From a2e385fea51333b9cfbf88b19cbb4f82a5677381 Mon Sep 17 00:00:00 2001

From: Fam Zheng <famz@redhat.com>

Date: Fri, 17 Aug 2018 03:08:36 +0200

Subject: [PATCH 5/5] aio: Do aio_notify_accept only during blocking aio_poll

RH-Author: Fam Zheng <famz@redhat.com>

Message-id: <20180817030836.20581-3-famz@redhat.com>

Patchwork-id: 81862

O-Subject: [RHEL-7.6 qemu-kvm-rhev PATCH 2/2] aio: Do aio_notify_accept only during blocking aio_poll

Bugzilla: 1562750

RH-Acked-by: Paolo Bonzini <pbonzini@redhat.com>

RH-Acked-by: Thomas Huth <thuth@redhat.com>

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

An aio_notify() pairs with an aio_notify_accept(). The former should

happen in the main thread or a vCPU thread, and the latter should be

done in the IOThread.

There is one rare case that the main thread or vCPU thread may "steal"

the aio_notify() event just raised by itself, in bdrv_set_aio_context()

[1]. The sequence is like this:

    main thread                     IO Thread

    ===============================================================

    bdrv_drained_begin()

      aio_disable_external(ctx)

                                    aio_poll(ctx, true)

                                      ctx->notify_me += 2

    ...

    bdrv_drained_end()

      ...

        aio_notify()

    ...

    bdrv_set_aio_context()

      aio_poll(ctx, false)

[1]     aio_notify_accept(ctx)

                                      ppoll() /* Hang! */

[1] is problematic. It will clear the ctx->notifier event so that

the blocked ppoll() will not return.

(For the curious, this bug was noticed when booting a number of VMs

simultaneously in RHV.  One or two of the VMs will hit this race

condition, making the VIRTIO device unresponsive to I/O commands. When

it hangs, Seabios is busy waiting for a read request to complete (read

MBR), right after initializing the virtio-blk-pci device, using 100%

guest CPU. See also https://bugzilla.redhat.com/show_bug.cgi?id=1562750

for the original bug analysis.)

aio_notify() only injects an event when ctx->notify_me is set,

correspondingly aio_notify_accept() is only useful when ctx->notify_me

_was_ set. Move the call to it into the "blocking" branch. This will

effectively skip [1] and fix the hang.

Furthermore, blocking aio_poll is only allowed on home thread

(in_aio_context_home_thread), because otherwise two blocking

aio_poll()'s can steal each other's ctx->notifier event and cause

hanging just like described above.

Cc: qemu-stable@nongnu.org

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>

Signed-off-by: Fam Zheng <famz@redhat.com>

Message-Id: <20180809132259.18402-3-famz@redhat.com>

Signed-off-by: Fam Zheng <famz@redhat.com>

(cherry picked from commit b37548fcd1b8ac2e88e185a395bef851f3fc4e65)

Signed-off-by: Fam Zheng <famz@redhat.com>

Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>

---

 util/aio-posix.c | 4 ++--

 util/aio-win32.c | 3 ++-

 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/util/aio-posix.c b/util/aio-posix.c

index f650c7c..f05d3a8 100644

--- a/util/aio-posix.c

+++ b/util/aio-posix.c

@@ -591,6 +591,7 @@ bool aio_poll(AioContext *ctx, bool blocking)

      * so disable the optimization now.

      */

     if (blocking) {

+        assert(in_aio_context_home_thread(ctx));

         atomic_add(&ctx->notify_me, 2);

     }

@@ -633,6 +634,7 @@ bool aio_poll(AioContext *ctx, bool blocking)

     if (blocking) {

         atomic_sub(&ctx->notify_me, 2);

+        aio_notify_accept(ctx);

     }

     /* Adjust polling time */

@@ -676,8 +678,6 @@ bool aio_poll(AioContext *ctx, bool blocking)

         }

     }

-    aio_notify_accept(ctx);

-

     /* if we have any readable fds, dispatch event */

     if (ret > 0) {

         for (i = 0; i < npfd; i++) {

diff --git a/util/aio-win32.c b/util/aio-win32.c

index a67b00c..ac5524c 100644

--- a/util/aio-win32.c

+++ b/util/aio-win32.c

@@ -373,11 +373,12 @@ bool aio_poll(AioContext *ctx, bool blocking)

         ret = WaitForMultipleObjects(count, events, FALSE, timeout);

         if (blocking) {

             assert(first);

+            assert(in_aio_context_home_thread(ctx));

             atomic_sub(&ctx->notify_me, 2);

+            aio_notify_accept(ctx);

         }

         if (first) {

-            aio_notify_accept(ctx);

             progress |= aio_bh_poll(ctx);

             first = false;

         }

-- 

1.8.3.1