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Date: Thu, 28 Sep 2017 09:05:14 -0700 From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> To: "Levin, Alexander (Sasha Levin)" <alexander.levin@...izon.com> Cc: Sasha Levin <levinsasha928@...il.com>, "linux-kernel@...r.kernel.org List" <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...nel.org>, "jiangshanlai@...il.com" <jiangshanlai@...il.com>, "dipankar@...ibm.com" <dipankar@...ibm.com>, Andrew Morton <akpm@...ux-foundation.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Josh Triplett <josh@...htriplett.org>, Thomas Gleixner <tglx@...utronix.de>, Peter Zijlstra <peterz@...radead.org>, Steven Rostedt <rostedt@...dmis.org>, "dhowells@...hat.com" <dhowells@...hat.com>, Eric Dumazet <edumazet@...gle.com>, Frédéric Weisbecker <fweisbec@...il.com>, Oleg Nesterov <oleg@...hat.com>, "bobby.prani@...il.com" <bobby.prani@...il.com> Subject: Re: [PATCH v3 tip/core/rcu 40/40] rcu: Make non-preemptive schedule be Tasks RCU quiescent state On Thu, Sep 28, 2017 at 03:38:16PM +0000, Levin, Alexander (Sasha Levin) wrote: > On Thu, Sep 28, 2017 at 05:30:55AM -0700, Paul E. McKenney wrote: > >On Thu, Sep 28, 2017 at 02:37:20AM -0700, Sasha Levin wrote: > >> On Wed, Apr 19, 2017 at 9:58 AM, Paul E. McKenney > >> <paulmck@...ux.vnet.ibm.com> wrote: > >> > Currently, a call to schedule() acts as a Tasks RCU quiescent state > >> > only if a context switch actually takes place. However, just the > >> > call to schedule() guarantees that the calling task has moved off of > >> > whatever tracing trampoline that it might have been one previously. > >> > This commit therefore plumbs schedule()'s "preempt" parameter into > >> > rcu_note_context_switch(), which then records the Tasks RCU quiescent > >> > state, but only if this call to schedule() was -not- due to a preemption. > >> > > >> > To avoid adding overhead to the common-case context-switch path, > >> > this commit hides the rcu_note_context_switch() check under an existing > >> > non-common-case check. > >> > > >> > Suggested-by: Steven Rostedt <rostedt@...dmis.org> > >> > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com> > >> > >> Hey Paul, > >> > >> I'm seeing the following on the latest -next kernel, and suspect it's > >> caused bit this patch: > > > >Hmmm... kernel/rcu/tree_plugin.h:329 thinks that someone slept (as opposed > >to was preempted) in an RCU read-side critical section. > > > >If this is reproducible, could you please enable lockdep if you are not > >already doing so? > > lockdep was on, as far as I can tell. > > It happened once in ~3 weeks of fuzzing, so not sure how easily I can reproduce. Oh, wait... Looking at the stack trace: > Call Trace: > __dump_stack lib/dump_stack.c:16 [inline] > dump_stack+0x11d/0x1e5 lib/dump_stack.c:52 > panic+0x1bc/0x3b7 kernel/panic.c:181 > __warn+0x1c4/0x1d9 kernel/panic.c:542 > report_bug+0x211/0x2d0 lib/bug.c:183 > fixup_bug+0x3f/0x90 arch/x86/kernel/traps.c:178 > do_trap_no_signal arch/x86/kernel/traps.c:212 [inline] > do_trap+0x260/0x390 arch/x86/kernel/traps.c:261 > do_error_trap+0x133/0x380 arch/x86/kernel/traps.c:298 > do_invalid_op+0x1b/0x20 arch/x86/kernel/traps.c:311 > invalid_op+0x18/0x20 arch/x86/entry/entry_64.S:905 > RIP: 0010:rcu_preempt_note_context_switch kernel/rcu/tree_plugin.h:329 [inline] > RIP: 0010:rcu_note_context_switch+0x16c/0x2210 kernel/rcu/tree.c:458 > RSP: 0018:ffff88003b2debc8 EFLAGS: 00010002 > RAX: 0000000000000001 RBX: 1ffff1000765bd85 RCX: 0000000000000000 > RDX: 1ffff100075d7882 RSI: ffffffffb5c7da20 RDI: ffff88003aebc410 > RBP: ffff88003b2def30 R08: dffffc0000000000 R09: 0000000000000001 > R10: 0000000000000000 R11: 0000000000000000 R12: ffff88003b2def08 > R13: 0000000000000000 R14: ffff88003aebc040 R15: ffff88003aebc040 Up to here is all about the kernel complaining. > __schedule+0x201/0x2240 kernel/sched/core.c:3292 > schedule+0x113/0x460 kernel/sched/core.c:3421 > kvm_async_pf_task_wait+0x43f/0x940 arch/x86/kernel/kvm.c:158 It is kvm_async_pf_task_wait() that calls schedule(), but it carefully sets state to make that legal. Except... > do_async_page_fault+0x72/0x90 arch/x86/kernel/kvm.c:271 > async_page_fault+0x22/0x30 arch/x86/entry/entry_64.S:1069 > RIP: 0010:format_decode+0x240/0x830 lib/vsprintf.c:1996 > RSP: 0018:ffff88003b2df520 EFLAGS: 00010283 > RAX: 000000000000003f RBX: ffffffffb5d1e141 RCX: ffff88003b2df670 > RDX: 0000000000000001 RSI: dffffc0000000000 RDI: ffffffffb5d1e140 > RBP: ffff88003b2df560 R08: dffffc0000000000 R09: 0000000000000000 > R10: ffff88003b2df718 R11: 0000000000000000 R12: ffff88003b2df5d8 > R13: 0000000000000064 R14: ffffffffb5d1e140 R15: 0000000000000000 > vsnprintf+0x173/0x1700 lib/vsprintf.c:2136 We took a page fault in vsnprintf() while doing link_path_walk(), which looks to be within an RCU read-side critical section. Maybe the page fault confused lockdep? Sigh. It is going to be a real pain if all printk()s need to be outside of RCU read-side critical sections due to the possibility of page faults... Thanx, Paul > sprintf+0xbe/0xf0 lib/vsprintf.c:2386 > proc_self_get_link+0xfb/0x1c0 fs/proc/self.c:23 > get_link fs/namei.c:1047 [inline] > link_path_walk+0x1041/0x1490 fs/namei.c:2127 > path_openat+0x2d0/0x3600 fs/namei.c:3527 > do_filp_open+0x263/0x3c0 fs/namei.c:3562 > do_sys_open+0x515/0x6f0 fs/open.c:1059 > SYSC_openat fs/open.c:1086 [inline] > SyS_openat+0x30/0x40 fs/open.c:1080 > do_syscall_64+0x26a/0x800 arch/x86/entry/common.c:287 > entry_SYSCALL64_slow_path+0x25/0x25 > RIP: 0033:0x4889da > RSP: 002b:000000c4261eb8f0 EFLAGS: 00000206 ORIG_RAX: 0000000000000101 > RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00000000004889da > RDX: 0000000000000002 RSI: 000000c42338d2c0 RDI: ffffffffffffff9c > RBP: 000000c4261eb980 R08: 0000000000000000 R09: 0000000000000000 > R10: 0000000000000000 R11: 0000000000000206 R12: 0000000000000000 > R13: 00000000ffffffee R14: 00000000000012c0 R15: 0000000000000100 > Dumping ftrace buffer: > (ftrace buffer empty) > Kernel Offset: 0x30000000 from 0xffffffff81000000 (relocation range: > 0xffffffff80000000-0xffffffffbfffffff) > Rebooting in 86400 seconds.. So kvm_async_pf_task_wait() does a schedule(), and it carefully
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