On Mon, Sep 18, 2017 at 07:37:22PM +0000, Mathieu Desnoyers wrote: > ----- On Sep 18, 2017, at 3:29 PM, Paul E. McKenney paulmck@xxxxxxxxxxxxxxxxxx wrote: > > > On Mon, Sep 18, 2017 at 03:04:21PM -0400, Alan Stern 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? > > > > The reason for a TIF flag is to keep this per-architecture, as only > > powerpc and x86 need it. > > > > The reason for clearing it during fork() is that fork() creates a new > > process initially having but a single thread, which might or might > > not use sys_membarrier(). Usually not, as most instances of fork() > > are quickly followed by exec(). In addition, if we give an error for > > unregistered use of private sys_membarrier(), clearing on fork() gets an > > unambiguous error instead of a silent likely failure (due to libraries > > being confused by the fork()). > > I think clearing that state on fork() would be unexpected. The child process > inherits from the parent flag in my current implementation. Clearing the > flag is only provided through exec(). > > Libraries don't get re-initialized on fork, only on exec(). Therefore, it > makes sense for the child process to inherit the state from its parent. Fair enough! Thanx, Paul
