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().