On 03/30/2017 11:19 AM, Linus Torvalds wrote:

like "dd" growing an atomic flag and setting it on stdout),

'dd' typically assumes that if a flag O_FOO is not #defined by 
<fcntl.h>, then 'dd' can "#define O_FOO 0" and the rest of dd's code can 
assume O_FOO works "well enough" on older systems. I hope this 
backward-compatibility hack will suffice for O_ATOMIC too.

I'm assuming you'd also possible want to be able to use F_SETFL to set
O_ATOMIC after the fact

Just for fun, one thread can set O_ATOMIC at the same time another 
thread is doing a 'write'....