Messages in this thread | | | Date | Thu, 09 Aug 2007 03:31:10 -0400 | From | Chris Snook <> | Subject | Re: [PATCH] make atomic_t volatile on all architectures |
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Linus Torvalds wrote: > > On Wed, 8 Aug 2007, Chris Snook wrote: >> Some architectures currently do not declare the contents of an atomic_t to be >> volatile. This causes confusion since atomic_read() might not actually read >> anything if an optimizing compiler re-uses a value stored in a register, which >> can break code that loops until something external changes the value of an >> atomic_t. > > I'd be *much* happier with "atomic_read()" doing the "volatile" instead. > > The fact is, volatile on data structures is a bug. It's a wart in the C > language. It shouldn't be used. > > Volatile accesses in *code* can be ok, and if we have "atomic_read()" > expand to a "*(volatile int *)&(x)->value", then I'd be ok with that. > > But marking data structures volatile just makes the compiler screw up > totally, and makes code for initialization sequences etc much worse. > > Linus
Fair enough. Casting to (volatile int *) will give us the behavior people expect when using atomic_t without needing to use inefficient barriers.
While we have the hood up, should we convert all the atomic_t's to non-volatile and put volatile casts in all the atomic_reads? I don't know enough about the various arches to say with confidence that those changes alone will preserve existing behavior. We might need some arch-specific tweaking of the atomic operations.
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