Re: Rseq registration: Google tcmalloc vs glibc

[Mathieu Desnoyers <mathieu dot desnoyers at efficios dot com>]

----- On Feb 26, 2020, at 2:12 PM, Chris Kennelly ckennelly@google.com wrote:

> On Wed, Feb 26, 2020 at 1:56 PM Mathieu Desnoyers
> <mathieu.desnoyers@efficios.com> wrote:
>>
>> ----- On Feb 26, 2020, at 12:27 PM, Chris Kennelly ckennelly@google.com wrote:
>>
>> > On Wed, Feb 26, 2020 at 12:01 PM Mathieu Desnoyers
>> > <mathieu.desnoyers@efficios.com> wrote:
>> >>
>> >> ----- On Feb 25, 2020, at 10:38 PM, Chris Kennelly ckennelly@google.com wrote:
>> >>
>> >> > On Tue, Feb 25, 2020 at 10:25 PM Joel Fernandes <joel@joelfernandes.org> wrote:
>> >> >>
>> >> >> On Fri, Feb 21, 2020 at 11:13 AM Mathieu Desnoyers
>> >> >> <mathieu.desnoyers@efficios.com> wrote:
>> >> >> >
>> >> >> > ----- On Feb 21, 2020, at 10:49 AM, Joel Fernandes, Google
>> >> >> > joel@joelfernandes.org wrote:
>> >> >> >
>> >> >> > [...]
>> >> >> > >>
>> >> >> > >> 3) Use the  __rseq_abi TLS cpu_id field to know whether Rseq has been
>> >> >> > >> registered.
>> >> >> > >>
>> >> >> > >> - Current protocol in the most recent glibc integration patch set.
>> >> >> > >> - Not supported yet by Linux kernel rseq selftests,
>> >> >> > >> - Not supported yet by tcmalloc,
>> >> >> > >>
>> >> >> > >> Use the per-thread state to figure out whether each thread need to register
>> >> >> > >> Rseq individually.
>> >> >> > >>
>> >> >> > >> Works for integration between a library which exists for the entire lifetime
>> >> >> > >> of the executable (e.g. glibc) and other libraries. However, it does not
>> >> >> > >> allow a set of libraries which are dlopen'd/dlclose'd to co-exist without
>> >> >> > >> having a library like glibc handling the registration present.
>> >> >> > >
>> >> >> > > Mathieu, could you share more details about why during dlopen/close
>> >> >> > > libraries we cannot use the same __rseq_abi TLS to detect that rseq was
>> >> >> > > registered?
>> >> >> >
>> >> >> > Sure,
>> >> >> >
>> >> >> > A library which is only loaded and never closed during the execution of the
>> >> >> > program can let the kernel implicitly unregister rseq at thread exit. For
>> >> >> > the dlopen/dlclose use-case, we need to be able to explicitly unregister
>> >> >> > each thread's __rseq_abi which sit in a library which is going to be
>> >> >> > dlclose'd.
>> >> >>
>> >> >> Mathieu, Thanks a lot for the explanation, it makes complete sense. It
>> >> >> sounds from Chris's reply that tcmalloc already checks
>> >> >> __rseq_abi.cpu_id and is not dlopened/closed. Considering these, it
>> >> >> seems to already handle things properly - CMIIW.
>> >> >
>> >> > I'll make a note about this, since we can probably benefit from some
>> >> > more comments about the assumptions/invariants the fastpath uses.
>> >>
>> >> I suspect the integration with glibc and with dlopen'd/dlclose'd libraries will
>> >> not
>> >> behave correctly with the current tcmalloc implementation.
>> >>
>> >> Based on the tcmalloc code-base, InitFastPerCpu is only called from IsFast. As
>> >> long
>> >> as this is the only expected caller, having IsFast comparing the RseqCpuId
>> >> detects
>> >> whether glibc (or some other library) has already registered rseq for the
>> >> current
>> >> thread.
>> >>
>> >> However, if the application chooses to invoke InitFastPerCpu() directly, things
>> >> become
>> >> expected, because it invokes:
>> >>
>> >>   absl::base_internal::LowLevelCallOnce(&init_per_cpu_once, InitPerCpu);
>> >>
>> >> which AFAIU invokes InitPerCpu once after execution of the current program.
>> >> Which
>> >> does:
>> >>
>> >> static bool InitThreadPerCpu() {
>> >>   if (__rseq_refcount++ > 0) {
>> >>     return true;
>> >>   }
>> >>
>> >>   auto ret = syscall(__NR_rseq, &__rseq_abi, sizeof(__rseq_abi), 0,
>> >>                      PERCPU_RSEQ_SIGNATURE);
>> >>   if (ret == 0) {
>> >>     return true;
>> >>   } else {
>> >>     __rseq_refcount--;
>> >>   }
>> >>
>> >>   return false;
>> >> }
>> >>
>> >> static void InitPerCpu() {
>> >>   // Based on the results of successfully initializing the first thread, mark
>> >>   // init_status to initialize all subsequent threads.
>> >>   if (InitThreadPerCpu()) {
>> >>     init_status = kFastMode;
>> >>   }
>> >> }
>> >>
>> >> In a scenario where glibc has already registered Rseq, the __rseq_refcount will
>> >> be incremented, the __NR_rseq syscall will fail with -1, errno=EBUSY, so the
>> >> refcount
>> >> will be immediately decremented and it will return false. Therefore,
>> >> "init_status" will
>> >> never be set fo kFastMode, leaving it in kSlowMode for the entire lifetime of
>> >> this
>> >> program. That being said, even though this state can come as a surprise, it
>> >> seems to
>> >> be entirely bypassed by the fast-paths IsFast() and IsFastNoInit(), so maybe it
>> >> won't
>> >> have any observable side-effects other than leaving init_status in a state that
>> >> does not
>> >> match reality.
>> >
>> > I agree that this could potentially violate inviarants, but
>> > InitFastPerCpu is not intended to be called by the application.
>>
>> OK, explicitly documenting this would be a good thing. In my own projects,
>> I prefix those symbols with double-underscores (__) to indicate that those
>> are not meant to be called by other means than the static inlines in the API.
>>
>> There may be use-cases which justify exposing InitFastPerCpu as a public API for
>> applications though, especially for those which require some level of
>> real-time guarantees from the malloc/free APIs. I've run into this situation
>> with liburcu which I maintain.
>>
>> >
>> >> In the other use-case where tcmalloc co-exist with a dlopened/dlclosed library,
>> >> but glibc
>> >> does not provide Rseq registration, we run into issues as well if the dlopened
>> >> library
>> >> registers rseq first for a given thread. The IsFastNoInit() expects that if Rseq
>> >> has been
>> >> observed as registered in the past for a thread, it stays registered. However,
>> >> if a
>> >> dlclosed library unregisters Rseq, we need to be prepared to re-register it. So
>> >> either
>> >> tcmalloc needs to express its use of Rseq by incrementing __rseq_refcount even
>> >> when Rseq
>> >> is registered (this would hurt the fast-path however, and I would hate to have
>> >> to do this),
>> >> or tcmalloc needs to be able to handle the fact that Rseq may be unregistered by
>> >> a dlclosed
>> >> library which was the actual owner of the Rseq registration.
>> >
>> > We have a bit of an opportunity to figure out whether this is the
>> > first time--from TCMalloc's perspective--a thread is doing per-CPU and
>> > bump the __rseq_count accordingly.  I think this could be done off of
>> > the fast path.
>>
>> Is there an explicit tcmalloc API call that each thread need to do before
>> starting
>> to use tcmalloc to allocate and free memory ? If not, you'll probably need to
>> add
>> at least a load of __rseq_refcount (or some other TLS variable), test and
>> conditional
>> branch on the fast-path, which is an additional cost I would ideally prefer to
>> avoid.
>> Or do you have something else in mind ?
> 
> No explicit call is necessary.  This is something that can be done in
> the slow path, since we can recognize the transition from slow -> fast
> path for that thread

Got it, it should work. Thanks!

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com