1.. SPDX-License-Identifier: GPL-2.0 2 3======================== 4membarrier() System Call 5======================== 6 7MEMBARRIER_CMD_{PRIVATE,GLOBAL}_EXPEDITED - Architecture requirements 8===================================================================== 9 10Memory barriers before updating rq->curr 11---------------------------------------- 12 13The commands MEMBARRIER_CMD_PRIVATE_EXPEDITED and MEMBARRIER_CMD_GLOBAL_EXPEDITED 14require each architecture to have a full memory barrier after coming from 15user-space, before updating rq->curr. This barrier is implied by the sequence 16rq_lock(); smp_mb__after_spinlock() in __schedule(). The barrier matches a full 17barrier in the proximity of the membarrier system call exit, cf. 18membarrier_{private,global}_expedited(). 19 20Memory barriers after updating rq->curr 21--------------------------------------- 22 23The commands MEMBARRIER_CMD_PRIVATE_EXPEDITED and MEMBARRIER_CMD_GLOBAL_EXPEDITED 24require each architecture to have a full memory barrier after updating rq->curr, 25before returning to user-space. The schemes providing this barrier on the various 26architectures are as follows. 27 28 - alpha, arc, arm, hexagon, mips rely on the full barrier implied by 29 spin_unlock() in finish_lock_switch(). 30 31 - arm64 relies on the full barrier implied by switch_to(). 32 33 - powerpc, riscv, s390, sparc, x86 rely on the full barrier implied by 34 switch_mm(), if mm is not NULL; they rely on the full barrier implied 35 by mmdrop(), otherwise. On powerpc and riscv, switch_mm() relies on 36 membarrier_arch_switch_mm(). 37 38The barrier matches a full barrier in the proximity of the membarrier system call 39entry, cf. membarrier_{private,global}_expedited(). 40