021bf17528
On current kernels (5.8 for example) under some conditions I don't fully grok it might happen that a detached loopback block device still has partition block devices around. Accessing these partition block devices results in EIO errors (that also fill up dmesg). These devices cannot be claned up with LOOP_CLR_FD (since the main device already is officially detached), nor with LOOP_CTL_DELETE (returns EBUSY as long as the partitions still exist). This is a kernel bug. But it appears to apply to all recent kernels. I cannot really pin down what triggers this, suffice to say our heavy-duty test can trigger it. Either way, let's do something about it: when we notice this state we'll attach an empty file to it, which is guaranteed to have to part table. This makes the partitions go away. After closing/reoping the device we hence are good to go again. ugly workaround, but I think OK enough to use. The net result is: with this commit, we'll guarantee that by the time we attach a file to the loopback device we have zero kernel partitions associated with it. Thus if we then wait for the kernel partitions we need to appear we should have entirely reliable behaviour even if loopback devices by the name are heavily recycled and udev events reach us very late. Fixes: #16858
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