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Our block cache consistency mechanism allows readers to try and read stale block references. They check block headers of the block they read to discover if it has been modified and they should retry the read with newer block references. For this to be correct the block contents can't change under the readers. That's obviously true in the simple imagined case of one node writing and another node reading. But we also have the case where the stale reader and dirtying writer can be concurrent tasks in the same mount which share a block cache. There were a two failure cases that derive from the order of readers and writers working with blocks. If the reader goes first, the writer could find the existing block in the cache and modify it while the reader assumes that it is read only. The fix is to have the writer always remove any existing cached block and insert a newly allocated block into the cache with the header fields already changed. Any existing readers will still have their cached block references and any new readers will see the modified headers and return -ESTALE. The next failure comes from readers trying to invalidate dirty blocks when they see modified headers. They assumed that the existing cached block was old and could be dropped so that a new current version could be read. But in this case a local writer has clobbered the reader's stale block and the reader should immediately return -ESTALE. Signed-off-by: Zach Brown <zab@versity.com>