Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757375Ab0G2PPD (ORCPT ); Thu, 29 Jul 2010 11:15:03 -0400 Received: from mx1.redhat.com ([209.132.183.28]:24664 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753848Ab0G2PPA (ORCPT ); Thu, 29 Jul 2010 11:15:00 -0400 Date: Thu, 29 Jul 2010 17:12:29 +0200 From: Oleg Nesterov To: "Zhang, Yanmin" , Andrew Morton Cc: Roland McGrath , LKML , andi.kleen@intel.com, stable@kernel.org Subject: [PATCH] ptrace: optimize exit_ptrace() for the likely case Message-ID: <20100729151229.GA24754@redhat.com> References: <1279176663.2096.1264.camel@ymzhang.sh.intel.com> <20100721144944.5351c741.akpm@linux-foundation.org> <20100721222529.EFBAA400B6@magilla.sf.frob.com> <20100722090524.GA6647@redhat.com> <1279874705.2096.1274.camel@ymzhang.sh.intel.com> <20100723173453.GA29831@redhat.com> <1280120732.2085.3.camel@ymzhang.sh.intel.com> <20100726085324.GA32223@redhat.com> <1280193353.2085.15.camel@ymzhang.sh.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1280193353.2085.15.camel@ymzhang.sh.intel.com> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2936 Lines: 86 (replaces ptrace-dont-run-write_locktasklist_lock-if-the-parent-doesnt-ptrace-other-processes.patch) exit_ptrace() takes tasklist_lock unconditionally. We need this lock to avoid the race with ptrace_traceme(), it acts as a barrier. Change its caller, forget_original_parent(), to call exit_ptrace() under tasklist_lock. Change exit_ptrace() to drop and reacquire this lock if needed. This allows us to add the fastpath list_empty(ptraced) check. In the likely no-tracees case exit_ptrace() just returns and we avoid the lock() + unlock() sequence. "Zhang, Yanmin" suggested to add this check, and he reports that this change adds about 11% improvement in some tests. Suggested-and-tested-by: "Zhang, Yanmin" Signed-off-by: Oleg Nesterov --- kernel/ptrace.c | 12 +++++++++--- kernel/exit.c | 7 +++++-- 2 files changed, 14 insertions(+), 5 deletions(-) --- 35-rc3/kernel/ptrace.c~exit_ptrace_fastpath_check 2010-05-28 13:41:41.000000000 +0200 +++ 35-rc3/kernel/ptrace.c 2010-07-29 16:37:13.000000000 +0200 @@ -324,26 +324,32 @@ int ptrace_detach(struct task_struct *ch } /* - * Detach all tasks we were using ptrace on. + * Detach all tasks we were using ptrace on. Called with tasklist held + * for writing, and returns with it held too. But note it can release + * and reacquire the lock. */ void exit_ptrace(struct task_struct *tracer) { struct task_struct *p, *n; LIST_HEAD(ptrace_dead); - write_lock_irq(&tasklist_lock); + if (likely(list_empty(&tracer->ptraced))) + return; + list_for_each_entry_safe(p, n, &tracer->ptraced, ptrace_entry) { if (__ptrace_detach(tracer, p)) list_add(&p->ptrace_entry, &ptrace_dead); } - write_unlock_irq(&tasklist_lock); + write_unlock_irq(&tasklist_lock); BUG_ON(!list_empty(&tracer->ptraced)); list_for_each_entry_safe(p, n, &ptrace_dead, ptrace_entry) { list_del_init(&p->ptrace_entry); release_task(p); } + + write_lock_irq(&tasklist_lock); } int ptrace_readdata(struct task_struct *tsk, unsigned long src, char __user *dst, int len) --- 35-rc3/kernel/exit.c~exit_ptrace_fastpath_check 2010-05-28 13:41:41.000000000 +0200 +++ 35-rc3/kernel/exit.c 2010-07-29 16:38:37.000000000 +0200 @@ -771,9 +771,12 @@ static void forget_original_parent(struc struct task_struct *p, *n, *reaper; LIST_HEAD(dead_children); - exit_ptrace(father); - write_lock_irq(&tasklist_lock); + /* + * Note that exit_ptrace() and find_new_reaper() might + * drop tasklist_lock and reacquire it. + */ + exit_ptrace(father); reaper = find_new_reaper(father); list_for_each_entry_safe(p, n, &father->children, sibling) { -- 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/