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Date:   Thu, 22 Nov 2018 11:47:42 -0500 (EST)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     Florian Weimer <fweimer@redhat.com>
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Subject: Re: [RFC PATCH v4 1/5] glibc: Perform rseq(2) registration at nptl
 init and thread creation
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----- On Nov 22, 2018, at 11:28 AM, Florian Weimer fweimer@redhat.com wrote:

> * Mathieu Desnoyers:
> 
>> Here is one scenario: we have 2 early adopter libraries using rseq which
>> are deployed in an environment with an older glibc (which does not
>> support rseq).
>>
>> Of course, none of those libraries can be dlclose'd unless they somehow
>> track all registered threads.
> 
> Well, you can always make them NODELETE so that dlclose is not an issue.
> If the library is small enough, that shouldn't be a problem.

That's indeed what I do with lttng-ust, mainly due to use of pthread_key.

> 
>> But let's focus on how exactly those libraries can handle lazily
>> registering rseq. They can use pthread_key, and pthread_setspecific on
>> first use by the thread to setup a destructor function to be invoked
>> at thread exit. But each early adopter library is unaware of the
>> other, so if we just use a "is_initialized" flag, the first destructor
>> to run will unregister rseq while the second library may still be
>> using it.
> 
> I don't think you need unregistering if the memory is initial-exec TLS
> memory.  Initial-exec TLS memory is tied directly to the TCB and cannot
> be freed while the thread is running, so it should be safe to put the
> rseq area there even if glibc knows nothing about it.

Is it true for user-supplied stacks as well ?

> Then you'll only
> need a mechanism to find the address of the actually active rseq area
> (which you probably have to store in a TLS variable for performance
> reasons).  And that part you need whether you have reference counter or
> not.

I'm not sure I follow your thoughts here. Currently, the __rseq_abi
TLS symbol identifies a structure registered to the kernel. The
"currently active" rseq critical section is identified by the field
"rseq_cs" within the __rseq_abi structure.

So here when you say "actually active rseq area", do you mean the
currently registered struct rseq (__rseq_abi) or the currently running
rseq critical section ? (pointed to by __rseq_abi.rseq_cs)

One issue here is that early adopter libraries cannot always use
the IE model. I tried using it for other TLS variables in lttng-ust, and
it ended up hanging our CI tests when tracing a sample application with
lttng-ust under a Java virtual machine: being dlopen'd in a process that
possibly already exhausts the number of available backup TLS IE entries
seems to have odd effects. This is why I'm worried about using the IE model
within lttng-ust.

So using the IE model for glibc makes sense, because nobody dlopen
glibc AFAIK. But it's not so simple for early adopter libraries which
can be dlopen'd.

> 
>> The same problem arises if we have an application early adopter which
>> explicitly deal with rseq, with a library early adopter. The issue is
>> similar, except that the application will explicitly want to unregister
>> rseq before exiting the thread, which leaves a race window where rseq
>> is unregistered, but the library may still need to use it.
>>
>> The reference counter solves this: only the last rseq user for a thread
>> performs unregistration.
> 
> If you do explicit unregistration, you will run into issues related to
> destructor ordering.  You should really find a way to avoid that.

The per-thread reference counter is a way to avoid issues that arise from
lack of destructor ordering. Is it an acceptable approach for you, or
you have something else in mind ?

Thanks,

Mathieu

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