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SubjectRe: Restartable Sequences system call merged into Linux
----- On Jun 14, 2018, at 8:27 AM, Pavel Machek wrote:

> On Tue 2018-06-12 12:31:24, Mathieu Desnoyers wrote:
>> ----- On Jun 12, 2018, at 9:11 AM, Florian Weimer wrote:
>> > On 06/11/2018 10:04 PM, Mathieu Desnoyers wrote:
>> >> ----- On Jun 11, 2018, at 3:55 PM, Florian Weimer wrote:
>> >>
>> >>> On 06/11/2018 09:49 PM, Mathieu Desnoyers wrote:
>> >>>> It should be noted that there can be only one rseq TLS area registered per
>> >>>> thread,
>> >>>> which can then be used by many libraries and by the executable, so this is a
>> >>>> process-wide (per-thread) resource that we need to manage carefully.
>> >>>
>> >>> Is it possible to resize the area after thread creation, perhaps even
>> >>> from other threads?
>> >>
>> >> I'm not sure why we would want to resize it. The per-thread area is fixed-size.
>> >> Its layout is here: include/uapi/linux/rseq.h: struct rseq
>> >
>> > Looks I was mistaken and this is very similar to the robust mutex list.
>> >
>> > Should we treat it the same way? Always allocate it for each new thread
>> > and register it with the kernel?
>> That would be an efficient way to do it, indeed. There is very little
>> performance overhead to have rseq registered for all threads, whether or
>> not they intend to run rseq critical sections.
> People with slow / low memory machines would prefer not to see
> overhead they don't need...

In terms of memory usage, if people don't want the extra few bytes of memory
used by rseq in the kernel, they should use CONFIG_RSEQ=n.

In terms of overhead, let's have a closer look at what it means: when a thread
is registered to rseq, but does not enter rseq critical sections, only this
extra work is done by the kernel:

- rseq_preempt(): on preemption, the scheduler sets the TIF_NOTIFY_RESUME thread
flag, so rseq_handle_notify_resume() can check whether it's in a rseq critical
section when returning to user-space,
- rseq_signal_deliver(): on signal delivery, rseq_handle_notify_resume() checks
whether it's in a rseq critical section,
- rseq_migrate: on migration, the scheduler sets TIF_NOTIFY_RESUME as well,

>> I have a few possible approaches in mind (feel free to suggest other
>> options):
>> A) glibc exposes a strong __rseq_abi TLS symbol:
>> - should ideally *not* be global-dynamic for performance reasons,
>> - registration to kernel can either be handled explicitly by requiring
>> application or libraries to call an API, or implicitly at thread
>> creation,
> I'd prefer explicit API call.

I have use-cases where a library wants to link against librseq and have rseq
critical sections, without requiring the application to explicitly add rseq
registration calls on thread creation/destruction. Is there a way to register
callbacks to glibc which could be invoked on thread creation/destruction ?

Then if we include dynamic loading of libraries (dlopen/dlclose) in the
picture, this gets even worse, as we'd need to be able to iterate on all
existing threads to invoke registration/unregistration callbacks.

One alternative approach would be to let the user library lazily register rseq
when needed, and use a pthread_key for unregistration. However, this does not
allow dlclose of the user library without figuring a way to iterate on all

Another alternative would be to somehow let glibc handle the registration,
perhaps only doing it for applications expressing their interest for rseq.

Thoughts ?



>> B) exposes a strong __rseq_abi symbol:
> Works for me.
> Pavel
> --
> (english)
> (cesky, pictures)

Mathieu Desnoyers
EfficiOS Inc.

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