* Mathieu Desnoyers: > ----- On Nov 22, 2018, at 11:28 AM, Florian Weimer fweimer@xxxxxxxxxx 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 ? I'm not entirely sure because the glibc terminology is confusing, but I think it places intial-exec TLS into the static TLS area (so that it has a fixed offset from the TCB). The static TLS area is placed on the user-supplied stack. >> 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) __rseq_abi. > 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. You can work around this by preloading the library. I'm not sure if this is a compelling reason not to use initial-exec TLS memory. >>> 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 ? Only for the involved libraries. It will not help if other TLS destructors run and use these libraries. Thanks, Florian
