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SubjectRe: [RFC][PATCH]: documentation,atomic: Add a new atomic_t document
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On 06/09/17 02:24, Peter Zijlstra wrote:
>
> --- /dev/null 2017-05-05 13:16:22.636212333 +0200
> +++ b/Documentation/atomic_t.txt 2017-06-09 11:05:31.501599153 +0200
> @@ -0,0 +1,147 @@
> +
> +The one detail to this is that atomic_set() should be observable to the RmW
> +ops. That is:
> +
> + CPU0 CPU1
> +
> + val = atomic_read(&X)
> + do {
> + atomic_set(&X, 0)
> + new = val + 1;
> + } while (!atomic_try_cmpxchg(&X, &val, new));
> +
> +Should cause the cmpxchg to *FAIL* (when @val != 0). This is typically true;

should

> +on 'normal' platforms; a regular competing STORE will invalidate a LL/SC.

too many semi-colons above.

> +
> +The obvious case where this is not so is where we need to implement atomic ops
> +with a spinlock hashtable; the typical solution is to then implement
> +atomic_set() with atomic_xchg().
> +
> +
> +RmW ops:
> +
> +These come in various forms:
> +
> + - plain operations without return value: atomic_{}()
> +
> + - operations which return the modified value: atomic_{}_return()
> +
> + these are limited to the arithmetic operations because those are
> + reversible. Bitops are irreversible and therefore the modified value
> + is of dubious utility.
> +
> + - operations which return the original value: atomic_fetch_{}()
> +
> + - swap operations: xchg(), cmpxchg() and try_cmpxchg()
> +
> + - misc; the special purpose operations that are commonly used and would,
> + given the interface, normally be implemented using (try_)cmpxchg loops but
> + are time critical and can, (typically) on LL/SC architectures, be more
> + efficiently implemented.
> +
> +
> +All these operations are SMP atomic; that is, the operations (for a single
> +atomic variable) can be fully ordered and no intermediate state is lost or
> +visible.
> +
> +
> +Ordering: (go read memory-barriers.txt first)
> +
> +The rule of thumb:
> +
> + - non-RmW operations are unordered;
> +
> + - RmW operations that have no return value are unordered;
> +
> + - RmW operations that have a return value are Sequentially Consistent;
> +
> + - RmW operations that are conditional are unordered on FAILURE, otherwise the
> + above rules apply.
> +
> +Except of course when an operation has an explicit ordering like:
> +
> + {}_relaxed: unordered
> + {}_acquire: the R of the RmW is an ACQUIRE
> + {}_release: the W of the RmW is a RELEASE
> +
> +NOTE: our ACQUIRE/RELEASE are RCpc
> +
> +
> +The barriers:
> +
> + smp_mb__{before,after}_atomic()
> +
> +only apply to the RmW ops and can be used to augment/upgrade the ordering
> +inherit to the used atomic op. These barriers provide a full smp_mb().

inherent ?

> +
> +These helper barriers exist because architectures have varying implicit
> +ordering on their SMP atomic primitives. For example our TSO architectures
> +provide SC atomics and these barriers are no-ops.
> +
> +So while something like:
> +
> + smp_mb__before_atomic();
> + val = atomic_dec_return_relaxed(&X);
> +
> +is a 'typical' RELEASE pattern (please use atomic_dec_return_release()), the
> +barrier is strictly stronger than a RELEASE.
>


--
~Randy

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