From: Christian Borntraeger <borntraeger@de.ibm.com>
Date: Tue, 28 Jul 2015 13:03:05 +0000 (+0200)
Subject: KVM: s390: Fix hang VCPU hang/loop regression
X-Git-Tag: omap-for-v4.3/fixes-merge-window~136^2~1^2
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=586b7ccdb7143b6a9b975d2c6ad52b6ca5c162b9;p=pandora-kernel.git

KVM: s390: Fix hang VCPU hang/loop regression

commit 785dbef407d8 ("KVM: s390: optimize round trip time in request
handling") introduced a regression. This regression was seen with
CPU hotplug in the guest and switching between 1 or 2 CPUs. This will
set/reset the IBS control via synced request.

Whenever we make a synced request, we first set the vcpu->requests
bit and then block the vcpu. The handler, on the other hand, unblocks
itself, processes vcpu->requests (by clearing them) and unblocks itself
once again.

Now, if the requester sleeps between setting of vcpu->requests and
blocking, the handler will clear the vcpu->requests bit and try to
unblock itself (although no bit is set). When the requester wakes up,
it blocks the VCPU and we have a blocked VCPU without requests.

Solution is to always unset the block bit.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Fixes: 785dbef407d8 ("KVM: s390: optimize round trip time in request handling")
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