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SubjectRe: [PATCH] make atomic_t volatile on all architectures
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.

-- Chris
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