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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Mon, 22 Jul 2019 13:07:35 +0100 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp22035.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x6MC7Y7F52560152 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Mon, 22 Jul 2019 12:07:34 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id A64ACB205F; Mon, 22 Jul 2019 12:07:34 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 7AEF8B2065; Mon, 22 Jul 2019 12:07:34 +0000 (GMT) Received: from paulmck-ThinkPad-W541 (unknown [9.85.189.166]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Mon, 22 Jul 2019 12:07:34 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id C0C1816C0D7D; Mon, 22 Jul 2019 05:07:35 -0700 (PDT) Date: Mon, 22 Jul 2019 05:07:35 -0700 From: "Paul E. McKenney" To: Masami Hiramatsu Cc: Catalin Marinas , Will Deacon , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Naresh Kamboju , Dan Rue , Matt Hart , Anders Roxell , Daniel Diaz Subject: Re: [PATCH v2 3/4] arm64: Make debug exception handlers visible from RCU Reply-To: paulmck@linux.ibm.com References: <156378170297.12011.17385386326930403235.stgit@devnote2> <156378173770.12011.3832608237079432765.stgit@devnote2> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <156378173770.12011.3832608237079432765.stgit@devnote2> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 19072212-0052-0000-0000-000003E3FA13 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00011474; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000287; SDB=6.01235804; UDB=6.00651291; IPR=6.01017153; MB=3.00027836; MTD=3.00000008; XFM=3.00000015; UTC=2019-07-22 12:07:38 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19072212-0053-0000-0000-000061CB59F5 Message-Id: <20190722120735.GA14271@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-07-22_09:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1907220144 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jul 22, 2019 at 04:48:58PM +0900, Masami Hiramatsu wrote: > Make debug exceptions visible from RCU so that synchronize_rcu() > correctly track the debug exception handler. > > This also introduces sanity checks for user-mode exceptions as same > as x86's ist_enter()/ist_exit(). > > The debug exception can interrupt in idle task. For example, it warns > if we put a kprobe on a function called from idle task as below. > The warning message showed that the rcu_read_lock() caused this > problem. But actually, this means the RCU is lost the context which > is already in NMI/IRQ. > > /sys/kernel/debug/tracing # echo p default_idle_call >> kprobe_events > /sys/kernel/debug/tracing # echo 1 > events/kprobes/enable > /sys/kernel/debug/tracing # [ 135.122237] > [ 135.125035] ============================= > [ 135.125310] WARNING: suspicious RCU usage > [ 135.125581] 5.2.0-08445-g9187c508bdc7 #20 Not tainted > [ 135.125904] ----------------------------- > [ 135.126205] include/linux/rcupdate.h:594 rcu_read_lock() used illegally while idle! > [ 135.126839] > [ 135.126839] other info that might help us debug this: > [ 135.126839] > [ 135.127410] > [ 135.127410] RCU used illegally from idle CPU! > [ 135.127410] rcu_scheduler_active = 2, debug_locks = 1 > [ 135.128114] RCU used illegally from extended quiescent state! > [ 135.128555] 1 lock held by swapper/0/0: > [ 135.128944] #0: (____ptrval____) (rcu_read_lock){....}, at: call_break_hook+0x0/0x178 > [ 135.130499] > [ 135.130499] stack backtrace: > [ 135.131192] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.2.0-08445-g9187c508bdc7 #20 > [ 135.131841] Hardware name: linux,dummy-virt (DT) > [ 135.132224] Call trace: > [ 135.132491] dump_backtrace+0x0/0x140 > [ 135.132806] show_stack+0x24/0x30 > [ 135.133133] dump_stack+0xc4/0x10c > [ 135.133726] lockdep_rcu_suspicious+0xf8/0x108 > [ 135.134171] call_break_hook+0x170/0x178 > [ 135.134486] brk_handler+0x28/0x68 > [ 135.134792] do_debug_exception+0x90/0x150 > [ 135.135051] el1_dbg+0x18/0x8c > [ 135.135260] default_idle_call+0x0/0x44 > [ 135.135516] cpu_startup_entry+0x2c/0x30 > [ 135.135815] rest_init+0x1b0/0x280 > [ 135.136044] arch_call_rest_init+0x14/0x1c > [ 135.136305] start_kernel+0x4d4/0x500 > [ 135.136597] > > So make debug exception visible to RCU can fix this warning. > > Reported-by: Naresh Kamboju > Cc: Paul E. McKenney >From an RCU viewpoint: Acked-by: Paul E. McKenney > Signed-off-by: Masami Hiramatsu > --- > arch/arm64/mm/fault.c | 40 ++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 40 insertions(+) > > diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c > index 9568c116ac7f..a6b244240db6 100644 > --- a/arch/arm64/mm/fault.c > +++ b/arch/arm64/mm/fault.c > @@ -777,6 +777,42 @@ void __init hook_debug_fault_code(int nr, > debug_fault_info[nr].name = name; > } > > +/* > + * In debug exception context, we explicitly disable preemption. > + * This serves two purposes: it makes it much less likely that we would > + * accidentally schedule in exception context and it will force a warning > + * if we somehow manage to schedule by accident. > + */ > +static void debug_exception_enter(struct pt_regs *regs) > +{ > + if (user_mode(regs)) { > + RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU"); > + } else { > + /* > + * We might have interrupted pretty much anything. In > + * fact, if we're a debug exception, we can even interrupt > + * NMI processing. We don't want in_nmi() to return true, > + * but we need to notify RCU. > + */ > + rcu_nmi_enter(); > + } > + > + preempt_disable(); > + > + /* This code is a bit fragile. Test it. */ > + RCU_LOCKDEP_WARN(!rcu_is_watching(), "exception_enter didn't work"); > +} > +NOKPROBE_SYMBOL(debug_exception_enter); > + > +static void debug_exception_exit(struct pt_regs *regs) > +{ > + preempt_enable_no_resched(); > + > + if (!user_mode(regs)) > + rcu_nmi_exit(); > +} > +NOKPROBE_SYMBOL(debug_exception_exit); > + > #ifdef CONFIG_ARM64_ERRATUM_1463225 > DECLARE_PER_CPU(int, __in_cortex_a76_erratum_1463225_wa); > > @@ -824,6 +860,8 @@ asmlinkage void __exception do_debug_exception(unsigned long addr_if_watchpoint, > if (interrupts_enabled(regs)) > trace_hardirqs_off(); > > + debug_exception_enter(regs); > + > if (user_mode(regs) && !is_ttbr0_addr(pc)) > arm64_apply_bp_hardening(); > > @@ -832,6 +870,8 @@ asmlinkage void __exception do_debug_exception(unsigned long addr_if_watchpoint, > inf->sig, inf->code, (void __user *)pc, esr); > } > > + debug_exception_exit(regs); > + > if (interrupts_enabled(regs)) > trace_hardirqs_on(); > } >