The following commit has been merged into the locking/kcsan branch of tip:

Commit-ID:     bbfa112b46bdbbdfc2f5bfb9c2dcbef780ff6417
Gitweb:        
https://git.kernel.org/tip/bbfa112b46bdbbdfc2f5bfb9c2dcbef780ff6417
Author:        Will Deacon <w...@kernel.org>
AuthorDate:    Mon, 11 May 2020 21:41:42 +01:00
Committer:     Thomas Gleixner <t...@linutronix.de>
CommitterDate: Tue, 12 May 2020 11:04:13 +02:00

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

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 <torva...@linux-foundation.org>
Signed-off-by: Will Deacon <w...@kernel.org>
Signed-off-by: Thomas Gleixner <t...@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <pet...@infradead.org>
Acked-by: Mark Rutland <mark.rutl...@arm.com>
Cc: Michael Ellerman <m...@ellerman.id.au>
Cc: Arnd Bergmann <a...@arndb.de>
Cc: Christian Borntraeger <borntrae...@de.ibm.com>
Link: https://lkml.kernel.org/r/20200511204150.27858-11-w...@kernel.org

---
 include/linux/compiler.h | 141 ++++++++++++++------------------------
 1 file changed, 55 insertions(+), 86 deletions(-)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 9bd0f76..1b4e64d 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -177,28 +177,57 @@ 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>
+/*
+ * Prevent the compiler from merging or refetching reads or writes. The
+ * compiler is also forbidden from reordering successive instances of
+ * READ_ONCE and WRITE_ONCE, but only when the compiler is aware of some
+ * particular ordering. One way to make the compiler aware of ordering is to
+ * put the two invocations of READ_ONCE or WRITE_ONCE in different C
+ * statements.
+ *
+ * These two macros will also work on aggregate data types like structs or
+ * unions.
+ *
+ * Their two major use cases are: (1) Mediating communication between
+ * process-level code and irq/NMI handlers, all running on the same CPU,
+ * and (2) Ensuring that the compiler does not fold, spindle, or otherwise
+ * mutilate accesses that either do not require ordering or that interact
+ * with an explicit memory barrier or atomic instruction that provides the
+ * required ordering.
+ */
+#include <asm/barrier.h>
+#include <linux/kasan-checks.h>
 #include <linux/kcsan-checks.h>
 
-#define __READ_ONCE_SIZE                                               \
+#define __READ_ONCE(x) (*(volatile typeof(x) *)&(x))
+
+#define READ_ONCE(x)                                                   \
 ({                                                                     \
-       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();                                              \
-       }                                                               \
+       typeof(x) *__xp = &(x);                                         \
+       kcsan_check_atomic_read(__xp, sizeof(*__xp));                   \
+       __kcsan_disable_current();                                      \
+       ({                                                              \
+               typeof(x) __x = __READ_ONCE(*__xp);                     \
+               __kcsan_enable_current();                               \
+               smp_read_barrier_depends();                             \
+               __x;                                                    \
+       });                                                             \
 })
 
+#define WRITE_ONCE(x, val)                                             \
+do {                                                                   \
+       typeof(x) *__xp = &(x);                                         \
+       kcsan_check_atomic_write(__xp, sizeof(*__xp));                  \
+       __kcsan_disable_current();                                      \
+       *(volatile typeof(x) *)__xp = (val);                            \
+       __kcsan_enable_current();                                       \
+} while (0)
+
 #ifdef CONFIG_KASAN
 /*
- * We can't declare function 'inline' because __no_sanitize_address confilcts
+ * 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
+ *     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
@@ -225,78 +254,26 @@ void ftrace_likely_update(struct ftrace_likely_data *f, 
int val,
 #define __no_sanitize_or_inline __always_inline
 #endif
 
-static __no_kcsan_or_inline
-void __read_once_size(const volatile void *p, void *res, int size)
-{
-       kcsan_check_atomic_read(p, size);
-       __READ_ONCE_SIZE;
-}
-
 static __no_sanitize_or_inline
-void __read_once_size_nocheck(const volatile void *p, void *res, int size)
+unsigned long __read_once_word_nocheck(const void *addr)
 {
-       __READ_ONCE_SIZE;
-}
-
-static __no_kcsan_or_inline
-void __write_once_size(volatile void *p, void *res, int size)
-{
-       kcsan_check_atomic_write(p, 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();
-       }
+       return __READ_ONCE(*(unsigned long *)addr);
 }
 
 /*
- * Prevent the compiler from merging or refetching reads or writes. The
- * compiler is also forbidden from reordering successive instances of
- * READ_ONCE and WRITE_ONCE, but only when the compiler is aware of some
- * particular ordering. One way to make the compiler aware of ordering is to
- * put the two invocations of READ_ONCE or WRITE_ONCE in different C
- * 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.
- *
- * Their two major use cases are: (1) Mediating communication between
- * process-level code and irq/NMI handlers, all running on the same CPU,
- * and (2) Ensuring that the compiler does not fold, spindle, or otherwise
- * mutilate accesses that either do not require ordering or that interact
- * with an explicit memory barrier or atomic instruction that provides the
- * required ordering.
+ * Use READ_ONCE_NOCHECK() instead of READ_ONCE() if you need to load a
+ * word from memory atomically but without telling KASAN/KCSAN. This is
+ * usually used by unwinding code when walking the stack of a running process.
  */
-#include <asm/barrier.h>
-#include <linux/kasan-checks.h>
-
-#define __READ_ONCE(x, check)                                          \
+#define READ_ONCE_NOCHECK(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;                                                      \
+       unsigned long __x;                                              \
+       compiletime_assert(sizeof(x) == sizeof(__x),                    \
+               "Unsupported access size for READ_ONCE_NOCHECK().");    \
+       __x = __read_once_word_nocheck(&(x));                           \
+       smp_read_barrier_depends();                                     \
+       __x;                                                            \
 })
-#define READ_ONCE(x) __READ_ONCE(x, 1)
-
-/*
- * Use READ_ONCE_NOCHECK() instead of READ_ONCE() if you need
- * to hide memory access from KASAN.
- */
-#define READ_ONCE_NOCHECK(x) __READ_ONCE(x, 0)
 
 static __no_kasan_or_inline
 unsigned long read_word_at_a_time(const void *addr)
@@ -305,14 +282,6 @@ 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;                                      \
-})
-
 /**
  * data_race - mark an expression as containing intentional data races
  *

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