From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:53503) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1bOxSG-0002fM-Ry for qemu-devel@nongnu.org; Sun, 17 Jul 2016 21:34:46 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1bOxSB-0007Od-U8 for qemu-devel@nongnu.org; Sun, 17 Jul 2016 21:34:43 -0400 Received: from ozlabs.org ([103.22.144.67]:33470) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1bOxSB-0007OW-5O for qemu-devel@nongnu.org; Sun, 17 Jul 2016 21:34:39 -0400 Date: Mon, 18 Jul 2016 11:17:25 +1000 From: David Gibson Message-ID: <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> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="BZaMRJmqxGScZ8Mx" Content-Disposition: inline In-Reply-To: <20160716001156.227ab4c9@bahia.lan> 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: Greg Kurz Cc: Bharata B Rao , Peter Maydell , Riku Voipio , QEMU Developers , Igor Mammedov --BZaMRJmqxGScZ8Mx Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable 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: >=20 > > 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: =20 > > > > On 14 July 2016 at 08:57, David Gibson wrote: =20 > > > >> With CONFIG_USER_ONLY, generation of cpu_index values is done diff= erently > > > >> 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 wil= l 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 f= ixes it > > > >> by making CONFIG_USER_ONLY use the same bitmap based allocation th= at 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 =3D 0; > > > >> - > > > >> - CPU_FOREACH(some_cpu) { > > > >> - cpu_index++; > > > >> - } > > > >> - return cpu_index; > > > >> -} > > > >> - > > > >> -static void cpu_release_index(CPUState *cpu) > > > >> -{ > > > >> - return; > > > >> -} > > > >> -#endif =20 > > > > > > > > Won't this change impose a maximum limit of 256 simultaneous > > > > threads? That seems a little low for comfort. =20 > > >=20 > > > This was the reason why the bitmap logic wasn't applied to > > > CONFIG_USER_ONLY when it was introduced. > > >=20 > > > https://lists.gnu.org/archive/html/qemu-devel/2015-05/msg01980.html = =20 > >=20 > > Ah.. good point. > >=20 > > 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? > >=20 >=20 > 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). > > > But then we didn't have actual removal, but we do now. =20 > >=20 > > You mean patch 1/2 in this set? Or something else? > >=20 > > 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. > >=20 >=20 --=20 David Gibson | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_ | _way_ _around_! http://www.ozlabs.org/~dgibson --BZaMRJmqxGScZ8Mx Content-Type: application/pgp-signature; name="signature.asc" -----BEGIN PGP SIGNATURE----- Version: GnuPG v1 iQIcBAEBAgAGBQJXjC4lAAoJEGw4ysog2bOSh0kP/2NnZKkyf7+kIsM9ZuLKukjI 3peZqLGaZsBuLGKolHbLvU8iLfUr2FGTz1fl3WGkX8M8IKo+DiF2WFXxy+JZ+xcG BdH6gjkPRR0HcCYRHDxohgP4zE24yOZKD3W04YOlCJDdJQV0coHWng456TAAY/5E P6X6d2NkGFsDUtCbLOUA5JJ88JzftSX9HPF+qzZiAsnqnh5SMS4zY7o9aVVjrueO 1/Pi/fESKg7yhpTVzEIYU5UtfupMI79GfmnTmHrcNhLF0jYVAIYWgc/AoMpWV16O 1twclL2jl694hOxIUS4AGwSH2WJne66MQeJirIdpbZIsVSBX8EUMBu62mMvT+w2W 7ml3pV0AfWGP/wvehk5Px7uLCx3NeFoL1L4FB0HrRHgmClaeCaoO9yXyC2nECJeL q5HZxn0MYWphBFChEOPB1IUVM3B0Jke2E3gOwz9EEB2ZYD4j37pOGRC7iYIXlh9L 8kNknmmmnUPWUSZ65M8kfWWqokf+dmhVn9iTyFRduDE0DgCGzegVSZlnXPDJ6e9T KeEXRJWUooQ5ifFa5UtXYgl32NbMn9dXMGh+fCLqEXOjNdRgPY+ZzHENBOIVYzg9 GefI077XQVS3M7R9PbVInD9GQiL1QmV1WAQe7NE0eGeP2ElAFtZ+X+xkJRwF8AB+ T4I4qczxBmgMYj26dAoZ =gjLZ -----END PGP SIGNATURE----- --BZaMRJmqxGScZ8Mx--