Date:   Fri, 28 Jan 2022 10:26:20 -0800
From:   Ian Rogers <irogers@...gle.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Darren Hart <dvhart@...radead.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        André Almeida <andrealmeid@...labora.com>,
        James Clark <james.clark@....com>,
        John Garry <john.garry@...wei.com>,
        Riccardo Mancini <rickyman7@...il.com>,
        Yury Norov <yury.norov@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jin Yao <yao.jin@...ux.intel.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Leo Yan <leo.yan@...aro.org>, Andi Kleen <ak@...ux.intel.com>,
        Thomas Richter <tmricht@...ux.ibm.com>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Madhavan Srinivasan <maddy@...ux.ibm.com>,
        Shunsuke Nakamura <nakamura.shun@...itsu.com>,
        Song Liu <song@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Miaoqian Lin <linmq006@...il.com>,
        Stephen Brennan <stephen.s.brennan@...cle.com>,
        Kajol Jain <kjain@...ux.ibm.com>,
        Alexey Bayduraev <alexey.v.bayduraev@...ux.intel.com>,
        German Gomez <german.gomez@....com>,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        Eric Dumazet <edumazet@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        masami.hiramatsu.pt@...achi.com, eranian@...gle.com
Subject: Re: [PATCH v2 0/4] Reference count checker and related fixes

On Fri, Jan 28, 2022 at 7:35 AM Masami Hiramatsu <mhiramat@...nel.org> wrote:
>
> On Thu, 27 Jan 2022 22:24:59 -0800
> Ian Rogers <irogers@...gle.com> wrote:
>
> > On Thu, Jan 27, 2022 at 9:24 PM Masami Hiramatsu <mhiramat@...nel.org> wrote:
> > >
> > > On Thu, 27 Jan 2022 13:33:23 -0800
> > > Ian Rogers <irogers@...gle.com> wrote:
> > >
> > > > On Tue, Jan 25, 2022 at 12:46 PM Ian Rogers <irogers@...gle.com> wrote:
> > > > >
> > > > > This v2 patch set has the main reference count patch for cpu map from
> > > > > the first set and then adds reference count checking to nsinfo. The
> > > > > reference count checking on nsinfo helped diagnose a data race bug
> > > > > which is fixed in the independent patches 2 and 3.
> > > > >
> > > > > The perf tool has a class of memory problems where reference counts
> > > > > are used incorrectly. Memory/address sanitizers and valgrind don't
> > > > > provide useful ways to debug these problems, you see a memory leak
> > > > > where the only pertinent information is the original allocation
> > > > > site. What would be more useful is knowing where a get fails to have a
> > > > > corresponding put, where there are double puts, etc.
> > > > >
> > > > > This work was motivated by the roll-back of:
> > > > > https://lore.kernel.org/linux-perf-users/20211118193714.2293728-1-irogers@google.com/
> > > > > where fixing a missed put resulted in a use-after-free in a different
> > > > > context. There was a sense in fixing the issue that a game of
> > > > > wac-a-mole had been embarked upon in adding missed gets and puts.
> > > > >
> > > > > The basic approach of the change is to add a level of indirection at
> > > > > the get and put calls. Get allocates a level of indirection that, if
> > > > > no corresponding put is called, becomes a memory leak (and associated
> > > > > stack trace) that leak sanitizer can report. Similarly if two puts are
> > > > > called for the same get, then a double free can be detected by address
> > > > > sanitizer. This can also detect the use after put, which should also
> > > > > yield a segv without a sanitizer.
> > > > >
> > > > > Adding reference count checking to cpu map was done as a proof of
> > > > > concept, it yielded little other than a location where the use of get
> > > > > could be cleaner by using its result. Reference count checking on
> > > > > nsinfo identified a double free of the indirection layer and the
> > > > > related threads, thereby identifying a data race as discussed here:
> > > > > https://lore.kernel.org/linux-perf-users/CAP-5=fWZH20L4kv-BwVtGLwR=Em3AOOT+Q4QGivvQuYn5AsPRg@mail.gmail.com/
> > > > > Accordingly the dso->lock was extended and use to cover the race.
> > > > >
> > > > > An alternative that was considered was ref_tracker:
> > > > >  https://lwn.net/Articles/877603/
> > > > > ref_tracker requires use of a reference counted struct to also use a
> > > > > cookie/tracker. The cookie is combined with data in a ref_tracker_dir
> > > > > to spot double puts. When an object is finished with leaks can be
> > > > > detected, as with this approach when leak analysis happens. This
> > > > > approach was preferred as it doesn't introduce cookies, spots use
> > > > > after put and appears moderately more neutral to the API. Weaknesses
> > > > > of the implemented approcah are not being able to do adhoc leak
> > > > > detection and a preference for adding an accessor API to structs. I
> > > > > believe there are other issues and welcome suggestions.
> > > >
> > > > And so we've been here before (Dec 2015 to be exact). Namhyung pointed me to:
> > > > https://lore.kernel.org/all/20151209021047.10245.8918.stgit@localhost.localdomain/
> > > > by Masami Hiramatsu. In this work he adds a leak sanitizer style
> > > > reference count checker that will describe locations of puts and gets
> > > > for diagnosis. Firstly that's an awesome achievement! This work is
> > > > different in that it isn't trying to invent a leak sanitizer, it is
> > > > just using the existing one. By adding a level of indirection this
> > > > work can catch use after put and pairs gets with puts to make lifetime
> > > > analysis more automatic. An advantage of Masami's work is that it
> > > > doesn't change data-structures and after the initial patch-set is
> > > > somewhat transparent. Overall I prefer the approach in these patches,
> > > > future patches can look to clean up the API as Masami has.
> > >
> > > Thanks for referring my series :-D The series aimed to solve the refcount
> > > usage issue in the perf which lead the object leaks. At that moment,
> > > I found that there were 2 patterns, refcount start from 0 and start from 1.
> > > That made me confused what I should do for using a object.
> > > But the perf uses linux/refcount.h now, I hope such issue has already gone.
> > > (but the object leakage seems not fixed fully yet, as you found.)
> > >
> > > BTW, I think the introducing UNWRAP_* macro may put a burden on future
> > > development. If it is inevitable, we should consider it as carefully as
> > > possible. Or, it may cause another issue (it is easily missed that the new
> > > patch does not use UNWRAP_* for object reference, because it is natual.)
> > >
> > > So I agree with you that you to clean up the API. :-)
> > > I think we can make yet another refcount.h for user space debugging and
> > > replace it with the linux/refcount.h.
> >
> > Thanks Masami,
> >
> > Agreed on the UNWRAP_ macros, hence wanting to hide them behind
> > accessors. Making accessors could be automated with macros, for
> > example, have a list of variables, have a macro declare the struct
> > using the list, another macro can use the list to declare accessors. I
> > didn't find adding the UNWRAP_ macros in this change particularly
> > burdensome as any use of the wrapping pointer as the original type
> > caused a compile time error telling you what and where to fix. The
> > macro is extra stuff in the way of using just the raw object, but
> > that's fairly typical in C++ with shared_ptr, scoped_lock, etc.
>
> Hi Ian,
>
> Hmm, but such a macro is not usual for C which perf is written in.
> If I understand correctly, you might want to use memory leak
> analyzer to detect refcount leak, and that analyzer will show
> what data structure is leaked.

Firstly, thanks for the conversation - this is really useful to
improve the code!

I think in an ideal world we'd somehow educate things like address
sanitizer of reference counted data structures and they would do a
better job of tracking gets and puts. The problem is pairing gets and
puts. In C++ you use RAII types so that the destructor ensures a put -
this can be complex when using data types like lists where you want to
move or swap things onto the list, to keep the single pointer
property. In the C code in Linux we use gotos, similarly to how defer
is used in Go. Anyway, the ref_tracker that Eric Dumazet added solved
the get/put pairing problem by adding a cookie that is passed around.
The problem with that is that then the cookie becomes part of the API.
To avoid that the approach here is just to change the original data
type and add in a layer of indirection, that layer has become the
cookie. A benefit of this approach is that once the cookie/indirection
is freed, use of it becomes an obvious failure - we leverage address
sanitizer for use after free.

> If so, maybe you can do the same thing by introducing a dummy
> list node for each data structure which you want to debug.
>
> struct perf_cpu_map__refdebug {
>         struct perf_cpu_map__refdebug *orig;
> };
>
> And expand refcount_t as.
>
> typedef struct refcount_struct {
>         atomic_t refs;
> #ifdef REFCNT_CHECKING
>         void *orig;
> #endif
> } refcount_t;
>
> And change the get/put as below
>
> struct perf_cpu_map *perf_cpu_map__get(struct perf_cpu_map *map)
> {
>         if (map) {
> #ifdef REFCNT_CHECKING
>                 struct perf_cpu_map__refdebug *new_node;
> #endif
>                 refcount_inc(&map->refcnt);
> #ifdef REFCNT_CHECKING
>                 new_node = malloc(sizeof(*new_node));
>                 new_node->orig = map->refcnt->orig;
>                 map->refcnt->orig = new_node;
> #endif
>         }
>         return map;
> }
>
> void perf_cpu_map__put(struct perf_cpu_map *map)
> {
>         if (map) {
>                 if (refcount_dec_and_test(&map->refcnt))
>                         cpu_map__delete(map);
>                 else {
> #ifdef REFCNT_CHECKING
>                         struct perf_cpu_map__refdebug *node = map->refcnt->orig;
>
>                         map->refcnt->orig = node->orig;
>                         free(node);
> #endif
>                 }
>         }
> }
>
> This need a bit complex get/put, but no need to change other parts.

Adding a list like this gives an ability to say something like of the
current reference count of 3 what indirection objects exist. This
could be useful for diagnosis but you probably want to pair it with a
stack trace, and the approach here is punting that problem to the
address/leak sanitizer. I'm also concerned that there should be a lock
around the list. I think pursuing this ends up with something like
ref_tracker.

If we're using indirection, as in my proposal, then adding a common
indirection struct is problematic as anything declared to be a "struct
cpumap" now needs to be either the indirection or the original type -
hence using macros to hide that in the code. If we embed the
information into the refcount_t then we end up with something like
ref_tracker, API problems and losing use-after-put checking. Outside
of the macros, I think there is a simplicity to the approach I've put
forward.

> > The
> > question is, is it worth it to make sure use of the reference counted
> > object is correct and misuse is easier to diagnose?
>
> You mean the stackdump for every get/put as I did? That's a good
> question. Let's think what may happen.
>
> For example, if funcA() expects its caller funcB() will put the object
> but actually funcB() doesn't, or the funcC() which is the another
> caller of funcA()) doesn't expect the funcA() gets the object.
>
> funcA() {
>         get(obj);
>         return obj;
> }
>
> funcB() {
>         obj = funcA();
>         ...
>         // wrong! it should do put(obj);
> }
>
> funcC() {
>         obj = funcA();
>         get(obj);               // this is wrong get().
>         ...
>         put(obj);
> }
>
> If we just list the non-released object, both logs seems same because
> funcB()'s get/put pair will be skipped. If the analyzer shows the
> stacktrace when the object was got, maybe we can notice the difference
> of funcB() and funcC() path, but this is the simplest case. funcA()
> can be called from funcB/C via several different functions.
> But perhaps I'm too worried.

So in the logs we should see for funcB:

Memory leak of ... at:
malloc...
get...
funcA
funcB
...

as the put on the indirection object was missed and this is now a leak
of the indirection object. For funcC we should see:

Memory leak of ... at:
malloc..
get..
funcA
funcC

So from the stack traces we can see that there is an unpaired get
happening in funcA called from either funcB and funcC, which means we
need to a put there. In the funcC case we can see the put was missed
from a call to funcA, rather than a get it made.

As the code in perf is complex, multi-threaded and sometimes
unintentionally racy a get may happen on 1 thread, the object is
placed in a global, the object is put by another thread and also
accessed by a 3rd thread. This is what was happening in the
dso->nsinfo case. The bug there is that there was a double put
happening by the third thread because of a race. Leak sanitizer treats
memory visible from a global as not a leak, this can mean to get the
most information on leaks in perf we need to aggressively
free/delete/deconstruct when terminating so that leaks become visible.
This feels to me like good hygiene, but it could also be argued to be
a tax.

Anyway, I think I'm still at the same point I was when I posted these
changes. That an indirection object is the simplest, smallest,
cleanest way to get the most information. I think making the rest of
the reference counted data structures have this feature would be
great, so I'd like to merge the 4 patches here and work to add more. I
think we can also build on that foundation for extra debug
information.

Thanks,
Ian

> Thank you,
>
> > I think it is near
> > as least offensive as possible while providing the best information -
> > hence being able to solve the dso->nsinfo put data race, that has been
> > a problem to solve up to this point. I'm open to better suggestions
> > though :-)
> >
> > Thanks again,
> > Ian
> >
> > > Thank you,
> > >
> > > --
> > > Masami Hiramatsu <mhiramat@...nel.org>
>
>
> --
> Masami Hiramatsu <mhiramat@...nel.org>

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