It was introduced in commit ab129972c8
,
with the following motivation:
Because start_exclusive uses CPU_FOREACH, merge exclusive_lock with
qemu_cpu_list_lock: together with a call to exclusive_idle (via
cpu_exec_start/end) in cpu_list_add, this protects exclusive work
against concurrent CPU addition and removal.
However, it seems to be redundant, because the cpu-exclusive
infrastructure provides suffificent protection against the newly added
CPU starting execution while the cpu-exclusive work is running, and the
aforementioned traversing of the cpu list is protected by
qemu_cpu_list_lock.
Besides, this appears to be the only place where the cpu-exclusive
section is entered with the BQL taken, which has been found to trigger
AB-BA deadlock as follows:
vCPU thread main thread
----------- -----------
async_safe_run_on_cpu(self,
async_synic_update)
... [cpu hot-add]
process_queued_cpu_work()
qemu_mutex_unlock_iothread()
[grab BQL]
start_exclusive() cpu_list_add()
async_synic_update() finish_safe_work()
qemu_mutex_lock_iothread() cpu_exec_start()
So remove it. This paves the way to establishing a strict nesting rule
of never entering the exclusive section with the BQL taken.
Signed-off-by: Roman Kagan <rkagan@virtuozzo.com>
Message-Id: <20190523105440.27045-2-rkagan@virtuozzo.com>
346 lines
10 KiB
C
346 lines
10 KiB
C
/*
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* CPU thread main loop - common bits for user and system mode emulation
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*
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* Copyright (c) 2003-2005 Fabrice Bellard
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*
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* This library is free software; you can redistribute it and/or
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* modify it under the terms of the GNU Lesser General Public
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* License as published by the Free Software Foundation; either
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* version 2 of the License, or (at your option) any later version.
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*
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* This library 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 GNU
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* Lesser General Public License for more details.
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*
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* You should have received a copy of the GNU Lesser General Public
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* License along with this library; if not, see <http://www.gnu.org/licenses/>.
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*/
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#include "qemu/osdep.h"
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#include "qemu/main-loop.h"
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#include "exec/cpu-common.h"
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#include "qom/cpu.h"
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#include "sysemu/cpus.h"
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static QemuMutex qemu_cpu_list_lock;
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static QemuCond exclusive_cond;
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static QemuCond exclusive_resume;
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static QemuCond qemu_work_cond;
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/* >= 1 if a thread is inside start_exclusive/end_exclusive. Written
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* under qemu_cpu_list_lock, read with atomic operations.
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*/
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static int pending_cpus;
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void qemu_init_cpu_list(void)
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{
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/* This is needed because qemu_init_cpu_list is also called by the
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* child process in a fork. */
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pending_cpus = 0;
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qemu_mutex_init(&qemu_cpu_list_lock);
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qemu_cond_init(&exclusive_cond);
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qemu_cond_init(&exclusive_resume);
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qemu_cond_init(&qemu_work_cond);
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}
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void cpu_list_lock(void)
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{
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qemu_mutex_lock(&qemu_cpu_list_lock);
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}
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void cpu_list_unlock(void)
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{
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qemu_mutex_unlock(&qemu_cpu_list_lock);
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}
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static bool cpu_index_auto_assigned;
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static int cpu_get_free_index(void)
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{
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CPUState *some_cpu;
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int cpu_index = 0;
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cpu_index_auto_assigned = true;
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CPU_FOREACH(some_cpu) {
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cpu_index++;
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}
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return cpu_index;
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}
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void cpu_list_add(CPUState *cpu)
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{
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qemu_mutex_lock(&qemu_cpu_list_lock);
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if (cpu->cpu_index == UNASSIGNED_CPU_INDEX) {
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cpu->cpu_index = cpu_get_free_index();
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assert(cpu->cpu_index != UNASSIGNED_CPU_INDEX);
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} else {
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assert(!cpu_index_auto_assigned);
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}
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QTAILQ_INSERT_TAIL_RCU(&cpus, cpu, node);
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qemu_mutex_unlock(&qemu_cpu_list_lock);
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}
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void cpu_list_remove(CPUState *cpu)
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{
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qemu_mutex_lock(&qemu_cpu_list_lock);
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if (!QTAILQ_IN_USE(cpu, node)) {
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/* there is nothing to undo since cpu_exec_init() hasn't been called */
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qemu_mutex_unlock(&qemu_cpu_list_lock);
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return;
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}
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assert(!(cpu_index_auto_assigned && cpu != QTAILQ_LAST(&cpus)));
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QTAILQ_REMOVE_RCU(&cpus, cpu, node);
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cpu->cpu_index = UNASSIGNED_CPU_INDEX;
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qemu_mutex_unlock(&qemu_cpu_list_lock);
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}
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struct qemu_work_item {
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struct qemu_work_item *next;
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run_on_cpu_func func;
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run_on_cpu_data data;
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bool free, exclusive, done;
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};
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static void queue_work_on_cpu(CPUState *cpu, struct qemu_work_item *wi)
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{
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qemu_mutex_lock(&cpu->work_mutex);
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if (cpu->queued_work_first == NULL) {
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cpu->queued_work_first = wi;
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} else {
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cpu->queued_work_last->next = wi;
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}
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cpu->queued_work_last = wi;
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wi->next = NULL;
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wi->done = false;
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qemu_mutex_unlock(&cpu->work_mutex);
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qemu_cpu_kick(cpu);
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}
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void do_run_on_cpu(CPUState *cpu, run_on_cpu_func func, run_on_cpu_data data,
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QemuMutex *mutex)
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{
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struct qemu_work_item wi;
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if (qemu_cpu_is_self(cpu)) {
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func(cpu, data);
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return;
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}
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wi.func = func;
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wi.data = data;
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wi.done = false;
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wi.free = false;
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wi.exclusive = false;
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queue_work_on_cpu(cpu, &wi);
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while (!atomic_mb_read(&wi.done)) {
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CPUState *self_cpu = current_cpu;
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qemu_cond_wait(&qemu_work_cond, mutex);
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current_cpu = self_cpu;
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}
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}
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void async_run_on_cpu(CPUState *cpu, run_on_cpu_func func, run_on_cpu_data data)
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{
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struct qemu_work_item *wi;
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wi = g_malloc0(sizeof(struct qemu_work_item));
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wi->func = func;
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wi->data = data;
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wi->free = true;
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queue_work_on_cpu(cpu, wi);
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}
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/* Wait for pending exclusive operations to complete. The CPU list lock
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must be held. */
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static inline void exclusive_idle(void)
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{
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while (pending_cpus) {
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qemu_cond_wait(&exclusive_resume, &qemu_cpu_list_lock);
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}
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}
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/* Start an exclusive operation.
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Must only be called from outside cpu_exec. */
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void start_exclusive(void)
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{
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CPUState *other_cpu;
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int running_cpus;
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qemu_mutex_lock(&qemu_cpu_list_lock);
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exclusive_idle();
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/* Make all other cpus stop executing. */
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atomic_set(&pending_cpus, 1);
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/* Write pending_cpus before reading other_cpu->running. */
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smp_mb();
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running_cpus = 0;
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CPU_FOREACH(other_cpu) {
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if (atomic_read(&other_cpu->running)) {
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other_cpu->has_waiter = true;
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running_cpus++;
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qemu_cpu_kick(other_cpu);
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}
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}
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atomic_set(&pending_cpus, running_cpus + 1);
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while (pending_cpus > 1) {
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qemu_cond_wait(&exclusive_cond, &qemu_cpu_list_lock);
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}
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/* Can release mutex, no one will enter another exclusive
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* section until end_exclusive resets pending_cpus to 0.
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*/
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qemu_mutex_unlock(&qemu_cpu_list_lock);
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}
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/* Finish an exclusive operation. */
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void end_exclusive(void)
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{
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qemu_mutex_lock(&qemu_cpu_list_lock);
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atomic_set(&pending_cpus, 0);
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qemu_cond_broadcast(&exclusive_resume);
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qemu_mutex_unlock(&qemu_cpu_list_lock);
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}
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/* Wait for exclusive ops to finish, and begin cpu execution. */
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void cpu_exec_start(CPUState *cpu)
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{
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atomic_set(&cpu->running, true);
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/* Write cpu->running before reading pending_cpus. */
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smp_mb();
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/* 1. start_exclusive saw cpu->running == true and pending_cpus >= 1.
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* After taking the lock we'll see cpu->has_waiter == true and run---not
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* for long because start_exclusive kicked us. cpu_exec_end will
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* decrement pending_cpus and signal the waiter.
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*
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* 2. start_exclusive saw cpu->running == false but pending_cpus >= 1.
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* This includes the case when an exclusive item is running now.
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* Then we'll see cpu->has_waiter == false and wait for the item to
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* complete.
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*
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* 3. pending_cpus == 0. Then start_exclusive is definitely going to
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* see cpu->running == true, and it will kick the CPU.
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*/
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if (unlikely(atomic_read(&pending_cpus))) {
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qemu_mutex_lock(&qemu_cpu_list_lock);
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if (!cpu->has_waiter) {
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/* Not counted in pending_cpus, let the exclusive item
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* run. Since we have the lock, just set cpu->running to true
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* while holding it; no need to check pending_cpus again.
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*/
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atomic_set(&cpu->running, false);
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exclusive_idle();
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/* Now pending_cpus is zero. */
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atomic_set(&cpu->running, true);
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} else {
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/* Counted in pending_cpus, go ahead and release the
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* waiter at cpu_exec_end.
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*/
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}
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qemu_mutex_unlock(&qemu_cpu_list_lock);
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}
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}
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/* Mark cpu as not executing, and release pending exclusive ops. */
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void cpu_exec_end(CPUState *cpu)
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{
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atomic_set(&cpu->running, false);
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/* Write cpu->running before reading pending_cpus. */
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smp_mb();
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/* 1. start_exclusive saw cpu->running == true. Then it will increment
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* pending_cpus and wait for exclusive_cond. After taking the lock
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* we'll see cpu->has_waiter == true.
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*
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* 2. start_exclusive saw cpu->running == false but here pending_cpus >= 1.
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* This includes the case when an exclusive item started after setting
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* cpu->running to false and before we read pending_cpus. Then we'll see
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* cpu->has_waiter == false and not touch pending_cpus. The next call to
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* cpu_exec_start will run exclusive_idle if still necessary, thus waiting
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* for the item to complete.
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*
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* 3. pending_cpus == 0. Then start_exclusive is definitely going to
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* see cpu->running == false, and it can ignore this CPU until the
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* next cpu_exec_start.
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*/
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if (unlikely(atomic_read(&pending_cpus))) {
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qemu_mutex_lock(&qemu_cpu_list_lock);
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if (cpu->has_waiter) {
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cpu->has_waiter = false;
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atomic_set(&pending_cpus, pending_cpus - 1);
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if (pending_cpus == 1) {
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qemu_cond_signal(&exclusive_cond);
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}
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}
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qemu_mutex_unlock(&qemu_cpu_list_lock);
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}
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}
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void async_safe_run_on_cpu(CPUState *cpu, run_on_cpu_func func,
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run_on_cpu_data data)
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{
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struct qemu_work_item *wi;
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wi = g_malloc0(sizeof(struct qemu_work_item));
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wi->func = func;
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wi->data = data;
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wi->free = true;
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wi->exclusive = true;
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queue_work_on_cpu(cpu, wi);
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}
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void process_queued_cpu_work(CPUState *cpu)
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{
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struct qemu_work_item *wi;
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if (cpu->queued_work_first == NULL) {
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return;
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}
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qemu_mutex_lock(&cpu->work_mutex);
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while (cpu->queued_work_first != NULL) {
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wi = cpu->queued_work_first;
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cpu->queued_work_first = wi->next;
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if (!cpu->queued_work_first) {
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cpu->queued_work_last = NULL;
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}
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qemu_mutex_unlock(&cpu->work_mutex);
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if (wi->exclusive) {
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/* Running work items outside the BQL avoids the following deadlock:
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* 1) start_exclusive() is called with the BQL taken while another
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* CPU is running; 2) cpu_exec in the other CPU tries to takes the
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* BQL, so it goes to sleep; start_exclusive() is sleeping too, so
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* neither CPU can proceed.
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*/
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qemu_mutex_unlock_iothread();
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start_exclusive();
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wi->func(cpu, wi->data);
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end_exclusive();
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qemu_mutex_lock_iothread();
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} else {
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wi->func(cpu, wi->data);
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}
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qemu_mutex_lock(&cpu->work_mutex);
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if (wi->free) {
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g_free(wi);
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} else {
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atomic_mb_set(&wi->done, true);
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}
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}
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qemu_mutex_unlock(&cpu->work_mutex);
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qemu_cond_broadcast(&qemu_work_cond);
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}
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