On Sun, 17 Sep 2017, Paul E. McKenney wrote: > Hello! > > Rough notes from our discussion last Thursday. Please reply to the > group with any needed elaborations or corrections. > > Adding Andy and Michael on CC since this most closely affects their > architectures. Also adding Dave Watson and Maged Michael because > the preferred approach requires that processes wanting to use the > lightweight sys_membarrier() do a registration step. > > Thanx, Paul > > ------------------------------------------------------------------------ > > Problem: > > 1. The current sys_membarrier() introduces an smp_mb() that > is not otherwise required on powerpc. > > 2. The envisioned JIT variant of sys_membarrier() assumes that > the return-to-user instruction sequence handling any change > to the usermode instruction stream, and Andy Lutomirski's > upcoming changes invalidate this assumption. It is believed > that powerpc has a similar issue. > E. Require that threads register before using sys_membarrier() for > private or JIT usage. (The historical implementation using > synchronize_sched() would continue to -not- require registration, > both for compatibility and because there is no need to do so.) > > For x86 and powerpc, this registration would set a TIF flag > on all of the current process's threads. This flag would be > inherited by any later thread creation within that process, and > would be cleared by fork() and exec(). When this TIF flag is set, Why a TIF flag, and why clear it during fork()? If a process registers to use private expedited sys_membarrier, shouldn't that apply to threads it will create in the future just as much as to threads it has already created? > the return-to-user path would execute additional code that would > ensure that ordering and newly JITed code was handled correctly. > We believe that checks for these TIF flags could be combined with > existing checks to avoid adding any overhead in the common case > where the process was not using these sys_membarrier() features. > > For all other architecture, the registration step would be > a no-op. Don't we want to fail private expedited sys_membarrier calls if the process hasn't registered for them? This requires the registration call to set a flag for the process, even on architectures where no additional memory barriers are actually needed. It can't be a no-op. Alan Stern
