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From 42dd1357310bd1a68d6cacaa53cd5b1d1b02880d Mon Sep 17 00:00:00 2001
From: Stefan Hajnoczi <stefanha@redhat.com>
Date: Mon, 4 Dec 2023 11:42:56 -0500
Subject: [PATCH 077/101] scsi: only access SCSIDevice->requests from one
thread
RH-Author: Kevin Wolf <kwolf@redhat.com>
RH-MergeRequest: 214: Remove AioContext lock
RH-Jira: RHEL-15965
RH-Acked-by: Miroslav Rezanina <mrezanin@redhat.com>
RH-Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
RH-Commit: [8/26] 9df662e82a63e93d184b5763bebbe7e43bc9dabe (kmwolf/centos-qemu-kvm)
Stop depending on the AioContext lock and instead access
SCSIDevice->requests from only one thread at a time:
- When the VM is running only the BlockBackend's AioContext may access
the requests list.
- When the VM is stopped only the main loop may access the requests
list.
These constraints protect the requests list without the need for locking
in the I/O code path.
Note that multiple IOThreads are not supported yet because the code
assumes all SCSIRequests are executed from a single AioContext. Leave
that as future work.
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-ID: <20231204164259.1515217-2-stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
hw/scsi/scsi-bus.c | 181 ++++++++++++++++++++++++++++-------------
include/hw/scsi/scsi.h | 7 +-
2 files changed, 131 insertions(+), 57 deletions(-)
diff --git a/hw/scsi/scsi-bus.c b/hw/scsi/scsi-bus.c
index fc4b77fdb0..b649cdf555 100644
--- a/hw/scsi/scsi-bus.c
+++ b/hw/scsi/scsi-bus.c
@@ -85,6 +85,89 @@ SCSIDevice *scsi_device_get(SCSIBus *bus, int channel, int id, int lun)
return d;
}
+/*
+ * Invoke @fn() for each enqueued request in device @s. Must be called from the
+ * main loop thread while the guest is stopped. This is only suitable for
+ * vmstate ->put(), use scsi_device_for_each_req_async() for other cases.
+ */
+static void scsi_device_for_each_req_sync(SCSIDevice *s,
+ void (*fn)(SCSIRequest *, void *),
+ void *opaque)
+{
+ SCSIRequest *req;
+ SCSIRequest *next_req;
+
+ assert(!runstate_is_running());
+ assert(qemu_in_main_thread());
+
+ QTAILQ_FOREACH_SAFE(req, &s->requests, next, next_req) {
+ fn(req, opaque);
+ }
+}
+
+typedef struct {
+ SCSIDevice *s;
+ void (*fn)(SCSIRequest *, void *);
+ void *fn_opaque;
+} SCSIDeviceForEachReqAsyncData;
+
+static void scsi_device_for_each_req_async_bh(void *opaque)
+{
+ g_autofree SCSIDeviceForEachReqAsyncData *data = opaque;
+ SCSIDevice *s = data->s;
+ AioContext *ctx;
+ SCSIRequest *req;
+ SCSIRequest *next;
+
+ /*
+ * If the AioContext changed before this BH was called then reschedule into
+ * the new AioContext before accessing ->requests. This can happen when
+ * scsi_device_for_each_req_async() is called and then the AioContext is
+ * changed before BHs are run.
+ */
+ ctx = blk_get_aio_context(s->conf.blk);
+ if (ctx != qemu_get_current_aio_context()) {
+ aio_bh_schedule_oneshot(ctx, scsi_device_for_each_req_async_bh,
+ g_steal_pointer(&data));
+ return;
+ }
+
+ QTAILQ_FOREACH_SAFE(req, &s->requests, next, next) {
+ data->fn(req, data->fn_opaque);
+ }
+
+ /* Drop the reference taken by scsi_device_for_each_req_async() */
+ object_unref(OBJECT(s));
+}
+
+/*
+ * Schedule @fn() to be invoked for each enqueued request in device @s. @fn()
+ * runs in the AioContext that is executing the request.
+ */
+static void scsi_device_for_each_req_async(SCSIDevice *s,
+ void (*fn)(SCSIRequest *, void *),
+ void *opaque)
+{
+ assert(qemu_in_main_thread());
+
+ SCSIDeviceForEachReqAsyncData *data =
+ g_new(SCSIDeviceForEachReqAsyncData, 1);
+
+ data->s = s;
+ data->fn = fn;
+ data->fn_opaque = opaque;
+
+ /*
+ * Hold a reference to the SCSIDevice until
+ * scsi_device_for_each_req_async_bh() finishes.
+ */
+ object_ref(OBJECT(s));
+
+ aio_bh_schedule_oneshot(blk_get_aio_context(s->conf.blk),
+ scsi_device_for_each_req_async_bh,
+ data);
+}
+
static void scsi_device_realize(SCSIDevice *s, Error **errp)
{
SCSIDeviceClass *sc = SCSI_DEVICE_GET_CLASS(s);
@@ -144,20 +227,18 @@ void scsi_bus_init_named(SCSIBus *bus, size_t bus_size, DeviceState *host,
qbus_set_bus_hotplug_handler(BUS(bus));
}
-static void scsi_dma_restart_bh(void *opaque)
+void scsi_req_retry(SCSIRequest *req)
{
- SCSIDevice *s = opaque;
- SCSIRequest *req, *next;
-
- qemu_bh_delete(s->bh);
- s->bh = NULL;
+ req->retry = true;
+}
- aio_context_acquire(blk_get_aio_context(s->conf.blk));
- QTAILQ_FOREACH_SAFE(req, &s->requests, next, next) {
- scsi_req_ref(req);
- if (req->retry) {
- req->retry = false;
- switch (req->cmd.mode) {
+/* Called in the AioContext that is executing the request */
+static void scsi_dma_restart_req(SCSIRequest *req, void *opaque)
+{
+ scsi_req_ref(req);
+ if (req->retry) {
+ req->retry = false;
+ switch (req->cmd.mode) {
case SCSI_XFER_FROM_DEV:
case SCSI_XFER_TO_DEV:
scsi_req_continue(req);
@@ -166,37 +247,22 @@ static void scsi_dma_restart_bh(void *opaque)
scsi_req_dequeue(req);
scsi_req_enqueue(req);
break;
- }
}
- scsi_req_unref(req);
}
- aio_context_release(blk_get_aio_context(s->conf.blk));
- /* Drop the reference that was acquired in scsi_dma_restart_cb */
- object_unref(OBJECT(s));
-}
-
-void scsi_req_retry(SCSIRequest *req)
-{
- /* No need to save a reference, because scsi_dma_restart_bh just
- * looks at the request list. */
- req->retry = true;
+ scsi_req_unref(req);
}
static void scsi_dma_restart_cb(void *opaque, bool running, RunState state)
{
SCSIDevice *s = opaque;
+ assert(qemu_in_main_thread());
+
if (!running) {
return;
}
- if (!s->bh) {
- AioContext *ctx = blk_get_aio_context(s->conf.blk);
- /* The reference is dropped in scsi_dma_restart_bh.*/
- object_ref(OBJECT(s));
- s->bh = aio_bh_new_guarded(ctx, scsi_dma_restart_bh, s,
- &DEVICE(s)->mem_reentrancy_guard);
- qemu_bh_schedule(s->bh);
- }
+
+ scsi_device_for_each_req_async(s, scsi_dma_restart_req, NULL);
}
static bool scsi_bus_is_address_free(SCSIBus *bus,
@@ -1657,15 +1723,16 @@ void scsi_device_set_ua(SCSIDevice *sdev, SCSISense sense)
}
}
+static void scsi_device_purge_one_req(SCSIRequest *req, void *opaque)
+{
+ scsi_req_cancel_async(req, NULL);
+}
+
void scsi_device_purge_requests(SCSIDevice *sdev, SCSISense sense)
{
- SCSIRequest *req;
+ scsi_device_for_each_req_async(sdev, scsi_device_purge_one_req, NULL);
aio_context_acquire(blk_get_aio_context(sdev->conf.blk));
- while (!QTAILQ_EMPTY(&sdev->requests)) {
- req = QTAILQ_FIRST(&sdev->requests);
- scsi_req_cancel_async(req, NULL);
- }
blk_drain(sdev->conf.blk);
aio_context_release(blk_get_aio_context(sdev->conf.blk));
scsi_device_set_ua(sdev, sense);
@@ -1737,31 +1804,33 @@ static char *scsibus_get_fw_dev_path(DeviceState *dev)
/* SCSI request list. For simplicity, pv points to the whole device */
+static void put_scsi_req(SCSIRequest *req, void *opaque)
+{
+ QEMUFile *f = opaque;
+
+ assert(!req->io_canceled);
+ assert(req->status == -1 && req->host_status == -1);
+ assert(req->enqueued);
+
+ qemu_put_sbyte(f, req->retry ? 1 : 2);
+ qemu_put_buffer(f, req->cmd.buf, sizeof(req->cmd.buf));
+ qemu_put_be32s(f, &req->tag);
+ qemu_put_be32s(f, &req->lun);
+ if (req->bus->info->save_request) {
+ req->bus->info->save_request(f, req);
+ }
+ if (req->ops->save_request) {
+ req->ops->save_request(f, req);
+ }
+}
+
static int put_scsi_requests(QEMUFile *f, void *pv, size_t size,
const VMStateField *field, JSONWriter *vmdesc)
{
SCSIDevice *s = pv;
- SCSIBus *bus = DO_UPCAST(SCSIBus, qbus, s->qdev.parent_bus);
- SCSIRequest *req;
- QTAILQ_FOREACH(req, &s->requests, next) {
- assert(!req->io_canceled);
- assert(req->status == -1 && req->host_status == -1);
- assert(req->enqueued);
-
- qemu_put_sbyte(f, req->retry ? 1 : 2);
- qemu_put_buffer(f, req->cmd.buf, sizeof(req->cmd.buf));
- qemu_put_be32s(f, &req->tag);
- qemu_put_be32s(f, &req->lun);
- if (bus->info->save_request) {
- bus->info->save_request(f, req);
- }
- if (req->ops->save_request) {
- req->ops->save_request(f, req);
- }
- }
+ scsi_device_for_each_req_sync(s, put_scsi_req, f);
qemu_put_sbyte(f, 0);
-
return 0;
}
diff --git a/include/hw/scsi/scsi.h b/include/hw/scsi/scsi.h
index 3692ca82f3..10c4e8288d 100644
--- a/include/hw/scsi/scsi.h
+++ b/include/hw/scsi/scsi.h
@@ -69,14 +69,19 @@ struct SCSIDevice
{
DeviceState qdev;
VMChangeStateEntry *vmsentry;
- QEMUBH *bh;
uint32_t id;
BlockConf conf;
SCSISense unit_attention;
bool sense_is_ua;
uint8_t sense[SCSI_SENSE_BUF_SIZE];
uint32_t sense_len;
+
+ /*
+ * The requests list is only accessed from the AioContext that executes
+ * requests or from the main loop when IOThread processing is stopped.
+ */
QTAILQ_HEAD(, SCSIRequest) requests;
+
uint32_t channel;
uint32_t lun;
int blocksize;
--
2.39.3
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