Re: [PATCH 1/9] kernel: Provide READ_ONCE and ASSIGN_ONCE

[Christian Borntraeger]

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
>>

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