forked from luck/tmp_suning_uos_patched
22e4ebb975
Implement MEMBARRIER_CMD_PRIVATE_EXPEDITED with IPIs using cpumask built from all runqueues for which current thread's mm is the same as the thread calling sys_membarrier. It executes faster than the non-expedited variant (no blocking). It also works on NOHZ_FULL configurations. Scheduler-wise, it requires a memory barrier before and after context switching between processes (which have different mm). The memory barrier before context switch is already present. For the barrier after context switch: * Our TSO archs can do RELEASE without being a full barrier. Look at x86 spin_unlock() being a regular STORE for example. But for those archs, all atomics imply smp_mb and all of them have atomic ops in switch_mm() for mm_cpumask(), and on x86 the CR3 load acts as a full barrier. * From all weakly ordered machines, only ARM64 and PPC can do RELEASE, the rest does indeed do smp_mb(), so there the spin_unlock() is a full barrier and we're good. * ARM64 has a very heavy barrier in switch_to(), which suffices. * PPC just removed its barrier from switch_to(), but appears to be talking about adding something to switch_mm(). So add a smp_mb__after_unlock_lock() for now, until this is settled on the PPC side. Changes since v3: - Properly document the memory barriers provided by each architecture. Changes since v2: - Address comments from Peter Zijlstra, - Add smp_mb__after_unlock_lock() after finish_lock_switch() in finish_task_switch() to add the memory barrier we need after storing to rq->curr. This is much simpler than the previous approach relying on atomic_dec_and_test() in mmdrop(), which actually added a memory barrier in the common case of switching between userspace processes. - Return -EINVAL when MEMBARRIER_CMD_SHARED is used on a nohz_full kernel, rather than having the whole membarrier system call returning -ENOSYS. Indeed, CMD_PRIVATE_EXPEDITED is compatible with nohz_full. Adapt the CMD_QUERY mask accordingly. Changes since v1: - move membarrier code under kernel/sched/ because it uses the scheduler runqueue, - only add the barrier when we switch from a kernel thread. The case where we switch from a user-space thread is already handled by the atomic_dec_and_test() in mmdrop(). - add a comment to mmdrop() documenting the requirement on the implicit memory barrier. CC: Peter Zijlstra <peterz@infradead.org> CC: Paul E. McKenney <paulmck@linux.vnet.ibm.com> CC: Boqun Feng <boqun.feng@gmail.com> CC: Andrew Hunter <ahh@google.com> CC: Maged Michael <maged.michael@gmail.com> CC: gromer@google.com CC: Avi Kivity <avi@scylladb.com> CC: Benjamin Herrenschmidt <benh@kernel.crashing.org> CC: Paul Mackerras <paulus@samba.org> CC: Michael Ellerman <mpe@ellerman.id.au> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Tested-by: Dave Watson <davejwatson@fb.com>
153 lines
4.5 KiB
C
153 lines
4.5 KiB
C
/*
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* Copyright (C) 2010-2017 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
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*
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* membarrier system call
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*/
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#include <linux/syscalls.h>
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#include <linux/membarrier.h>
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#include <linux/tick.h>
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#include <linux/cpumask.h>
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#include "sched.h" /* for cpu_rq(). */
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/*
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* Bitmask made from a "or" of all commands within enum membarrier_cmd,
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* except MEMBARRIER_CMD_QUERY.
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*/
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#define MEMBARRIER_CMD_BITMASK \
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(MEMBARRIER_CMD_SHARED | MEMBARRIER_CMD_PRIVATE_EXPEDITED)
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static void ipi_mb(void *info)
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{
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smp_mb(); /* IPIs should be serializing but paranoid. */
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}
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static void membarrier_private_expedited(void)
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{
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int cpu;
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bool fallback = false;
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cpumask_var_t tmpmask;
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if (num_online_cpus() == 1)
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return;
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/*
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* Matches memory barriers around rq->curr modification in
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* scheduler.
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*/
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smp_mb(); /* system call entry is not a mb. */
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/*
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* Expedited membarrier commands guarantee that they won't
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* block, hence the GFP_NOWAIT allocation flag and fallback
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* implementation.
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*/
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if (!zalloc_cpumask_var(&tmpmask, GFP_NOWAIT)) {
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/* Fallback for OOM. */
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fallback = true;
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}
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cpus_read_lock();
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for_each_online_cpu(cpu) {
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struct task_struct *p;
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/*
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* Skipping the current CPU is OK even through we can be
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* migrated at any point. The current CPU, at the point
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* where we read raw_smp_processor_id(), is ensured to
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* be in program order with respect to the caller
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* thread. Therefore, we can skip this CPU from the
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* iteration.
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*/
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if (cpu == raw_smp_processor_id())
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continue;
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rcu_read_lock();
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p = task_rcu_dereference(&cpu_rq(cpu)->curr);
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if (p && p->mm == current->mm) {
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if (!fallback)
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__cpumask_set_cpu(cpu, tmpmask);
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else
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smp_call_function_single(cpu, ipi_mb, NULL, 1);
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}
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rcu_read_unlock();
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}
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if (!fallback) {
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smp_call_function_many(tmpmask, ipi_mb, NULL, 1);
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free_cpumask_var(tmpmask);
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}
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cpus_read_unlock();
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/*
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* Memory barrier on the caller thread _after_ we finished
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* waiting for the last IPI. Matches memory barriers around
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* rq->curr modification in scheduler.
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*/
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smp_mb(); /* exit from system call is not a mb */
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}
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/**
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* sys_membarrier - issue memory barriers on a set of threads
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* @cmd: Takes command values defined in enum membarrier_cmd.
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* @flags: Currently needs to be 0. For future extensions.
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*
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* If this system call is not implemented, -ENOSYS is returned. If the
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* command specified does not exist, not available on the running
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* kernel, or if the command argument is invalid, this system call
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* returns -EINVAL. For a given command, with flags argument set to 0,
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* this system call is guaranteed to always return the same value until
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* reboot.
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*
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* All memory accesses performed in program order from each targeted thread
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* is guaranteed to be ordered with respect to sys_membarrier(). If we use
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* the semantic "barrier()" to represent a compiler barrier forcing memory
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* accesses to be performed in program order across the barrier, and
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* smp_mb() to represent explicit memory barriers forcing full memory
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* ordering across the barrier, we have the following ordering table for
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* each pair of barrier(), sys_membarrier() and smp_mb():
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*
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* The pair ordering is detailed as (O: ordered, X: not ordered):
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*
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* barrier() smp_mb() sys_membarrier()
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* barrier() X X O
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* smp_mb() X O O
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* sys_membarrier() O O O
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*/
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SYSCALL_DEFINE2(membarrier, int, cmd, int, flags)
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{
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if (unlikely(flags))
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return -EINVAL;
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switch (cmd) {
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case MEMBARRIER_CMD_QUERY:
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{
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int cmd_mask = MEMBARRIER_CMD_BITMASK;
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if (tick_nohz_full_enabled())
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cmd_mask &= ~MEMBARRIER_CMD_SHARED;
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return cmd_mask;
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}
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case MEMBARRIER_CMD_SHARED:
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/* MEMBARRIER_CMD_SHARED is not compatible with nohz_full. */
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if (tick_nohz_full_enabled())
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return -EINVAL;
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if (num_online_cpus() > 1)
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synchronize_sched();
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return 0;
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case MEMBARRIER_CMD_PRIVATE_EXPEDITED:
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membarrier_private_expedited();
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return 0;
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default:
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return -EINVAL;
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}
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}
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