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Date: Thu, 7 Apr 2016 17:53:12 +0200 From: Peter Zijlstra <peterz@...radead.org> To: Andy Lutomirski <luto@...capital.net> Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>, Ingo Molnar <mingo@...hat.com>, Paul Turner <commonly@...il.com>, Andi Kleen <andi@...stfloor.org>, Chris Lameter <cl@...ux.com>, Dave Watson <davejwatson@...com>, Josh Triplett <josh@...htriplett.org>, Linux API <linux-api@...r.kernel.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Andrew Hunter <ahh@...gle.com>, Linus Torvalds <torvalds@...ux-foundation.org> Subject: Re: [RFC PATCH 0/3] restartable sequences v2: fast user-space percpu critical sections On Thu, Apr 07, 2016 at 08:44:38AM -0700, Andy Lutomirski wrote: > On Thu, Apr 7, 2016 at 8:24 AM, Peter Zijlstra <peterz@...radead.org> wrote: > > On Thu, Apr 07, 2016 at 07:35:26AM -0700, Andy Lutomirski wrote: > >> What I meant was: rather than shoving individual values into the TLABI > >> thing, shove in a pointer: > >> > >> struct commit_info { > >> u64 post_commit_rip; > >> u32 cpu; > >> u64 *event; > >> // whatever else; > >> }; > >> > >> and then put a commit_info* in TLABI. > >> > >> This would save some bytes in the TLABI structure. > > > > But would cost us extra indirections. The whole point was getting this > > stuff at a constant offset from the TLS segment register. > > I don't think the extra indirections would matter much. The kernel > would have to chase the pointer, but only in the very rare case where > it resumes userspace during a commit or on the immediately following > instruction. Its about userspace finding these values, not the kernel. > At the very least, post_commit_rip and the abort address (which I > forgot about) could both live in a static structure, Paul keeps the abort address in rcx. > and shoving a > pointer to *that* into TLABI space is one store instead of two. > > Ah, so what happens if the signal happens before the commit but after > > the load of the seqcount? > > > > Then, even if the signal motifies the count, we'll not observe. > > > > Where exactly? > > In my scheme, nothing except the kernel ever loads the seqcount. The > user code generates a fresh value, writes it to memory, and then, just > before commit, writes that same value to the TLABI area and then > double-checks that the value it wrote at the beginning is still there. > > If the signal modifies the count, then the user code won't directly > notice, but prepare_exit_to_usermode on the way out of the signal will > notice that the (restored) TLABI state doesn't match the counter that > the signal handler changed and will just to the abort address. OK, you lost me.. commit looks like: + __asm__ __volatile__ goto ( + "movq $%l[failed], %%rcx\n" + "movq $1f, %[commit_instr]\n" + "cmpq %[start_value], %[current_value]\n" If we get preempted/signaled here without the preemption/signal entry checking for the post_commit_instr, we'll fail hard. + "jnz %l[failed]\n" + "movq %[to_write], (%[target])\n" + "1: movq $0, %[commit_instr]\n" + : /* no outputs */ + : [start_value]"d"(start_value.storage), + [current_value]"m"(__rseq_state), + [to_write]"r"(to_write), + [target]"r"(p), + [commit_instr]"m"(__rseq_state.post_commit_instr) + : "rcx", "memory" + : failed + );
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