On Thu, Dec 04, 2014 at 10:24:47AM +0100, Christian Borntraeger wrote: > 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? Indeed it would! Color me blind and oblivious. > >> + 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. Very good! Thanx, Paul > >> + * > >> + * 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
