From b886411a682b56bfe674f0a35d40c67c8e9dc87a Mon Sep 17 00:00:00 2001

From: Stefan Hajnoczi <stefanha@redhat.com>

Date: Tue, 21 Feb 2023 16:22:17 -0500

Subject: [PATCH 02/12] dma-helpers: prevent dma_blk_cb() vs dma_aio_cancel()

 race

RH-Author: Stefan Hajnoczi <stefanha@redhat.com>

RH-MergeRequest: 155: virtio-scsi: reset SCSI devices from main loop thread

RH-Bugzilla: 2155748

RH-Acked-by: Eric Blake <eblake@redhat.com>

RH-Acked-by: Kevin Wolf <kwolf@redhat.com>

RH-Acked-by: Laszlo Ersek <lersek@redhat.com>

RH-Commit: [2/3] eeeea43c25d8f4fa84591b05547fb77e4058abff (stefanha/centos-stream-qemu-kvm)

dma_blk_cb() only takes the AioContext lock around ->io_func(). That

means the rest of dma_blk_cb() is not protected. In particular, the

DMAAIOCB field accesses happen outside the lock.

There is a race when the main loop thread holds the AioContext lock and

invokes scsi_device_purge_requests() -> bdrv_aio_cancel() ->

dma_aio_cancel() while an IOThread executes dma_blk_cb(). The dbs->acb

field determines how cancellation proceeds. If dma_aio_cancel() sees

dbs->acb == NULL while dma_blk_cb() is still running, the request can be

completed twice (-ECANCELED and the actual return value).

The following assertion can occur with virtio-scsi when an IOThread is

used:

  ../hw/scsi/scsi-disk.c:368: scsi_dma_complete: Assertion `r->req.aiocb != NULL' failed.

Fix the race by holding the AioContext across dma_blk_cb(). Now

dma_aio_cancel() under the AioContext lock will not see

inconsistent/intermediate states.

Cc: Paolo Bonzini <pbonzini@redhat.com>

Reviewed-by: Eric Blake <eblake@redhat.com>

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

Message-Id: <20230221212218.1378734-3-stefanha@redhat.com>

Signed-off-by: Kevin Wolf <kwolf@redhat.com>

(cherry picked from commit abfcd2760b3e70727bbc0792221b8b98a733dc32)

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

---

 hw/scsi/scsi-disk.c   |  4 +---

 softmmu/dma-helpers.c | 12 +++++++-----

 2 files changed, 8 insertions(+), 8 deletions(-)

diff --git a/hw/scsi/scsi-disk.c b/hw/scsi/scsi-disk.c

index 5327f93f4c..b12d8b0816 100644

--- a/hw/scsi/scsi-disk.c

+++ b/hw/scsi/scsi-disk.c

@@ -354,13 +354,12 @@ done:

     scsi_req_unref(&r->req);

 }

+/* Called with AioContext lock held */

 static void scsi_dma_complete(void *opaque, int ret)

 {

     SCSIDiskReq *r = (SCSIDiskReq *)opaque;

     SCSIDiskState *s = DO_UPCAST(SCSIDiskState, qdev, r->req.dev);

-    aio_context_acquire(blk_get_aio_context(s->qdev.conf.blk));

-

     assert(r->req.aiocb != NULL);

     r->req.aiocb = NULL;

@@ -370,7 +369,6 @@ static void scsi_dma_complete(void *opaque, int ret)

         block_acct_done(blk_get_stats(s->qdev.conf.blk), &r->acct);

     }

     scsi_dma_complete_noio(r, ret);

-    aio_context_release(blk_get_aio_context(s->qdev.conf.blk));

 }

 static void scsi_read_complete_noio(SCSIDiskReq *r, int ret)

diff --git a/softmmu/dma-helpers.c b/softmmu/dma-helpers.c

index 7820fec54c..2463964805 100644

--- a/softmmu/dma-helpers.c

+++ b/softmmu/dma-helpers.c

@@ -113,17 +113,19 @@ static void dma_complete(DMAAIOCB *dbs, int ret)

 static void dma_blk_cb(void *opaque, int ret)

 {

     DMAAIOCB *dbs = (DMAAIOCB *)opaque;

+    AioContext *ctx = dbs->ctx;

     dma_addr_t cur_addr, cur_len;

     void *mem;

     trace_dma_blk_cb(dbs, ret);

+    aio_context_acquire(ctx);

     dbs->acb = NULL;

     dbs->offset += dbs->iov.size;

     if (dbs->sg_cur_index == dbs->sg->nsg || ret < 0) {

         dma_complete(dbs, ret);

-        return;

+        goto out;

     }

     dma_blk_unmap(dbs);

@@ -164,9 +166,9 @@ static void dma_blk_cb(void *opaque, int ret)

     if (dbs->iov.size == 0) {

         trace_dma_map_wait(dbs);

-        dbs->bh = aio_bh_new(dbs->ctx, reschedule_dma, dbs);

+        dbs->bh = aio_bh_new(ctx, reschedule_dma, dbs);

         cpu_register_map_client(dbs->bh);

-        return;

+        goto out;

     }

     if (!QEMU_IS_ALIGNED(dbs->iov.size, dbs->align)) {

@@ -174,11 +176,11 @@ static void dma_blk_cb(void *opaque, int ret)

                                 QEMU_ALIGN_DOWN(dbs->iov.size, dbs->align));

     }

-    aio_context_acquire(dbs->ctx);

     dbs->acb = dbs->io_func(dbs->offset, &dbs->iov,

                             dma_blk_cb, dbs, dbs->io_func_opaque);

-    aio_context_release(dbs->ctx);

     assert(dbs->acb);

+out:

+    aio_context_release(ctx);

 }

 static void dma_aio_cancel(BlockAIOCB *acb)

-- 

2.39.1