On Fri, 7 Feb 2020, John Dorminy wrote: > There's no obvious way for the other process to wait for the discard > to be complete at the present time, though -- suppose there were two > holders of a given buffer, and one decided to discard -- how will the > second one to wait for the discard to complete, or even tell that it's > currently being discarded from another thread? I would naively guess > that __write_dirty_buffer() waits via wait_on_bit_io(&b->state, > B_WRITING, ...) to make sure nobody else is doing IO on the buffer > location at the moment, but a discard doesn't currently set that bit, > as far as I can see. > > (If there is a way to wait, perhaps it should be documented at > dm_bufio_discard_buffers() -- "If there is another process holding the > buffer, the other process should be sure to [do stuff] before issuing > a write, lest the write potentially be dropped or corrupted." If two processes write to the same buffer, it is undefined behavior. If both of them do this: 1) dm_bufio_get 2) ... write to the buffer 3) dm_bufio_mark_buffer_dirty 4) dm_bufio_release it is undefined what data the buffer would hold. It can even hold mixture of data written by those two processes. You must design your code in such a way that this doesn't happen. The same is with discards - if you want to use them, you must design your code so that it doesn't overlay discards with other I/O. If you can't design it this way, then don't use discard. Mikulas -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
