linux-imx

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             LINUX KERNEL MEMORY BARRIERS

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By: David Howells <dhowells@redhat.com>

    Paul E. McKenney <paulmck@linux.vnet.ibm.com>

    Will Deacon <will.deacon@arm.com>

    Peter Zijlstra <peterz@infradead.org>

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DISCLAIMER

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This document is not a specification; it is intentionally (for the sake of

brevity) and unintentionally (due to being human) incomplete. This document is

meant as a guide to using the various memory barriers provided by Linux, but

in case of any doubt (and there are many) please ask.  Some doubts may be

resolved by referring to the formal memory consistency model and related

documentation at tools/memory-model/.  Nevertheless, even this memory

model should be viewed as the collective opinion of its maintainers rather

than as an infallible oracle.

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To repeat, this document is not a specification of what Linux expects from

hardware.

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The purpose of this document is twofold:

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 (1) to specify the minimum functionality that one can rely on for any

     particular barrier, and

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 (2) to provide a guide as to how to use the barriers that are available.

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Note that an architecture can provide more than the minimum requirement

for any particular barrier, but if the architecture provides less than

that, that architecture is incorrect.

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Note also that it is possible that a barrier may be a no-op for an

architecture because the way that arch works renders an explicit barrier

unnecessary in that case.

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CONTENTS

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 (*) Abstract memory access model.

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     - Device operations.

     - Guarantees.

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 (*) What are memory barriers?

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     - Varieties of memory barrier.

     - What may not be assumed about memory barriers?

     - Data dependency barriers (historical).

     - Control dependencies.

     - SMP barrier pairing.

     - Examples of memory barrier sequences.

     - Read memory barriers vs load speculation.

     - Multicopy atomicity.

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 (*) Explicit kernel barriers.

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     - Compiler barrier.

     - CPU memory barriers.

     - MMIO write barrier.

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 (*) Implicit kernel memory barriers.

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     - Lock acquisition functions.

     - Interrupt disabling functions.

     - Sleep and wake-up functions.

     - Miscellaneous functions.

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 (*) Inter-CPU acquiring barrier effects.

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     - Acquires vs memory accesses.

     - Acquires vs I/O accesses.

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 (*) Where are memory barriers needed?

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     - Interprocessor interaction.

     - Atomic operations.

     - Accessing devices.

     - Interrupts.

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 (*) Kernel I/O barrier effects.

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 (*) Assumed minimum execution ordering model.

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 (*) The effects of the cpu cache.

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