Messages in this thread | | | Date | Mon, 06 Aug 2007 10:12:40 -0400 | From | Chris Snook <> | Subject | Re: why are some atomic_t's not volatile, while most are? |
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Jerry Jiang wrote: > Is there some feedback on this point ? > > Thank you > ./Jerry > > On Sun, 1 Jul 2007 08:49:37 -0400 (EDT) > "Robert P. J. Day" <rpjday@mindspring.com> wrote: > >> prompted by the earlier post on "volatile"s, is there a reason that >> most atomic_t typedefs use volatile int's, while the rest don't? >> >> $ grep "typedef.*struct" $(find . -name atomic.h) >> ./include/asm-v850/atomic.h:typedef struct { int counter; } atomic_t; >> ./include/asm-mips/atomic.h:typedef struct { volatile int counter; } atomic_t; >> ./include/asm-mips/atomic.h:typedef struct { volatile long counter; } atomic64_t; >> ... >> >> etc, etc. just curious.
If your architecture doesn't support SMP, the volatile keyword doesn't do anything except add a useless memory fetch. Also, some SMP architectures (i386, x86_64, s390) provide sufficiently strong guarantees about memory access ordering that it's not necessary as long as you're using the appropriate locked/atomic instructions in the atomic operations.
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