memcmp-sse4.S EqualHappy bug

[Simo Sorce]

On Fri, 2015-06-19 at 19:40 +0200, Torvald Riegel wrote:
> On Fri, 2015-06-19 at 13:18 -0400, Simo Sorce wrote:
> > On Fri, 2015-06-19 at 13:16 -0400, Simo Sorce wrote:
> > > On Fri, 2015-06-19 at 18:56 +0200, Torvald Riegel wrote:
> > > > On Fri, 2015-06-19 at 16:59 +0100, Dr. David Alan Gilbert wrote:
> > > > > * Torvald Riegel (triegel@redhat.com) wrote:
> > > > > > On Fri, 2015-06-19 at 10:42 -0400, Simo Sorce wrote:
> > > > > > > On Fri, 2015-06-19 at 16:07 +0200, Andrea Arcangeli wrote:
> > > > > > > > On Fri, Jun 19, 2015 at 09:38:51AM -0400, Carlos O'Donell wrote:
> > > > > > > > > I agree with aborting, but only as long as the hot path's performance
> > > > > > > > > is not impacted and I haven't thought about how to do that.
> > > > > > > > > 
> > > > > 
> > > > > > > Clearly people are better using atomic comparisons on canary values
> > > > > > > instead, but it seem easy to avoid false positives (returning 0 when
> > > > > > > memory is clearly different) and keep these things working, so why not
> > > > > > > do it ?
> > > > > > 
> > > > > > I see two separate issues here.  First, where do we draw the line, and
> > > > > > what do we guarantee.  I strongly believe that programs must not have
> > > > > > data races, and that they should use atomics or other synchronization
> > > > > > properly (this doesn't mean locks, but relaxed memory order atomics, for
> > > > > > example).
> > > > > > Second, do we try to keep buggy programs working.  If it has no cost to
> > > > > > do so (e.g., like it might be true in this case), then doing that can
> > > > > > help to trigger less errors.  But that doesn't mean the buggy programs
> > > > > > should get fixed eventually.
> > > > > 
> > > > > I find it difficult to understand the boundaries of what the C library
> > > > > is allowed to do in this type of optimisation.
> > > > > 
> > > > > For example, consider the following:
> > > > > 
> > > > >     char a[128];
> > > > >     char b[128];
> > > > > 
> > > > >     put some static data in a[0-63]
> > > > >     put some static data in b[0-63]
> > > > >     a[64]=0;
> > > > >     b[64]=0;
> > > > >     start a thread doing stuff in a[65..]
> > > > >     start a thread doing stuff in b[65..]
> > > > >     
> > > > >     if (!strcmp(a,b)) {
> > > > >       /* Do something */
> > > > >     }
> > > > > 
> > > > >    a) Is that behaviour defined?
> > > > 
> > > > Good question.  I think it should be.  This depends on both the data
> > > > race definition and what strcmp/strncmp/memcmp are specified to do using
> > > > the abstract machine.
> > > > 
> > > > The data race definition uses memory locations as granularity, which is
> > > > in 3.14 in C11.  Separate characters in an array should be separate
> > > > memory locations.
> > > > 
> > > > C11 isn't very specific regarding strcmp, and just says that it
> > > > "compares the string[s]" (7.24.4.2).  C++14 is a bit more specific
> > > > regarding basic_string::compare (21.4.7.9), saying that first the length
> > > > of the strings are determined, and then a strncmp is run using the
> > > > smaller of the two lengths.
> > > > 
> > > > Assuming the C++ specs, only the array indices [0..64] should be
> > > > accessed by the abstract machine.  So no data race with the stuff going
> > > > on in [65..).
> > > > 
> > > > >    b) Is it defined with strncmp(a,b,64) ?
> > > > 
> > > > Yes.
> > > > 
> > > > >    c) Is it defined with strncmp(a,b,128)?
> > > > 
> > > > Not sure.  C11 says that not more than "n characters" are compared, and
> > > > characters that follow the a null character aren't compared either.
> > > > This indicates it wouldn't be different from strncmp(a,b,64) in the
> > > > particular case.
> > > > Regarding C++11, I'm not sure.  The closest copies a substring
> > > > (conceptually) and then compares, but the substring copying has to
> > > > determine length of the string and then subtracting the max length.
> > > > This would do a strlen first, which wouldn't access past index 64.
> > > > Thus, should be fine too.
> > > > 
> > > > >    d) Is it defined with memcmp(a,b,64)?
> > > > 
> > > > No data race, IMO.
> > > > 
> > > > >    e) Is it defined with memcmp(a,b,128)?
> > > > 
> > > > Data race.  Undefined behavior.
> > > > 
> > > > >    f) If I moved that boundary off a nice round % 8 mark would
> > > > >       it matter?
> > > > 
> > > > No difference as far as the standard is concerned.
> > > > 
> > > > > I can imagine there may be lots of things that terminate
> > > > > a string and let other stuff happen in the allocated space
> > > > > after the end of the string in the belief that at the end
> > > > > of that string all is unknown.   Andrea's case is a bit different
> > > > > in that it's the later data that's static, but that doesn't
> > > > > sound like it should change the answer as to what's allowed.
> > > > 
> > > > I think it does, because the question is whether there is a data race on
> > > > the memory locations that the abstract machine would access.
> > > 
> > > Well given we are making examples.
> > > 
> > > Assume 2 structures like this:
> > > 
> > > struct test {
> > >     void *pointer;
> > >     char start[16];
> > >     char end[240];
> > > }
> > > 
> > > and
> > > 
> > > struct test a;
> > > struct test b;
> > > 
> > > memset(a.end, 1, 240);
> > > memset(b.end, 2, 240);
> > > 
> > > In what case it would be expected/legal for 
> > > memcmp(a.start, b.start, 256); to ever return 0 ?
> > 
> > Just to be clear, if there are data-races in [ab].pointer or [as].start
> > I see absolutely no problem in stopping short and returning a
> > positive/negative value. I just do not see how returning 0 would ever
> > make sense.
> 
> It would be legal if the invocation of memcmp and at least one of the
> invocations of memset would be concurrent (ie, not ordered by
> happens-before).

Nope, sorry , there is a misunderstanding, consider the memsets part of
initialization, *never* run in parallel with memcmp.

> I do see that this wouldn't be an intuitive outcome if one reasons based
> on assuming some simplified (or straightforward, or natural, or whatever
> the personal opinion might be...) implementation.  But a data race is
> undefined behavior, and that's where we're drawing the line.

Sure, I can understand that is the *whole* are under consideration is
racing. But here we are talking about a large chunk of the area that is
different and *never* raced upon.

I can accept that the C standard may still decide this is undefined
behavior even if just one small part of the total area sees concurrent
modifications, but again, you are returning "short" from the optimized
code, so it is unclear to me how you can return 0 when the memcmp code
hasn't checked all the memory, wouldn't it make more sense to return the
current position ? It is still undefined but at least it won't cause
false positives.

Simo.

-- 
Simo Sorce * Red Hat, Inc * New York