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block: Poll after drain on attaching a node
Commit dcf94a2 ('block: Don't poll in parent drain callbacks') removed polling in bdrv_child_cb_drained_begin() on the grounds that the original bdrv_drain() already will poll and BdrvChildRole.drained_begin calls must not cause graph changes (and therefore must not call aio_poll() or the recursion through the graph will break. This reasoning is correct for calls through bdrv_do_drained_begin(). However, BdrvChildRole.drained_begin is also called when a node that is already in a drained section (i.e. bdrv_do_drained_begin() has already returned and therefore can't poll any more) is attached to a new parent. In this case, we must explicitly poll to have all requests completed before the drained new child can be attached to the parent. In bdrv_replace_child_noperm(), we know that we're not inside the recursion of bdrv_do_drained_begin() because graph changes are not allowed there, and bdrv_replace_child_noperm() is a graph change. The call of BdrvChildRole.drained_begin() must therefore be followed by a BDRV_POLL_WHILE() that waits for the completion of requests. Reported-by: Max Reitz <mreitz@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
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