From a2e385fea51333b9cfbf88b19cbb4f82a5677381 Mon Sep 17 00:00:00 2001 From: Fam Zheng 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 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 RH-Acked-by: Thomas Huth RH-Acked-by: Laszlo Ersek 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 Signed-off-by: Fam Zheng Message-Id: <20180809132259.18402-3-famz@redhat.com> Signed-off-by: Fam Zheng (cherry picked from commit b37548fcd1b8ac2e88e185a395bef851f3fc4e65) Signed-off-by: Fam Zheng Signed-off-by: Miroslav Rezanina --- 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