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[Qemu-devel] [PULL 06/17] aio: Do aio_notify_accept only during blocking
From: |
Fam Zheng |
Subject: |
[Qemu-devel] [PULL 06/17] aio: Do aio_notify_accept only during blocking aio_poll |
Date: |
Wed, 15 Aug 2018 11:12:37 +0800 |
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: address@hidden
Suggested-by: Paolo Bonzini <address@hidden>
Signed-off-by: Fam Zheng <address@hidden>
Message-Id: <address@hidden>
Signed-off-by: Fam Zheng <address@hidden>
---
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 b5c7f463aa..b5c609b68b 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 e676a8d9b2..c58957cc4b 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;
}
--
2.17.1
- [Qemu-devel] [PULL 00/17] Block and testing patches, Fam Zheng, 2018/08/14
- [Qemu-devel] [PULL 01/17] tests/vm: Only use -cpu 'host' if KVM is available, Fam Zheng, 2018/08/14
- [Qemu-devel] [PULL 02/17] tests/vm: Add flex and bison to the vm image, Fam Zheng, 2018/08/14
- [Qemu-devel] [PULL 05/17] aio-posix: Don't count ctx->notifier as progress when polling, Fam Zheng, 2018/08/14
- [Qemu-devel] [PULL 04/17] nvme: simplify plug/unplug, Fam Zheng, 2018/08/14
- [Qemu-devel] [PULL 03/17] nvme: Fix nvme_init error handling, Fam Zheng, 2018/08/14
- [Qemu-devel] [PULL 06/17] aio: Do aio_notify_accept only during blocking aio_poll,
Fam Zheng <=
- [Qemu-devel] [PULL 09/17] tests: Allow overriding archive path with SRC_ARCHIVE, Fam Zheng, 2018/08/14
- [Qemu-devel] [PULL 10/17] tests: Add centos VM testing, Fam Zheng, 2018/08/14
- [Qemu-devel] [PULL 08/17] tests: Add an option for snapshot (default: off), Fam Zheng, 2018/08/14
- [Qemu-devel] [PULL 13/17] tests/vm: Propagate V=1 down into the make inside the VM, Fam Zheng, 2018/08/14
- [Qemu-devel] [PULL 11/17] tests: vm: Add vm-clean-all, Fam Zheng, 2018/08/14
- [Qemu-devel] [PULL 12/17] tests/vm: Pass the jobs parallelism setting to 'make check', Fam Zheng, 2018/08/14
- [Qemu-devel] [PULL 07/17] docker: Install more packages in centos7, Fam Zheng, 2018/08/14
- [Qemu-devel] [PULL 14/17] tests/vm: Bump guest RAM up from 2G to 4G, Fam Zheng, 2018/08/14
- [Qemu-devel] [PULL 15/17] tests/vm: Use make's --output-sync option, Fam Zheng, 2018/08/14
- [Qemu-devel] [PULL 17/17] aio-posix: Improve comment around marking node deleted, Fam Zheng, 2018/08/14