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device-core: use RCU for list of children of a bus 

This fixes the race between device emulation code that tries to find
a child device to dispatch the request to (e.g a scsi disk),
and hotplug of a new device to that bus.

Note that this doesn't convert all the readers of the list
but only these that might go over that list without BQL held.

This is a very small first step to make this code thread safe.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20200913160259.32145-5-mlevitsk@redhat.com>
[Use RCU_READ_LOCK_GUARD in more places, adjust testcase now that
 the delay in DEVICE_DELETED due to RCU is more consistent. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20201006123904.610658-9-mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Maxim Levitsky 2020-10-06 15:38:59 +03:00 committed by Paolo Bonzini
parent 7bed89958b
commit 2d24a64661
5 changed files with 63 additions and 29 deletions
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28
hw/core/bus.c
View File

@ -49,12 +49,14 @@ int qbus_walk_children(BusState *bus,
}
}
QTAILQ_FOREACH(kid, &bus->children, sibling) {
err = qdev_walk_children(kid->child,
pre_devfn, pre_busfn,
post_devfn, post_busfn, opaque);
if (err < 0) {
return err;
WITH_RCU_READ_LOCK_GUARD() {
QTAILQ_FOREACH_RCU(kid, &bus->children, sibling) {
err = qdev_walk_children(kid->child,
pre_devfn, pre_busfn,
post_devfn, post_busfn, opaque);
if (err < 0) {
return err;
}
}
}
@ -90,8 +92,10 @@ static void bus_reset_child_foreach(Object *obj, ResettableChildCallback cb,
BusState *bus = BUS(obj);
BusChild *kid;
QTAILQ_FOREACH(kid, &bus->children, sibling) {
cb(OBJECT(kid->child), opaque, type);
WITH_RCU_READ_LOCK_GUARD() {
QTAILQ_FOREACH_RCU(kid, &bus->children, sibling) {
cb(OBJECT(kid->child), opaque, type);
}
}
}
@ -194,9 +198,11 @@ static void bus_set_realized(Object *obj, bool value, Error **errp)
/* TODO: recursive realization */
} else if (!value && bus->realized) {
QTAILQ_FOREACH(kid, &bus->children, sibling) {
DeviceState *dev = kid->child;
qdev_unrealize(dev);
WITH_RCU_READ_LOCK_GUARD() {
QTAILQ_FOREACH_RCU(kid, &bus->children, sibling) {
DeviceState *dev = kid->child;
qdev_unrealize(dev);
}
}
if (bc->unrealize) {
bc->unrealize(bus);

37
hw/core/qdev.c
View File

@ -51,6 +51,12 @@ const VMStateDescription *qdev_get_vmsd(DeviceState *dev)
return dc->vmsd;
}
static void bus_free_bus_child(BusChild *kid)
{
object_unref(OBJECT(kid->child));
g_free(kid);
}
static void bus_remove_child(BusState *bus, DeviceState *child)
{
BusChild *kid;
@ -60,15 +66,16 @@ static void bus_remove_child(BusState *bus, DeviceState *child)
char name[32];
snprintf(name, sizeof(name), "child[%d]", kid->index);
QTAILQ_REMOVE(&bus->children, kid, sibling);
QTAILQ_REMOVE_RCU(&bus->children, kid, sibling);
bus->num_children--;
/* This gives back ownership of kid->child back to us. */
object_property_del(OBJECT(bus), name);
object_unref(OBJECT(kid->child));
g_free(kid);
return;
/* free the bus kid, when it is safe to do so*/
call_rcu(kid, bus_free_bus_child, rcu);
break;
}
}
}
@ -83,7 +90,7 @@ static void bus_add_child(BusState *bus, DeviceState *child)
kid->child = child;
object_ref(OBJECT(kid->child));
QTAILQ_INSERT_HEAD(&bus->children, kid, sibling);
QTAILQ_INSERT_HEAD_RCU(&bus->children, kid, sibling);
/* This transfers ownership of kid->child to the property. */
snprintf(name, sizeof(name), "child[%d]", kid->index);
@ -672,17 +679,19 @@ DeviceState *qdev_find_recursive(BusState *bus, const char *id)
DeviceState *ret;
BusState *child;
QTAILQ_FOREACH(kid, &bus->children, sibling) {
DeviceState *dev = kid->child;
WITH_RCU_READ_LOCK_GUARD() {
QTAILQ_FOREACH_RCU(kid, &bus->children, sibling) {
DeviceState *dev = kid->child;
if (dev->id && strcmp(dev->id, id) == 0) {
return dev;
}
if (dev->id && strcmp(dev->id, id) == 0) {
return dev;
}
QLIST_FOREACH(child, &dev->child_bus, sibling) {
ret = qdev_find_recursive(child, id);
if (ret) {
return ret;
QLIST_FOREACH(child, &dev->child_bus, sibling) {
ret = qdev_find_recursive(child, id);
if (ret) {
return ret;
}
}
}
}

12
hw/scsi/scsi-bus.c
View File

@ -400,7 +400,10 @@ static bool scsi_target_emulate_report_luns(SCSITargetReq *r)
id = r->req.dev->id;
found_lun0 = false;
n = 0;
QTAILQ_FOREACH(kid, &r->req.bus->qbus.children, sibling) {
RCU_READ_LOCK_GUARD();
QTAILQ_FOREACH_RCU(kid, &r->req.bus->qbus.children, sibling) {
DeviceState *qdev = kid->child;
SCSIDevice *dev = SCSI_DEVICE(qdev);
@ -421,7 +424,7 @@ static bool scsi_target_emulate_report_luns(SCSITargetReq *r)
memset(r->buf, 0, len);
stl_be_p(&r->buf[0], n);
i = found_lun0 ? 8 : 16;
QTAILQ_FOREACH(kid, &r->req.bus->qbus.children, sibling) {
QTAILQ_FOREACH_RCU(kid, &r->req.bus->qbus.children, sibling) {
DeviceState *qdev = kid->child;
SCSIDevice *dev = SCSI_DEVICE(qdev);
@ -430,6 +433,7 @@ static bool scsi_target_emulate_report_luns(SCSITargetReq *r)
i += 8;
}
}
assert(i == n + 8);
r->len = len;
return true;
@ -1572,7 +1576,8 @@ SCSIDevice *scsi_device_find(SCSIBus *bus, int channel, int id, int lun)
BusChild *kid;
SCSIDevice *target_dev = NULL;
QTAILQ_FOREACH(kid, &bus->qbus.children, sibling) {
RCU_READ_LOCK_GUARD();
QTAILQ_FOREACH_RCU(kid, &bus->qbus.children, sibling) {
DeviceState *qdev = kid->child;
SCSIDevice *dev = SCSI_DEVICE(qdev);
@ -1591,6 +1596,7 @@ SCSIDevice *scsi_device_find(SCSIBus *bus, int channel, int id, int lun)
}
}
}
return target_dev;
}

6
hw/scsi/virtio-scsi.c
View File

@ -367,12 +367,16 @@ static int virtio_scsi_do_tmf(VirtIOSCSI *s, VirtIOSCSIReq *req)
case VIRTIO_SCSI_T_TMF_I_T_NEXUS_RESET:
target = req->req.tmf.lun[1];
s->resetting++;
QTAILQ_FOREACH(kid, &s->bus.qbus.children, sibling) {
rcu_read_lock();
QTAILQ_FOREACH_RCU(kid, &s->bus.qbus.children, sibling) {
d = SCSI_DEVICE(kid->child);
if (d->channel == 0 && d->id == target) {
qdev_reset_all(&d->qdev);
}
}
rcu_read_unlock();
s->resetting--;
break;

9
include/hw/qdev-core.h
View File

@ -3,6 +3,8 @@
#include "qemu/queue.h"
#include "qemu/bitmap.h"
#include "qemu/rcu.h"
#include "qemu/rcu_queue.h"
#include "qom/object.h"
#include "hw/hotplug.h"
#include "hw/resettable.h"
@ -238,6 +240,7 @@ struct BusClass {
};
typedef struct BusChild {
struct rcu_head rcu;
DeviceState *child;
int index;
QTAILQ_ENTRY(BusChild) sibling;
@ -258,6 +261,12 @@ struct BusState {
int max_index;
bool realized;
int num_children;
/*
* children is a RCU QTAILQ, thus readers must use RCU to access it,
* and writers must hold the big qemu lock
*/
QTAILQ_HEAD(, BusChild) children;
QLIST_ENTRY(BusState) sibling;
ResettableState reset;