functions, or to remove other warnings. > > > > > > diff --git a/include/linux/percpu-defs.h b/include/linux/percpu-defs.h > > > index a6fabd865211..a49b6c702598 100644 > > > --- a/include/linux/percpu-defs.h > > > +++ b/include/linux/percpu-defs.h > > > @@ -229,7 +229,7 @@ do { \ > > > * pointer value. The weird cast keeps both GCC and sparse happy. > > > */ > > > #define SHIFT_PERCPU_PTR(__p, __offset) \ > > > - RELOC_HIDE((typeof(*(__p)) __kernel __force *)(__p), (__offset)) > > > + RELOC_HIDE((typeof(*(__p)) __kernel __percpu __force *)(__p), (__offset)) > > > > > > #define per_cpu_ptr(ptr, cpu) \ > > > ({ \ > > > -- > > > 2.23.0 > > > > > Hi Ben, > > I've reverted this commit. After spending a little more time on it, I > don't think this is the fix we want because after we call RELOC_HIDE, > we are shifting from __percpu to __kernel address spaces as we're adding > the offset here.
I agree. SHIFT_PERCPU_PTR()'s type is as designed. Also, it should be noted that this warning doesn't exist on x86.
The problem lies in include/asm-generic/percpu.h, in this case within: #define raw_cpu_generic_add_return(pcp, val) \ ({ \ typeof(&(pcp)) __p = raw_cpu_ptr(&(pcp)); \ \ *__p += val; \ *__p; \ })
There, the variable __p is declared as a __percpu pointer but: 1) the value assigned to it, the return value of raw_cpu_ptr(), is not a plain (__kernel) pointer. 2) the variable is dereferenced just after when a __percpu pointer is implicitly __noderef. It's thus not SHIFT_PERCPU_PTR() or raw_cpu_ptr() that must have their return type changed but the declaration of __p.
Here is a quick patch, only compile & sparse tested on mm/vmstat.c:
diff --git a/include/asm-generic/percpu.h b/include/asm-generic/percpu.h index c2de013b2..757b667ca 100644 --- a/include/asm-generic/percpu.h +++ b/include/asm-generic/percpu.h @@ -74,7 +74,7 @@ do { \ #define raw_cpu_generic_add_return(pcp, val) \ ({ \ - typeof(&(pcp)) __p = raw_cpu_ptr(&(pcp)); \ + typeof(pcp) __kernel __force *__p = raw_cpu_ptr(&(pcp)); \ \ *__p += val; \ *__p; \
Note: the same problem is present in raw_cpu_generic_xchg(), raw_cpu_generic_cmpxchg() & raw_cpu_generic_cmpxchg_double()
I need more testing and to evaluate possible side-effects before sending a proper patch.
Best regards, -- Luc Van Oostenryck
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