----- On Sep 18, 2017, at 3:04 PM, Alan Stern stern@xxxxxxxxxxxxxxxxxxx wrote: > 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? In my implementation posted today, I'm not clearing it on fork. The child inherits from the parent. Why TIF flag ? It appears to be a convenient way to add an architecture-specific single-bit state for each thread. We also don't want to do too much pointer chasing on the scheduler fast-path (current->mm->..). > >> 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. My implementation posted today fails the private expedited command if the process is not registered yet. We indeed add a new flag in mm_struct for all architectures to do so. So why not re-use this flag instead of the TIF on powerpc ? See my pointer chasing on fast-path argument above. Thanks, Mathieu > > Alan Stern -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
