Re: Compilers and RCU readers: Once more unto the breach!

[Linus Torvalds]

On Tue, May 19, 2015 at 5:55 PM, Paul E. McKenney
<paulmck@xxxxxxxxxxxxxxxxxx> wrote:
>
>         http://www.rdrop.com/users/paulmck/RCU/consume.2015.05.18a.pdf

>From a very quick read-through, the restricted dependency chain in 7.9
seems to be reasonable, and essentially covers "thats' what hardware
gives us anyway", making compiler writers happy.

I would clarify the language somewhat:

 - it says that the result of a cast of a pointer is a dependency. You
need to make an exception for casting to bool, methinks (because
that's effectively just a test-against-NULL, which you later describe
as terminating the dependency).

   Maybe get rid of the "any type", and just limit it to casts to
types of size intptr_t, ie ones that don't drop significant bits. All
the other rules talk about [u]intptr_t anyway.

 - you clarify that the trivial "& 0" and "| ~0" kill the dependency
chain, but if you really want to be a stickler, you might want to
extend it to a few more cases. Things like "& 1" (to extract a tag
from the lot bit of a tagged pointer) should likely also drop the
dependency, since a compiler would commonly end up using the end
result as a conditional even if the code was written to then use
casting etc to look like a dereference.

 - the "you can add/subtract integral values" still opens you up to
language lawyers claiming "(char *)ptr - (intptr_t)ptr" preserving the
dependency, which it clearly doesn't. But language-lawyering it does,
since all those operations (cast to pointer, cast to integer,
subtracting an integer) claim to be dependency-preserving operations.

So I think you want to limit the logical operators to things that
don't mask off too many bits, and you should probably limit the
add/subtract operations some way (maybe specify that the integer value
you add/subtract cannot be related to the pointer). But I think
limiting it to mostly pointer ops (and a _few_ integer operations to
do offsets and remove tag bits) is otherwise a good approach.

               Linus
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