----- On Oct 17, 2017, at 12:19 PM, Ben Maurer bmaurer@xxxxxx wrote: > Hey, > >> So far the restrictions I see for libraries using this symbol are: >> - They should never be unloaded, >> - They should never be loaded with dlopen RTLD_LOCAL flag. > > We talked a bit about this off-list but I wanted to state publicly that I think > this model works well for our use case. Specifically, > > (1) It reduces complexity by focusing on the common case -- long term we expect > glibc to manage the process of using this feature and registering/deregistering > threads for rseq. Unloading isn't a challenge in these situations, so why add > the complexity for it? > > (2) This still allows for early adopters to use rseq before there is glibc > support. I believe the vast majority of real world applications meet these two > criteria you've listed. If not, they can create a thin shared library that has > the sole purpose of providing the weak symbol and that never gets unloaded > > (3) This allows for applications to provide the __rseq_abi so that they can > ensure it uses the initial_exec tls model and optimize in-application assembly > code for it. This is a good optimization for server applications that tend to > statically link. Agreed with all the above, > > If others agree with this, would it make sense to remove the concept of > reference counting in the system call that defines and redefines the per-thread > area? Seems like it would remove complexity. I have a use-case for keeping the reference counting in place though. It's use of rseq in signal handlers. If we have two early-adopter libraries trying to lazy-register rseq, and one of those libraries can be called within a signal handler (e.g. lttng-ust), we run into a situation where signal handler could nest on top of the first library lazy-register (test, branch, register), and race against it. So having reference counting in place allows the kernel to deal with those multi-lib use-cases atomically wrt signal handlers from a thread perspective. And I don't want to require every early-adopter library to disable signals just in case some _other_ library would be invoked in a signal handler. Thoughts ? Thanks, Mathieu > > -b -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
