READ_ONCE: Drop pointer qualifiers when reading from scalar types (dee081bf) · Commits · 戴 / test

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>

include/linux/compiler.h

+3 −3

Original line number	Diff line number	Diff line
		@@ -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) \

include/linux/compiler_types.h

+21 −0

Original line number	Diff line number	Diff line
		@@ -210,6 +210,27 @@ struct ftrace_likely_data {
		/* Are two types/vars the same type (ignoring qualifiers)? */
		#define __same_type(a, b) __builtin_types_compatible_p(typeof(a), typeof(b))

		/*
		* __unqual_scalar_typeof(x) - Declare an unqualified scalar type, leaving
		* non-scalar types unchanged.
		*
		* We build this out of a couple of helper macros in a vain attempt to
		* help you keep your lunch down while reading it.
		*/
		#define __pick_scalar_type(x, type, otherwise) \
		__builtin_choose_expr(__same_type(x, type), (type)0, otherwise)

		#define __pick_integer_type(x, type, otherwise) \
		__pick_scalar_type(x, unsigned type, \
		__pick_scalar_type(x, signed type, otherwise))

		#define __unqual_scalar_typeof(x) typeof( \
		__pick_integer_type(x, char, \
		__pick_integer_type(x, short, \
		__pick_integer_type(x, int, \
		__pick_integer_type(x, long, \
		__pick_integer_type(x, long long, x))))))

		/* Is this type a native word size -- useful for atomic operations */
		#define __native_word(t) \
		(sizeof(t) == sizeof(char) \|\| sizeof(t) == sizeof(short) \|\| \

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