Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754353AbbEKOra (ORCPT ); Mon, 11 May 2015 10:47:30 -0400 Received: from relay6-d.mail.gandi.net ([217.70.183.198]:47731 "EHLO relay6-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753443AbbEKOr0 (ORCPT ); Mon, 11 May 2015 10:47:26 -0400 X-Originating-IP: 50.43.43.179 Date: Mon, 11 May 2015 07:47:17 -0700 From: Josh Triplett To: Vineet Gupta Cc: Andy Lutomirski , Ingo Molnar , "H. Peter Anvin" , Peter Zijlstra , Thomas Gleixner , Linus Torvalds , "linux-api@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "x86@kernel.org" Subject: Re: [PATCH 1/2] clone: Support passing tls argument via C rather than pt_regs magic Message-ID: <20150511144717.GC6130@x> References: <20150421174711.GA5127@jtriplet-mobl1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5065 Lines: 107 On Mon, May 11, 2015 at 02:31:39PM +0000, Vineet Gupta wrote: > On Tuesday 21 April 2015 11:17 PM, Josh Triplett wrote: > > clone with CLONE_SETTLS accepts an argument to set the thread-local > > storage area for the new thread. sys_clone declares an int argument > > tls_val in the appropriate point in the argument list (based on the > > various CLONE_BACKWARDS variants), but doesn't actually use or pass > > along that argument. Instead, sys_clone calls do_fork, which calls > > copy_process, which calls the arch-specific copy_thread, and copy_thread > > pulls the corresponding syscall argument out of the pt_regs captured at > > kernel entry (knowing what argument of clone that architecture passes > > tls in). > > > > Apart from being awful and inscrutable, that also only works because > > only one code path into copy_thread can pass the CLONE_SETTLS flag, and > > that code path comes from sys_clone with its architecture-specific > > argument-passing order. This prevents introducing a new version of the > > clone system call without propagating the same architecture-specific > > position of the tls argument. > > > > However, there's no reason to pull the argument out of pt_regs when > > sys_clone could just pass it down via C function call arguments. > > > > Introduce a new CONFIG_HAVE_COPY_THREAD_TLS for architectures to opt > > into, and a new copy_thread_tls that accepts the tls parameter as an > > additional unsigned long (syscall-argument-sized) argument. > > Change sys_clone's tls argument to an unsigned long (which does > > not change the ABI), and pass that down to copy_thread_tls. > > > > Architectures that don't opt into copy_thread_tls will continue to > > ignore the C argument to sys_clone in favor of the pt_regs captured at > > kernel entry, and thus will be unable to introduce new versions of the > > clone syscall. > > > > Signed-off-by: Josh Triplett > > Signed-off-by: Thiago Macieira > > Acked-by: Andy Lutomirski > > --- > > arch/Kconfig | 7 ++++++ > > include/linux/sched.h | 14 ++++++++++++ > > include/linux/syscalls.h | 6 +++--- > > kernel/fork.c | 55 +++++++++++++++++++++++++++++++----------------- > > 4 files changed, 60 insertions(+), 22 deletions(-) > > > > diff --git a/arch/Kconfig b/arch/Kconfig > > index 05d7a8a..4834a58 100644 > > --- a/arch/Kconfig > > +++ b/arch/Kconfig > > @@ -484,6 +484,13 @@ config HAVE_IRQ_EXIT_ON_IRQ_STACK > > This spares a stack switch and improves cache usage on softirq > > processing. > > > > +config HAVE_COPY_THREAD_TLS > > + bool > > + help > > + Architecture provides copy_thread_tls to accept tls argument via > > + normal C parameter passing, rather than extracting the syscall > > + argument from pt_regs. > > + > > # > > # ABI hall of shame > > # > > diff --git a/include/linux/sched.h b/include/linux/sched.h > > index a419b65..2cc88c6 100644 > > --- a/include/linux/sched.h > > +++ b/include/linux/sched.h > > @@ -2480,8 +2480,22 @@ extern struct mm_struct *mm_access(struct task_struct *task, unsigned int mode); > > /* Remove the current tasks stale references to the old mm_struct */ > > extern void mm_release(struct task_struct *, struct mm_struct *); > > > > +#ifdef CONFIG_HAVE_COPY_THREAD_TLS > > +extern int copy_thread_tls(unsigned long, unsigned long, unsigned long, > > + struct task_struct *, unsigned long); > > +#else > > extern int copy_thread(unsigned long, unsigned long, unsigned long, > > struct task_struct *); > > + > > +/* Architectures that haven't opted into copy_thread_tls get the tls argument > > + * via pt_regs, so ignore the tls argument passed via C. */ > > +static inline int copy_thread_tls( > > + unsigned long clone_flags, unsigned long sp, unsigned long arg, > > + struct task_struct *p, unsigned long tls) > > +{ > > + return copy_thread(clone_flags, sp, arg, p); > > +} > > +#endif > > Is this detour really needed. Can we not update copy_thread() of all arches in one > go and add the tls arg, w/o using it. > > And then arch maintainers can micro-optimize their code to use that arg vs. > pt_regs->rxx version at their own leisure. The only downside I see with that is > bigger churn (touches all arches), and a interim unused arg warning ? In addition to the cleanup and simplification, the purpose of this patch is specifically to make sure that any architecture opting into HAVE_COPY_THREAD_TLS does *not* care how tls is passed in, and in particular doesn't depend on it arriving in a specific syscall argument. I have patches in flight (for CLONE_FD and clone4) that depend on that assumption, by introducing additional syscalls (with tls passed differently) that call down through these same code paths. - Josh Triplett -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/