Re: LDD3 pitfalls (was Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures)

Stefan Richter wrote:

Nick Piggin wrote:

Stefan Richter wrote:

Nick Piggin wrote:

I don't know why people would assume volatile of atomics. AFAIK, most
of the documentation is pretty clear that all the atomic stuff can be
reordered etc. except for those that modify and return a value.

Which documentation is there?

Documentation/atomic_ops.txt

For driver authors, there is LDD3.  It doesn't specifically cover
effects of optimization on accesses to atomic_t.

For architecture port authors, there is Documentation/atomic_ops.txt.
Driver authors also can learn something from that document, as it
indirectly documents the atomic_t and bitops APIs.

"Semantics and Behavior of Atomic and Bitmask Operations" is
pretty direct :)

Sure, it says that it's for arch maintainers, but there is no
reason why users can't make use of it.

Note, LDD3 page 238 says:  "It is worth noting that most of the other
kernel primitives dealing with synchronization, such as spinlock and
atomic_t operations, also function as memory barriers."

I don't know about Linux 2.6.10 against which LDD3 was written, but
currently only _some_ atomic_t operations function as memory barriers.

Besides, judging from some posts in this thread, saying that atomic_t
operations dealt with synchronization may not be entirely precise.

atomic_t is often used as the basis for implementing more sophisticated synchronization mechanisms, such as rwlocks. Whether or not they are designed for that purpose, the atomic_* operations are de facto synchronization primitives.

	-- Chris
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---

• References:
  • Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures
    • From: Satyam Sharma <[email protected]>
  • Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures
    • From: Paul Mackerras <[email protected]>
  • Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures
    • From: Herbert Xu <[email protected]>
  • Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures
    • From: Paul Mackerras <[email protected]>
  • Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures
    • From: Herbert Xu <[email protected]>
  • Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures
    • From: Stefan Richter <[email protected]>
  • Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures
    • From: Herbert Xu <[email protected]>
  • Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures
    • From: Stefan Richter <[email protected]>
  • Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures
    • From: Stefan Richter <[email protected]>
  • Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures
    • From: Herbert Xu <[email protected]>
  • Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures
    • From: "Paul E. McKenney" <[email protected]>
  • Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures
    • From: Nick Piggin <[email protected]>
  • Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures
    • From: Stefan Richter <[email protected]>
  • Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures
    • From: Nick Piggin <[email protected]>
  • LDD3 pitfalls (was Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures)
    • From: Stefan Richter <[email protected]>

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