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READ_ONCE: Simplify implementations of {READ,WRITE}_ONCE()

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The implementations of {READ,WRITE}_ONCE() suffer from a significant
amount of indirection and complexity due to a historic GCC bug:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145

which was originally worked around by 230fa253df63 ("kernel: Provide
READ_ONCE and ASSIGN_ONCE").

Since GCC 4.8 is fairly vintage at this point and we emit a warning if
we detect it during the build, return {READ,WRITE}_ONCE() to their former
glory with an implementation that is easier to understand and, crucially,
more amenable to optimisation. A side effect of this simplification is
that WRITE_ONCE() no longer returns a value, but nobody seems to be
relying on that and the new behaviour is aligned with smp_store_release().

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Will Deacon <will@kernel.org>
This commit is contained in:
Will Deacon 2019-12-16 16:51:45 +00:00
parent c6a771d932
commit a5460b5e5f
1 changed files with 39 additions and 79 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

118
include/linux/compiler.h
View File

@ -177,60 +177,6 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
# define __UNIQUE_ID(prefix) __PASTE(__PASTE(__UNIQUE_ID_, prefix), __LINE__)
#endif
#include <uapi/linux/types.h>
#define __READ_ONCE_SIZE \
({ \
switch (size) { \
case 1: *(__u8 *)res = *(volatile __u8 *)p; break; \
case 2: *(__u16 *)res = *(volatile __u16 *)p; break; \
case 4: *(__u32 *)res = *(volatile __u32 *)p; break; \
case 8: *(__u64 *)res = *(volatile __u64 *)p; break; \
default: \
barrier(); \
__builtin_memcpy((void *)res, (const void *)p, size); \
barrier(); \
} \
})
static __always_inline
void __read_once_size(const volatile void *p, void *res, int size)
{
__READ_ONCE_SIZE;
}
#ifdef CONFIG_KASAN
/*
* We can't declare function 'inline' because __no_sanitize_address confilcts
* with inlining. Attempt to inline it may cause a build failure.
* https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67368
* '__maybe_unused' allows us to avoid defined-but-not-used warnings.
*/
# define __no_kasan_or_inline __no_sanitize_address notrace __maybe_unused
#else
# define __no_kasan_or_inline __always_inline
#endif
static __no_kasan_or_inline
void __read_once_size_nocheck(const volatile void *p, void *res, int size)
{
__READ_ONCE_SIZE;
}
static __always_inline void __write_once_size(volatile void *p, void *res, int size)
{
switch (size) {
case 1: *(volatile __u8 *)p = *(__u8 *)res; break;
case 2: *(volatile __u16 *)p = *(__u16 *)res; break;
case 4: *(volatile __u32 *)p = *(__u32 *)res; break;
case 8: *(volatile __u64 *)p = *(__u64 *)res; break;
default:
barrier();
__builtin_memcpy((void *)p, (const void *)res, size);
barrier();
}
}
/*
* Prevent the compiler from merging or refetching reads or writes. The
* compiler is also forbidden from reordering successive instances of
@ -240,11 +186,7 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
* statements.
*
* These two macros will also work on aggregate data types like structs or
* unions. If the size of the accessed data type exceeds the word size of
* the machine (e.g., 32 bits or 64 bits) READ_ONCE() and WRITE_ONCE() will
* fall back to memcpy(). There's at least two memcpy()s: one for the
* __builtin_memcpy() and then one for the macro doing the copy of variable
* - '__u' allocated on the stack.
* unions.
*
* Their two major use cases are: (1) Mediating communication between
* process-level code and irq/NMI handlers, all running on the same CPU,
@ -256,23 +198,49 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
#include <asm/barrier.h>
#include <linux/kasan-checks.h>
#define __READ_ONCE(x, check) \
#define __READ_ONCE(x) (*(volatile typeof(x) *)&(x))
#define READ_ONCE(x) \
({ \
union { typeof(x) __val; char __c[1]; } __u; \
if (check) \
__read_once_size(&(x), __u.__c, sizeof(x)); \
else \
__read_once_size_nocheck(&(x), __u.__c, sizeof(x)); \
smp_read_barrier_depends(); /* Enforce dependency ordering from x */ \
__u.__val; \
typeof(x) __x = __READ_ONCE(x); \
smp_read_barrier_depends(); \
__x; \
})
#define READ_ONCE(x) __READ_ONCE(x, 1)
#define WRITE_ONCE(x, val) \
do { \
*(volatile typeof(x) *)&(x) = (val); \
} while (0)
#ifdef CONFIG_KASAN
/*
* We can't declare function 'inline' because __no_sanitize_address conflicts
* with inlining. Attempt to inline it may cause a build failure.
* https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67368
* '__maybe_unused' allows us to avoid defined-but-not-used warnings.
*/
# define __no_kasan_or_inline __no_sanitize_address notrace __maybe_unused
#else
# define __no_kasan_or_inline __always_inline
#endif
static __no_kasan_or_inline
unsigned long __read_once_word_nocheck(const void *addr)
{
return __READ_ONCE(*(unsigned long *)addr);
}
/*
* Use READ_ONCE_NOCHECK() instead of READ_ONCE() if you need
* to hide memory access from KASAN.
* Use READ_ONCE_NOCHECK() instead of READ_ONCE() if you need to load a
* word from memory atomically but without telling KASAN. This is usually
* used by unwinding code when walking the stack of a running process.
*/
#define READ_ONCE_NOCHECK(x) __READ_ONCE(x, 0)
#define READ_ONCE_NOCHECK(x) \
({ \
unsigned long __x = __read_once_word_nocheck(&(x)); \
smp_read_barrier_depends(); \
__x; \
})
static __no_kasan_or_inline
unsigned long read_word_at_a_time(const void *addr)
@ -281,14 +249,6 @@ unsigned long read_word_at_a_time(const void *addr)
return *(unsigned long *)addr;
}
#define WRITE_ONCE(x, val) \
({ \
union { typeof(x) __val; char __c[1]; } __u = \
{ .__val = (__force typeof(x)) (val) }; \
__write_once_size(&(x), __u.__c, sizeof(x)); \
__u.__val; \
})
#endif /* __KERNEL__ */
/*