Re: [PATCH] Btrfs: fix race condition between writting and scrubing supers

[Bob Marley <bobmarley@shiftmail.org>]

On 22/10/2013 10:37, Stefan Behrens wrote:
> I don't believe that this issue can ever happen. I don't believe that
> somewhere on the path to the flash memory, to the magnetic disc or to
> the drive's cache memory, someone interrupts a 4KB write in the middle
> of operation to read from this 4KB area. This is not an issue IMHO.

I think I have read that unfortunately it can happen.
SAS and SATA specs for disks do not mandate that if a write is in-flight 
but still not completed, reads from the same sector should return the 
value it is being written; they can return the old value.
I also think that Linux does not check either.
Much worse, I think I have even read that two simultaneous in-flight 
writes to the same sector can be completed in any order by the disk, and 
since the write which wins is the latter being completed, this results 
in an indeterminate value persisting on that sector at the end. One 
needs to synchronize cache between the two writes to guarantee the 
outcome. Way worse is when the drives also cheat on synchronize cache, 
and that one is impossible to fix I believe.