On Thu, Nov 22, 2018 at 10:04:16AM -0500, Mathieu Desnoyers wrote: > ----- On Nov 22, 2018, at 9:36 AM, Rich Felker dalias@xxxxxxxx wrote: > > > On Wed, Nov 21, 2018 at 01:39:32PM -0500, Mathieu Desnoyers wrote: > >> Register rseq(2) TLS for each thread (including main), and unregister > >> for each thread (excluding main). "rseq" stands for Restartable > >> Sequences. > > > > Maybe I'm missing something obvious, but "unregister" does not seem to > > be a meaningful operation. Can you clarify what it's for? > > There are really two ways rseq TLS can end up being unregistered: either > through an explicit call to the rseq "unregister", or when the OS frees the > thread's task struct. > > You bring an interesting point here: do we need to explicitly unregister > rseq at thread exit, or can we leave that to the OS ? > > The key thing to look for here is whether it's valid to access the > TLS area of the thread from preemption or signal delivery happening > at the very end of START_THREAD_DEFN. If it's OK to access it until > the very end of the thread lifetime, then we could do without an > explicit unregistration. However, if at any given point of the late > thread lifetime we end up in a situation where reading or writing to > that TLS area can cause corruption, then we need to carefully > unregister it before that memory is reclaimed/reused. The thread memory cannot be reused until after kernel task exit, reported via the set_tid_address futex. Also, assuming signals are blocked (which is absolutely necessary for other reasons) nothing in userspace can touch the rseq state after this point anyway. I was more confused about the need for reference counting, though. Where would anything be able to observe a state other than "refcnt>0"? -- in which case tracking it makes no sense. If the goal is to make an ABI thatsupports environments where libc doesn't have rseq support, and a third-party library is providing a compatible ABI, it seems all that would be needed it a boolean thread-local "is_initialized" flag. There does not seem to be any safe way such a library could be dynamically unloaded (which would require unregistration in all threads) and thus no need for a count. Rich
