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SubjectRe: [lttng-dev] [PATCH] Add ACCESS_ONCE() to avoid compiler splitting assignments
On Sun, Jan 20, 2013 at 03:51:31PM -0500, Mathieu Desnoyers wrote:
> * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> > On Wed, Jan 16, 2013 at 07:50:54AM -0500, Mathieu Desnoyers wrote:
> > > * Mathieu Desnoyers (mathieu.desnoyers@efficios.com) wrote:
> > > > * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> > > > > As noted by Konstantin Khlebnikov, gcc can split assignment of
> > > > > constants to long variables (https://lkml.org/lkml/2013/1/15/141),
> > > > > though assignment of NULL (0) is OK. Assuming that a gcc bug is
> > > > > fixed (http://gcc.gnu.org/bugzilla/attachment.cgi?id=29169&action=diff
> > > > > has a patch), making the store be volatile keeps gcc from splitting.
> > > > >
> > > > > This commit therefore applies ACCESS_ONCE() to CMM_STORE_SHARED(),
> > > > > which is the underlying primitive used by rcu_assign_pointer().
> > > >
> > > > Hi Paul,
> > > >
> > > > I recognise that this is an issue in the Linux kernel, since a simple
> > > > store is used and expected to be performed atomically when aligned.
> > > > However, I think this does not affect liburcu, see below:
> > >
> > > Side question: what gcc versions may issue non-atomic volatile stores ?
> > > I think we should at least document those. Bug
> > > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55981 seems to target gcc
> > > 4.7.2, but I wonder when this issue first appeared on x86 and x86-64
> > > (and if it affects other architectures as well).
> >
> > I have no idea which versions are affected. The bug is in the x86
> > backend, so is specific to x86, but there might well be similar bugs
> > in other architectures.
>
> Results for my quick tests so far:
>
> gcc version 4.4.7 (Debian 4.4.7-2)
> gcc version 4.6.3 (Debian 4.6.3-11)
> gcc version 4.7.2 (Debian 4.7.2-5)
>
> for x86-64 are affected (all gcc installed currently on my Debian
> laptop). I made a simpler test case than the one shown on the bugzilla,
> which triggers the issue with -O0 and -O1 (-O2 and -Os optimisations
> seems not to show this issue with the simpler test case). So far, it
> seems to only affect stores from immediate value operands (and only some
> patterns).

Good to know! And more widespread than I would have guessed...

For whatever it is worth, there is an alleged patch here:

http://gcc.gnu.org/ml/gcc-patches/2013-01/msg01119.html

> Debian clang version 3.0-6 (tags/RELEASE_30/final) (based on LLVM 3.0)
> is not affected for x86-64.

Heh! Good to know as well. ;-)

Thanx, Paul

> My test case looks like this:
>
> * non-atomic-pointer.c:
>
> volatile void *var;
>
> void fct(void)
> {
> asm volatile ("/* test_begin */" : : : "memory");
> var = (void *) 0x100000002;
> var = (void *) 0x300000004;
> asm volatile ("/* test_end */" : : : "memory");
> }
>
>
> * build.sh:
>
> #!/bin/bash
>
> FLAGS="-S ${*}"
>
> function do_build
> {
> $1 ${FLAGS} -o non-atomic-pointer-$1.S.output non-atomic-pointer.c
> }
>
> do_build gcc-4.4
> do_build gcc-4.6
> do_build gcc-4.7
> do_build gcc-4.7
> do_build g++-4.6
> do_build g++-4.7
> do_build clang
>
>
> * check.py (might need adaptation for each architecture and O2, Os):
>
> #!/usr/bin/env python
>
> import sys, string, os, re
>
> # count the number of lines with "mov*" instructions between test_begin
> # and test_end. Should be 2. Report error if not.
>
> f = open (sys.argv[1], "r")
>
> lines = f.readlines()
>
> within_range = 0
> mov_count = 0
> error = 0
>
> re_mov = re.compile(' mov.*')
> re_begin = re.compile('.*test_begin.*')
> re_end = re.compile('.*test_end.*')
>
> output = ""
>
> for line in lines:
> if re_begin.match(line):
> within_range = 1
> elif re_end.match(line):
> within_range = 0
> elif (within_range and re_mov.match(line)):
> output += line
> mov_count += 1
>
> if mov_count > 2:
> error = 1
>
> f.close()
>
> if error > 0:
> print "[Error] expect 2 mov, found " + str(mov_count)
> print output
> 1
> else:
> 0
>
> >
> > > Thanks,
> > >
> > > Mathieu
> > >
> > > >
> > > > >
> > > > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > > > >
> > > > > diff --git a/urcu/system.h b/urcu/system.h
> > > > > index 2a45f22..7a1887e 100644
> > > > > --- a/urcu/system.h
> > > > > +++ b/urcu/system.h
> > > > > @@ -49,7 +49,7 @@
> > > > > */
> > > > > #define CMM_STORE_SHARED(x, v) \
> > > > > ({ \
> > > > > - __typeof__(x) _v = _CMM_STORE_SHARED(x, v); \
> > > > > + __typeof__(x) CMM_ACCESS_ONCE(_v) = _CMM_STORE_SHARED(x, v); \
> > > >
> > > > Here, the macro "_CMM_STORE_SHARED(x, v)" is doing the actual store.
> > > > It stores v into "x". So adding a CMM_ACCESS_ONCE(_v), as you propose
> > > > here, is really only making sure the return value (usually unused),
> > > > located on the stack, is accessed with a volatile access, which does not
> > > > make much sense.
> > > >
> > > > What really matters is the _CMM_STORE_SHARED() macro:
> > > >
> > > > #define _CMM_STORE_SHARED(x, v) ({ CMM_ACCESS_ONCE(x) = (v); })
> > > >
> > > > which already uses a volatile access for the store. So this seems to be
> > > > a case where our preemptive use of volatile for stores in addition to
> > > > loads made us bug-free for a gcc behavior unexpected at the time we
> > > > implemented this macro. Just a touch of paranoia seems to be a good
> > > > thing sometimes. ;-)
> > > >
> > > > Thoughts ?
> >
> > Here is my thought: You should ignore my "fix". Please accept my
> > apologies for my confusion!
>
> No worries! Thanks for bringing this issue to my attention.
>
> I'm thinking we might want to create a test throwing code with random
> immediate values into my python checker script, for various
> architectures for a couple of weeks, and see what breaks.
>
> Also, it might be good to test whether some register inputs can generate
> code that write into an unsigned long non-atomically. This would be even
> more worrying.
>
> Thanks,
>
> Mathieu
>
> >
> > Thanx, Paul
> >
> > > > Thanks,
> > > >
> > > > Mathieu
> > > >
> > > > > cmm_smp_wmc(); \
> > > > > _v; \
> > > > > })
> > > > >
> > > > >
> > > > > _______________________________________________
> > > > > lttng-dev mailing list
> > > > > lttng-dev@lists.lttng.org
> > > > > http://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev
> > > >
> > > > --
> > > > Mathieu Desnoyers
> > > > EfficiOS Inc.
> > > > http://www.efficios.com
> > > >
> > > > _______________________________________________
> > > > lttng-dev mailing list
> > > > lttng-dev@lists.lttng.org
> > > > http://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev
> > >
> > > --
> > > Mathieu Desnoyers
> > > EfficiOS Inc.
> > > http://www.efficios.com
> > >
> >
>
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com
>



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