From mboxrd@z Thu Jan 1 00:00:00 1970 From: Eric Dumazet Subject: Re: [PATCH] x86: percpu_to_op() misses memory and flags clobbers Date: Wed, 01 Apr 2009 19:13:38 +0200 Message-ID: <49D3A0C2.9000403@cosmosbay.com> References: <49D32212.80607@cosmosbay.com> <49D32DC2.9010003@goop.org> <49D33E80.70802@cosmosbay.com> <20090401161218.GB3859@elte.hu> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: Jeremy Fitzhardinge , Tejun Heo , linux kernel , Linux Netdev List , Joe Perches , Rusty Russell To: Ingo Molnar Return-path: In-Reply-To: <20090401161218.GB3859@elte.hu> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org Ingo Molnar a =E9crit : > * Eric Dumazet wrote: >=20 >> Jeremy Fitzhardinge a =E9crit : >>> Eric Dumazet wrote: >>>> While playing with new percpu_{read|write|add|sub} stuff in networ= k tree, >>>> I found x86 asm was a litle bit optimistic. >>>> >>>> We need to tell gcc that percpu_{write|add|sub|or|xor} are modyfin= g >>>> memory and possibly eflags. We could add another parameter to >>>> percpu_to_op() >>>> to separate the plain "mov" case (not changing eflags), >>>> but let keep it simple for the moment. >>>> =20 >>> Did you observe an actual failure that this patch fixed? >>> >> Not in current tree, as we dont use yet percpu_xxxx() very much. >> >> If deployed for SNMP mibs with hundred of call sites, >> can you guarantee it will work as is ? >=20 > Do we "guarantee" it for you? No. >=20 > Is it expected to work just fine? Yes. >=20 > Are there any known bugs in this area? No. Good to know. So I shut up. I am a jerk and should blindly trust linux kernel, sorry. >=20 > Will we fix it if it's demonstrated to be broken? Of course! :-) >=20 > [ Btw., it's definitely cool that you will make heavy use for it for=20 > SNMP mib statistics - please share with us your experiences with=20 > the facilities - good or bad experiences alike! ] I tried but I miss kind of an indirect percpu_add() function. because of Net namespaces, mibs are dynamically allocated, and current percpu_add() works on static percpu only (because of added per_cpu__ prefix) #define percpu_add(var, val) percpu_to_op("add", per_cpu__##var, val) I tried adding : #define dyn_percpu_add(var, val) percpu_to_op("add", var, val) But I dont know it this is the plan ? Should we get rid of "per_cpu__" prefix and use a special ELF section/ marker instead ? I have a patch to add percpu_inc() and percpu_dec(), I am not sure its worth it... [PATCH] percpu: Adds percpu_inc() and percpu_dec() Increments and decrements are quite common operations for SNMP mibs. Signed-off-by: Eric Dumazet diff --git a/arch/x86/include/asm/percpu.h b/arch/x86/include/asm/percp= u.h index aee103b..248be11 100644 --- a/arch/x86/include/asm/percpu.h +++ b/arch/x86/include/asm/percpu.h @@ -103,6 +103,29 @@ do { \ } \ } while (0) =20 +#define percpu_to_op0(op, var) \ +do { \ + switch (sizeof(var)) { \ + case 1: \ + asm(op "b "__percpu_arg(0) \ + : "+m" (var)); \ + break; \ + case 2: \ + asm(op "w "__percpu_arg(0) \ + : "+m" (var)); \ + break; \ + case 4: \ + asm(op "l "__percpu_arg(0) \ + : "+m" (var)); \ + break; \ + case 8: \ + asm(op "q "__percpu_arg(0) \ + : "+m" (var)); \ + break; \ + default: __bad_percpu_size(); \ + } \ +} while (0) + #define percpu_from_op(op, var) \ ({ \ typeof(var) ret__; \ @@ -139,6 +162,8 @@ do { \ #define percpu_and(var, val) percpu_to_op("and", per_cpu__##var, val) #define percpu_or(var, val) percpu_to_op("or", per_cpu__##var, val) #define percpu_xor(var, val) percpu_to_op("xor", per_cpu__##var, val) +#define percpu_inc(var) percpu_to_op0("inc", per_cpu__##var) +#define percpu_dec(var) percpu_to_op0("dec", per_cpu__##var) =20 /* This is not atomic against other CPUs -- CPU preemption needs to be= off */ #define x86_test_and_clear_bit_percpu(bit, var) \ diff --git a/include/asm-generic/percpu.h b/include/asm-generic/percpu.= h index 00f45ff..c57357e 100644 --- a/include/asm-generic/percpu.h +++ b/include/asm-generic/percpu.h @@ -120,6 +120,14 @@ do { \ # define percpu_sub(var, val) __percpu_generic_to_op(var, (val), -=3D= ) #endif =20 +#ifndef percpu_inc +# define percpu_inc(var) do { percpu_add(var, 1); } while (0) +#endif + +#ifndef percpu_dec +# define percpu_dec(var) do { percpu_sub(var, 1); } while (0) +#endif + #ifndef percpu_and # define percpu_and(var, val) __percpu_generic_to_op(var, (val), &=3D= ) #endif