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READ_ONCE: Drop pointer qualifiers when reading from scalar types

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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.

Introduce '__unqual_scalar_typeof()' which takes an expression and, if
the expression is an optionally qualified 8, 16, 32 or 64-bit scalar
type, evaluates to the unqualified type. Other input types, including
aggregates, remain unchanged. Hopefully READ_ONCE() on volatile aggregate
pointers isn't something we do on a fast-path.

Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Reported-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Will Deacon <will@kernel.org>
This commit is contained in:
Will Deacon 2019-12-19 14:22:31 +00:00
parent 9e343b467c
commit dee081bf8f
2 changed files with 24 additions and 3 deletions
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6
include/linux/compiler.h
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@ -203,13 +203,13 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
* atomicity or dependency ordering guarantees. Note that this may result
* in tears!
*/
#define __READ_ONCE(x) (*(const volatile typeof(x) *)&(x))
#define __READ_ONCE(x) (*(const volatile __unqual_scalar_typeof(x) *)&(x))
#define __READ_ONCE_SCALAR(x) \
({ \
typeof(x) __x = __READ_ONCE(x); \
__unqual_scalar_typeof(x) __x = __READ_ONCE(x); \
smp_read_barrier_depends(); \
__x; \
(typeof(x))__x; \
})
#define READ_ONCE(x) \