ext3, BKL, journal replay, multiple non-bind mounts of same device

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Since it is possible for the same block device to be mounted multiple 
times concurrently by the same filesystem, and since ext3 explicitly 
disables the BKL during its fill_super operation which would prevent this, 
what is the result of mounting it multiple times this way? Especially if 
the filesystem is dirty and a journal is replayed. (In any case, what 
operation is being performed by ext3/ext4 that requires the BKL to be 
dropped? What's the need to even consider the BKL during fill_super?)

And in general, how does a filesystem deal with being mounted multiple 
times in this way? In my testing and exploration so far, everything seems 
to generally work, but I haven't tried deliberately using different 
instances of the mount concurrently. Do we end up with locks not being 
held properly on the superblock because the super_block structure 
instances don't know about each other? Has dealing with this behavior of 
bd_claim really been considered before, and if so, what's the general 
scheme for handling it?
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