From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mathieu Desnoyers Subject: Re: [PATCH 08/11] membarrier: Provide core serializing command (v2) Date: Mon, 29 Jan 2018 19:20:52 +0000 (UTC) Message-ID: <579395772.11661.1517253652787.JavaMail.zimbra@efficios.com> References: <20180123155733.3404-1-mathieu.desnoyers@efficios.com> <20180123155733.3404-9-mathieu.desnoyers@efficios.com> <20180129180414.GO2249@hirez.programming.kicks-ass.net> <20180129181529.GG2295@hirez.programming.kicks-ass.net> <485936677.11601.1517250965043.JavaMail.zimbra@efficios.com> <20180129190923.GP2249@hirez.programming.kicks-ass.net> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20180129190923.GP2249-Nxj+rRp3nVydTX5a5knrm8zTDFooKrT+cvkQGrU6aU0@public.gmane.org> Sender: linux-api-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Peter Zijlstra Cc: Ingo Molnar , Thomas Gleixner , linux-kernel , linux-api , Andy Lutomirski , "Paul E. McKenney" , Boqun Feng , Andrew Hunter , maged michael , Avi Kivity , Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , Dave Watson , "H. Peter Anvin" , Andrea Parri , "Russell King, ARM Linux" , Greg Hackmann , Will Deacon , David List-Id: linux-api@vger.kernel.org ----- On Jan 29, 2018, at 2:09 PM, Peter Zijlstra peterz-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org wrote: > On Mon, Jan 29, 2018 at 06:36:05PM +0000, Mathieu Desnoyers wrote: >> ----- On Jan 29, 2018, at 1:15 PM, Peter Zijlstra peterz-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org wrote: > >> > Aaah, its the case where we do not pass through switch_mm(), the partial >> > comment got to me. I only realized after reading the next patch. >> >> Indeed, if we read the entire comment, it's made clear that this case is for >> when switch_mm is not invoked, where the current mm is changed without going >> through switch_mm(), when scheduling between uthread->kthread->uthread for >> instance. >> >> /* >> * When transitioning from a kernel thread to a userspace >> * thread, mmdrop()'s implicit full barrier is required by the >> * membarrier system call, because the current active_mm can >> * become the current mm without going through switch_mm(). >> * membarrier also requires a core serializing instruction >> * before going back to user-space after storing to rq->curr. >> */ >> >> Is there something I should improve in the wording of this added >> sentence to make it clearer ? > > Can be improved I think, its got two unqualified "membarrier"s in and > its a bit mixed up. I'm having a major case of the mondays (brain just > won't start today), but maybe something like: > > When we switched through a kernel thread, the loop in > membarrier_{private,global}_expedited() can have observed that > kernel thread and not issued an IPI. We will also not pass > through switch_mm(). Membarrier requires a barrier after writing > rq->curr and returning to userspace, so provide them here: > > - a full memory barrier for {PRIVATE,GLOBAL}_EXPEDITED > - a sync_core for SYNC_CORE Editing to remove use of "we" and clarify, which ends up as: /* * When switching through a kernel thread, the loop in * membarrier_{private,global}_expedited() may have observed that * kernel thread and not issued an IPI. It is therefore possible to * schedule between user->kernel->user threads without passing though * switch_mm(). Membarrier requires a barrier after storing to * rq->curr, before returning to userspace, so provide them here: * * - a full memory barrier for {PRIVATE,GLOBAL}_EXPEDITED, implicitly * provided by mmdrop(), * - a sync_core for SYNC_CORE. */ > > Also I think changing the changlog to state where we need core-sync > would be good. Currently the x86 patch does that, but not this one, > while this introduces the feature. Planning to add this: Architectures selecting this feature need to either document that they issue core serializing instructions when returning to user-space, or implement their architecture-specific sync_core_before_usermode(). Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com