From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:43847) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1bP2wI-0003YZ-K0 for qemu-devel@nongnu.org; Mon, 18 Jul 2016 03:26:08 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1bP2wE-0000xM-Qn for qemu-devel@nongnu.org; Mon, 18 Jul 2016 03:26:06 -0400 Received: from mx1.redhat.com ([209.132.183.28]:58094) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1bP2wE-0000xE-Iv for qemu-devel@nongnu.org; Mon, 18 Jul 2016 03:26:02 -0400 Date: Mon, 18 Jul 2016 09:25:58 +0200 From: Igor Mammedov Message-ID: <20160718092558.09f7df04@nial.brq.redhat.com> In-Reply-To: <20160718011725.GE16769@voom.fritz.box> References: <1468483025-1084-1-git-send-email-david@gibson.dropbear.id.au> <1468483025-1084-3-git-send-email-david@gibson.dropbear.id.au> <20160714115945.GQ14615@voom.fritz.box> <20160716001156.227ab4c9@bahia.lan> <20160718011725.GE16769@voom.fritz.box> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [RFC 2/2] linux-user: Fix cpu_index generation List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: David Gibson Cc: Greg Kurz , Bharata B Rao , Peter Maydell , Riku Voipio , QEMU Developers On Mon, 18 Jul 2016 11:17:25 +1000 David Gibson wrote: > On Sat, Jul 16, 2016 at 12:11:56AM +0200, Greg Kurz wrote: > > On Thu, 14 Jul 2016 21:59:45 +1000 > > David Gibson wrote: > > > > > On Thu, Jul 14, 2016 at 03:50:56PM +0530, Bharata B Rao wrote: > > > > On Thu, Jul 14, 2016 at 3:24 PM, Peter Maydell wrote: > > > > > On 14 July 2016 at 08:57, David Gibson wrote: > > > > >> With CONFIG_USER_ONLY, generation of cpu_index values is done differently > > > > >> than for full system targets. This method turns out to be broken, since > > > > >> it can fairly easily result in duplicate cpu_index values for > > > > >> simultaneously active cpus (i.e. threads in the emulated process). > > > > >> > > > > >> Consider this sequence: > > > > >> Create thread 1 > > > > >> Create thread 2 > > > > >> Exit thread 1 > > > > >> Create thread 3 > > > > >> > > > > >> With the current logic thread 1 will get cpu_index 1, thread 2 will get > > > > >> cpu_index 2 and thread 3 will also get cpu_index 2 (because there are 2 > > > > >> threads in the cpus list at the point of its creation). > > > > >> > > > > >> We mostly get away with this because cpu_index values aren't that important > > > > >> for userspace emulation. Still, it can't be good, so this patch fixes it > > > > >> by making CONFIG_USER_ONLY use the same bitmap based allocation that full > > > > >> system targets already use. > > > > >> > > > > >> Signed-off-by: David Gibson > > > > >> --- > > > > >> exec.c | 19 ------------------- > > > > >> 1 file changed, 19 deletions(-) > > > > >> > > > > >> diff --git a/exec.c b/exec.c > > > > >> index 011babd..e410dab 100644 > > > > >> --- a/exec.c > > > > >> +++ b/exec.c > > > > >> @@ -596,7 +596,6 @@ AddressSpace *cpu_get_address_space(CPUState *cpu, int asidx) > > > > >> } > > > > >> #endif > > > > >> > > > > >> -#ifndef CONFIG_USER_ONLY > > > > >> static DECLARE_BITMAP(cpu_index_map, MAX_CPUMASK_BITS); > > > > >> > > > > >> static int cpu_get_free_index(Error **errp) > > > > >> @@ -617,24 +616,6 @@ static void cpu_release_index(CPUState *cpu) > > > > >> { > > > > >> bitmap_clear(cpu_index_map, cpu->cpu_index, 1); > > > > >> } > > > > >> -#else > > > > >> - > > > > >> -static int cpu_get_free_index(Error **errp) > > > > >> -{ > > > > >> - CPUState *some_cpu; > > > > >> - int cpu_index = 0; > > > > >> - > > > > >> - CPU_FOREACH(some_cpu) { > > > > >> - cpu_index++; > > > > >> - } > > > > >> - return cpu_index; > > > > >> -} > > > > >> - > > > > >> -static void cpu_release_index(CPUState *cpu) > > > > >> -{ > > > > >> - return; > > > > >> -} > > > > >> -#endif > > > > > > > > > > Won't this change impose a maximum limit of 256 simultaneous > > > > > threads? That seems a little low for comfort. > > > > > > > > This was the reason why the bitmap logic wasn't applied to > > > > CONFIG_USER_ONLY when it was introduced. > > > > > > > > https://lists.gnu.org/archive/html/qemu-devel/2015-05/msg01980.html > > > > > > Ah.. good point. > > > > > > Hrm, ok, my next idea would be to just (globally) sequentially > > > allocate cpu_index values for CONFIG_USER, and never try to re-use > > > them. Does that seem reasonable? > > > > > > > Isn't it only deferring the problem to later ? > > You mean that we could get duplicate indexes after the value wraps > around? > > I suppose, but duplicates after spawning 4 billion threads seems like > a substantial improvement over duplicates after spawning 3 in the > wrong order.. > > > Maybe it is possible to define MAX_CPUMASK_BITS to a much higher > > value fo CONFIG_USER only instead ? > > Perhaps. It does mean carrying around a huge bitmap, though. > > Another option is to remove cpu_index entirely for the user only > case. I have some patches for this, which are very ugly but it's > possible they can be cleaned up to something reasonable (the biggest > chunk is moving a bunch of ARM stuff under #ifndef CONFIG_USER_ONLY > for what I think are registers that aren't accessible in user mode). could we remove cpu_index altogether for bot *-user and *-softmmu targets? > > > > > > But then we didn't have actual removal, but we do now. > > > > > > You mean patch 1/2 in this set? Or something else? > > > > > > Even so, 256 does seem a bit low for a number of simultaneously active > > > threads - there are some bug hairy multi-threaded programs out there. > > > > > > > >