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From e6ae3c4666f35c30ac171e9994b5116d9594c8ce Mon Sep 17 00:00:00 2001
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From: Kevin Wolf <kwolf@redhat.com>
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Date: Wed, 31 May 2017 10:55:21 +0200
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Subject: [PATCH 03/13] mirror: Drop permissions on s->target on completion
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RH-Author: Kevin Wolf <kwolf@redhat.com>
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Message-id: <1496228121-28770-2-git-send-email-kwolf@redhat.com>
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Patchwork-id: 75443
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O-Subject: [RHEL-7.4 qemu-kvm-rhev PATCH] mirror: Drop permissions on s->target on completion
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Bugzilla: 1456456
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RH-Acked-by: Jeffrey Cody <jcody@redhat.com>
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RH-Acked-by: Fam Zheng <famz@redhat.com>
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RH-Acked-by: Max Reitz <mreitz@redhat.com>
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RH-Acked-by: John Snow <jsnow@redhat.com>
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This fixes an assertion failure that was triggered by qemu-iotests 129
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on some CI host, while the same test case didn't seem to fail on other
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hosts.
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Essentially the problem is that the blk_unref(s->target) in
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mirror_exit() doesn't necessarily mean that the BlockBackend goes away
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immediately. It is possible that the job completion was triggered nested
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in mirror_drain(), which looks like this:
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BlockBackend *target = s->target;
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blk_ref(target);
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blk_drain(target);
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blk_unref(target);
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In this case, the write permissions for s->target are retained until
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after blk_drain(), which makes removing mirror_top_bs fail for the
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active commit case (can't have a writable backing file in the chain
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without the filter driver).
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Explicitly dropping the permissions first means that the additional
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reference doesn't hurt and the job can complete successfully even if
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called from the nested blk_drain().
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Cc: qemu-stable@nongnu.org
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Signed-off-by: Kevin Wolf <kwolf@redhat.com>
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Acked-by: Paolo Bonzini <pbonzini@redhat.com>
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Reviewed-by: Max Reitz <mreitz@redhat.com>
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(cherry picked from commit 63c8ef289087a225d445319d047501d4fe593687)
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Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>
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---
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block/mirror.c | 7 ++++++-
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1 file changed, 6 insertions(+), 1 deletion(-)
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diff --git a/block/mirror.c b/block/mirror.c
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index 2173a2f..4e8f124 100644
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--- a/block/mirror.c
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+++ b/block/mirror.c
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@@ -514,7 +514,12 @@ static void mirror_exit(BlockJob *job, void *opaque)
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/* Remove target parent that still uses BLK_PERM_WRITE/RESIZE before
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* inserting target_bs at s->to_replace, where we might not be able to get
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- * these permissions. */
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+ * these permissions.
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+ *
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+ * Note that blk_unref() alone doesn't necessarily drop permissions because
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+ * we might be running nested inside mirror_drain(), which takes an extra
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+ * reference, so use an explicit blk_set_perm() first. */
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+ blk_set_perm(s->target, 0, BLK_PERM_ALL, &error_abort);
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blk_unref(s->target);
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s->target = NULL;
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--
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1.8.3.1
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