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.. SPDX-License-Identifier: GPL-2.0
========================
membarrier() System Call
========================
MEMBARRIER_CMD_{PRIVATE,GLOBAL}_EXPEDITED - Architecture requirements
=====================================================================
Memory barriers before updating rq->curr
----------------------------------------
The commands MEMBARRIER_CMD_PRIVATE_EXPEDITED and MEMBARRIER_CMD_GLOBAL_EXPEDITED
require each architecture to have a full memory barrier after coming from
user-space, before updating rq->curr. This barrier is implied by the sequence
rq_lock(); smp_mb__after_spinlock() in __schedule(). The barrier matches a full
barrier in the proximity of the membarrier system call exit, cf.
membarrier_{private,global}_expedited().
Memory barriers after updating rq->curr
---------------------------------------
The commands MEMBARRIER_CMD_PRIVATE_EXPEDITED and MEMBARRIER_CMD_GLOBAL_EXPEDITED
require each architecture to have a full memory barrier after updating rq->curr,
before returning to user-space. The schemes providing this barrier on the various
architectures are as follows.
- alpha, arc, arm, hexagon, mips rely on the full barrier implied by
spin_unlock() in finish_lock_switch().
- arm64 relies on the full barrier implied by switch_to().
- powerpc, riscv, s390, sparc, x86 rely on the full barrier implied by
switch_mm(), if mm is not NULL; they rely on the full barrier implied
by mmdrop(), otherwise. On powerpc and riscv, switch_mm() relies on
membarrier_arch_switch_mm().
The barrier matches a full barrier in the proximity of the membarrier system call
entry, cf. membarrier_{private,global}_expedited().
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