Messages in this thread | | | Subject | Re: [PATCH v4 08/11] READ_ONCE: Drop pointer qualifiers when reading from scalar types | From | Rasmus Villemoes <> | Date | Wed, 22 Apr 2020 12:25:03 +0200 |
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On 21/04/2020 17.15, Will Deacon wrote: > Passing a volatile-qualified pointer to READ_ONCE() is an absolute > trainwreck for code generation: the use of 'typeof()' to define a > temporary variable inside the macro means that the final evaluation in > macro scope ends up forcing a read back from the stack. When stack > protector is enabled (the default for arm64, at least), this causes > the compiler to vomit up all sorts of junk. > > Unfortunately, dropping pointer qualifiers inside the macro poses quite > a challenge, especially since the pointed-to type is permitted to be an > aggregate, and this is relied upon by mm/ code accessing things like > 'pmd_t'. Based on numerous hacks and discussions on the mailing list, > this is the best I've managed to come up with.
Hm, maybe this can be brought to work, only very lightly tested. It basically abuses what -Wignored-qualifiers points out:
warning: type qualifiers ignored on function return type
Example showing the idea:
const int c(void); volatile int v(void);
int hack(int x, int y) { typeof(c()) a = x; typeof(v()) b = y;
a += b; b += a; a += b; return a; }
Since that compiles, a cannot be const-qualified, and the generated code certainly suggests that b is not volatile-qualified. So something like
#define unqual_type(x) _unqual_type(x, unique_id_dance) #define _unqual_type(x, id) typeof( ({ typeof(x) id(void); id(); }) )
and perhaps some _Pragma("GCC diagnostic push")/_Pragma("GCC diagnostic ignored -Wignored-qualifiers")/_Pragma("GCC diagnostic pop") could prevent the warning (which is in -Wextra, so I don't think it would appear in a normal build anyway).
No idea how well any of this would work across gcc versions or with clang.
Rasmus
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