Hi!

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You can do it seqlock-style, kind of - you reserve the first byte of
the page or so as a "is this page initialized" marker, and after every
read from the page, you do a compiler barrier and check whether that
byte has been cleared.

This is certainly possible yet wery awkwar interface to use IMHO.
MADV_EXTERNALY_VOLATILE would express the actual semantic much better.
I might not still understand the expected usecase but if the target
application has to be changed anyway then why not simply use a
transparent and proper signaling mechanism like poll on a fd. That

The goal is to have cryprographically-safe get_random_number() with 0
syscalls.

You'd need to do:

   if (!poll(did_i_migrate)) {
         use_prng_seed();
	 if (poll(did_i_migrate)) {
	       /* oops_they_migrated_me_in_middle_of_computation,
 	          lets_redo_it() */
 		  goto retry:
	 }
   }

Which means two syscalls..

Is this a real problem though? Do we have any actual numbers? E.g. how
often does the migration happen so that 2 syscalls would be visible in
actual workloads?

Please go through the thread and try to understand it.

You'd need syscalls per get_randomness(), not per migration.

									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html