On Fri, 2020-10-09 at 18:23 +0200, Peter Zijlstra wrote: > On Fri, Oct 09, 2020 at 06:58:37AM -0700, Paul E. McKenney wrote: > > On Fri, Oct 09, 2020 at 09:41:24AM -0400, Qian Cai wrote: > > > On Fri, 2020-10-09 at 07:58 +0000, tip-bot2 for Peter Zijlstra wrote: > > > > The following commit has been merged into the locking/core branch of > > > > tip: > > > > > > > > Commit-ID: 4d004099a668c41522242aa146a38cc4eb59cb1e > > > > Gitweb: > > > > https://git.kernel.org/tip/4d004099a668c41522242aa146a38cc4eb59cb1e > > > > Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx> > > > > AuthorDate: Fri, 02 Oct 2020 11:04:21 +02:00 > > > > Committer: Ingo Molnar <mingo@xxxxxxxxxx> > > > > CommitterDate: Fri, 09 Oct 2020 08:53:30 +02:00 > > > > > > > > lockdep: Fix lockdep recursion > > > > > > > > Steve reported that lockdep_assert*irq*(), when nested inside lockdep > > > > itself, will trigger a false-positive. > > > > > > > > One example is the stack-trace code, as called from inside lockdep, > > > > triggering tracing, which in turn calls RCU, which then uses > > > > lockdep_assert_irqs_disabled(). > > > > > > > > Fixes: a21ee6055c30 ("lockdep: Change hardirq{s_enabled,_context} to > > > > per-cpu > > > > variables") > > > > Reported-by: Steven Rostedt <rostedt@xxxxxxxxxxx> > > > > Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> > > > > Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx> > > > > > > Reverting this linux-next commit fixed booting RCU-list warnings > > > everywhere. > > > > Is it possible that the RCU-list warnings were being wrongly suppressed > > without a21ee6055c30? As in are you certain that these RCU-list warnings > > are in fact false positives? > > > [ 4.002695][ T0] init_timer_key+0x29/0x220 > > > [ 4.002695][ T0] identify_cpu+0xfcb/0x1980 > > > [ 4.002695][ T0] identify_secondary_cpu+0x1d/0x190 > > > [ 4.002695][ T0] smp_store_cpu_info+0x167/0x1f0 > > > [ 4.002695][ T0] start_secondary+0x5b/0x290 > > > [ 4.002695][ T0] secondary_startup_64_no_verify+0xb8/0xbb > > They're actually correct warnings, this is trying to use RCU before that > CPU is reported to RCU. > > Possibly something like the below works, but I've not tested it, nor > have I really thought hard about it, bring up tricky and this is just > moving code. I don't think this will always work. Basically, anything like printk() would trigger the warning because it tries to acquire a lock. For example, on arm64: [ 0.418627] lockdep_rcu_suspicious+0x134/0x14c [ 0.418629] __lock_acquire+0x1c30/0x2600 [ 0.418631] lock_acquire+0x274/0xc48 [ 0.418632] _raw_spin_lock+0xc8/0x140 [ 0.418634] vprintk_emit+0x90/0x3d0 [ 0.418636] vprintk_default+0x34/0x40 [ 0.418638] vprintk_func+0x378/0x590 [ 0.418640] printk+0xa8/0xd4 [ 0.418642] __cpuinfo_store_cpu+0x71c/0x868 [ 0.418644] cpuinfo_store_cpu+0x2c/0xc8 [ 0.418645] secondary_start_kernel+0x244/0x318 Back to x86, we have: start_secondary() smp_callin() apic_ap_setup() setup_local_APIC() printk() in certain conditions. which is before smp_store_cpu_info(). Can't we add a rcu_cpu_starting() at the very top for each start_secondary(), secondary_start_kernel(), smp_start_secondary() etc, so we don't worry about any printk() later? > > --- > > diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c > index 35ad8480c464..9173d64ee69d 100644 > --- a/arch/x86/kernel/cpu/common.c > +++ b/arch/x86/kernel/cpu/common.c > @@ -1670,6 +1670,9 @@ void __init identify_boot_cpu(void) > void identify_secondary_cpu(struct cpuinfo_x86 *c) > { > BUG_ON(c == &boot_cpu_data); > + > + rcu_cpu_starting(smp_processor_id()); > + > identify_cpu(c); > #ifdef CONFIG_X86_32 > enable_sep_cpu(); > diff --git a/arch/x86/kernel/cpu/mtrr/mtrr.c b/arch/x86/kernel/cpu/mtrr/mtrr.c > index 6a80f36b5d59..5f436cb4f7c4 100644 > --- a/arch/x86/kernel/cpu/mtrr/mtrr.c > +++ b/arch/x86/kernel/cpu/mtrr/mtrr.c > @@ -794,8 +794,6 @@ void mtrr_ap_init(void) > if (!use_intel() || mtrr_aps_delayed_init) > return; > > - rcu_cpu_starting(smp_processor_id()); > - > /* > * Ideally we should hold mtrr_mutex here to avoid mtrr entries > * changed, but this routine will be called in cpu boot time, >
