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Re: Compilers and RCU readers: Once more unto the breach!
- From: Linus Torvalds <torvalds at linux-foundation dot org>
- To: Paul McKenney <paulmck at linux dot vnet dot ibm dot com>
- Cc: Linux Kernel Mailing List <linux-kernel at vger dot kernel dot org>, c++std-parallel at accu dot org, "linux-arch at vger dot kernel dot org" <linux-arch at vger dot kernel dot org>, "gcc at gcc dot gnu dot org" <gcc at gcc dot gnu dot org>, p796231 <Peter dot Sewell at cl dot cam dot ac dot uk>, "mark dot batty at cl dot cam dot ac dot uk" <Mark dot Batty at cl dot cam dot ac dot uk>, Peter Zijlstra <peterz at infradead dot org>, Will Deacon <will dot deacon at arm dot com>, Ramana Radhakrishnan <Ramana dot Radhakrishnan at arm dot com>, David Howells <dhowells at redhat dot com>, Andrew Morton <akpm at linux-foundation dot org>, Ingo Molnar <mingo at kernel dot org>, michaelw at ca dot ibm dot com
- Date: Tue, 19 May 2015 18:57:02 -0700
- Subject: Re: Compilers and RCU readers: Once more unto the breach!
- Authentication-results: sourceware.org; auth=none
- References: <20150520005510 dot GA23559 at linux dot vnet dot ibm dot com>
On Tue, May 19, 2015 at 5:55 PM, Paul E. McKenney
>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.