Am 04.12.2014 um 01:07 schrieb Paul E. McKenney: > On Wed, Dec 03, 2014 at 11:30:13PM +0100, Christian Borntraeger wrote: >> ACCESS_ONCE does not work reliably on non-scalar types. For >> example gcc 4.6 and 4.7 might remove the volatile tag for such >> accesses during the SRA (scalar replacement of aggregates) step >> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145) >> >> Let's provide READ_ONCE/ASSIGN_ONCE that will do all accesses via >> scalar types as suggested by Linus Torvalds. Accesses larger than >> the machines word size cannot be guaranteed to be atomic. These >> macros will use memcpy and emit a build warning. >> >> Signed-off-by: Christian Borntraeger <borntraeger@xxxxxxxxxx> > > With or without some nits below addressed: > > Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> > >> --- >> include/linux/compiler.h | 64 ++++++++++++++++++++++++++++++++++++++++++++++++ >> 1 file changed, 64 insertions(+) >> >> diff --git a/include/linux/compiler.h b/include/linux/compiler.h >> index d5ad7b1..947710e 100644 >> --- a/include/linux/compiler.h >> +++ b/include/linux/compiler.h >> @@ -186,6 +186,70 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect); >> # define __UNIQUE_ID(prefix) __PASTE(__PASTE(__UNIQUE_ID_, prefix), __LINE__) >> #endif >> >> +#include <linux/types.h> >> + >> +void data_access_exceeds_word_size(void) >> +__compiletime_warning("data access exceeds word size and won't be atomic"); >> + >> +static __always_inline void __read_once_size(volatile void *p, void *res, int 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; >> +#ifdef CONFIG_64BIT >> + case 8: *(u64 *)res = *(volatile u64 *)p; break; >> +#endif > > You could get rid of the #ifdef above by doing "case sizeof(long)" > and switching the casts from u64 to unsigned long. Wouldnt that cause a compile warning because we have two case statements with the same size? > >> + default: >> + barrier(); >> + __builtin_memcpy((void *)res, (const void *)p, size); >> + data_access_exceeds_word_size(); >> + barrier(); >> + } >> +} >> + >> +static __always_inline void __assign_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; >> +#ifdef CONFIG_64BIT >> + case 8: *(volatile u64 *)p = *(u64 *)res; break; >> +#endif > > Ditto. > >> + default: >> + barrier(); >> + __builtin_memcpy((void *)p, (const void *)res, size); >> + data_access_exceeds_word_size(); >> + barrier(); >> + } >> +} >> + >> +/* >> + * Prevent the compiler from merging or refetching reads or writes. The compiler >> + * is also forbidden from reordering successive instances of READ_ONCE, >> + * ASSIGN_ONCE and ACCESS_ONCE (see below), 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, ASSIGN_ONCE or ACCESS_ONCE() in >> + * different C statements. >> + * >> + * In contrast to ACCESS_ONCE 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. 32bit or 64bit), the access might happen >> + * in multiple chunks, though. > > This last sentence might be more clear if it was something like the > following: > > If the size of the accessed data type exceeeds the word size of > the machine (e.g., 32 bits or 64 bits), ACCESS_ONCE() might > carry out the access in multiple chunks, but READ_ONCE() and > ASSIGN_ONCE() will give a link-time error. The code in v4 now combines Linus (memcpy) and your (error) suggestion. We do a memcpy and a compile time _warning_ So I will do * In contrast to ACCESS_ONCE 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 ASSIGN_ONCE() will fall back to memcpy and print a * compile-time warning. > >> + * >> + * These macros do absolutely -nothing- to prevent the CPU from reordering, >> + * merging, or refetching absolutely anything at any time. Their main intended >> + * use is to mediate communication between process-level code and irq/NMI >> + * handlers, all running on the same CPU. > > This last sentence is now obsolete. How about something like this? > > 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. sounds good. Will change. Thanks > >> + */ >> + >> +#define READ_ONCE(p) \ >> + ({ typeof(p) __val; __read_once_size(&p, &__val, sizeof(__val)); __val; }) >> + >> +#define ASSIGN_ONCE(val, p) \ >> + ({ typeof(p) __val; __val = val; __assign_once_size(&p, &__val, sizeof(__val)); __val; }) >> + >> #endif /* __KERNEL__ */ >> >> #endif /* __ASSEMBLY__ */ >> -- >> 1.9.3 >> -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
