aio: Do aio_notify_accept only during blocking aio_poll
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 b37548fcd1
)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
stable-3.0
parent
98f6f21261
commit
8f0e1478d0
|
@ -591,6 +591,7 @@ bool aio_poll(AioContext *ctx, bool blocking)
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* so disable the optimization now.
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*/
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if (blocking) {
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assert(in_aio_context_home_thread(ctx));
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atomic_add(&ctx->notify_me, 2);
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}
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@ -633,6 +634,7 @@ bool aio_poll(AioContext *ctx, bool blocking)
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if (blocking) {
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atomic_sub(&ctx->notify_me, 2);
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aio_notify_accept(ctx);
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}
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/* Adjust polling time */
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@ -676,8 +678,6 @@ bool aio_poll(AioContext *ctx, bool blocking)
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}
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}
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aio_notify_accept(ctx);
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/* if we have any readable fds, dispatch event */
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if (ret > 0) {
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for (i = 0; i < npfd; i++) {
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@ -373,11 +373,12 @@ bool aio_poll(AioContext *ctx, bool blocking)
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ret = WaitForMultipleObjects(count, events, FALSE, timeout);
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if (blocking) {
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assert(first);
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assert(in_aio_context_home_thread(ctx));
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atomic_sub(&ctx->notify_me, 2);
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aio_notify_accept(ctx);
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}
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if (first) {
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aio_notify_accept(ctx);
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progress |= aio_bh_poll(ctx);
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first = false;
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}
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