Messages in this thread | | | Date | Fri, 30 Oct 2020 08:15:05 +0000 | From | Will Deacon <> | Subject | Re: [PATCH] arm64/smp: Move rcu_cpu_starting() earlier |
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On Thu, Oct 29, 2020 at 09:17:35AM -0400, Qian Cai wrote: > On Thu, 2020-10-29 at 09:10 +0000, Will Deacon wrote: > > On Wed, Oct 28, 2020 at 02:26:14PM -0400, Qian Cai wrote: > > > The call to rcu_cpu_starting() in secondary_start_kernel() is not early > > > enough in the CPU-hotplug onlining process, which results in lockdep > > > splats as follows: > > > > > > WARNING: suspicious RCU usage > > > ----------------------------- > > > kernel/locking/lockdep.c:3497 RCU-list traversed in non-reader section!! > > > > > > other info that might help us debug this: > > > > > > RCU used illegally from offline CPU! > > > rcu_scheduler_active = 1, debug_locks = 1 > > > no locks held by swapper/1/0. > > > > > > Call trace: > > > dump_backtrace+0x0/0x3c8 > > > show_stack+0x14/0x60 > > > dump_stack+0x14c/0x1c4 > > > lockdep_rcu_suspicious+0x134/0x14c > > > __lock_acquire+0x1c30/0x2600 > > > lock_acquire+0x274/0xc48 > > > _raw_spin_lock+0xc8/0x140 > > > vprintk_emit+0x90/0x3d0 > > > vprintk_default+0x34/0x40 > > > vprintk_func+0x378/0x590 > > > printk+0xa8/0xd4 > > > __cpuinfo_store_cpu+0x71c/0x868 > > > cpuinfo_store_cpu+0x2c/0xc8 > > > secondary_start_kernel+0x244/0x318 > > > > > > This is avoided by moving the call to rcu_cpu_starting up near the > > > beginning of the secondary_start_kernel() function. > > > > Hmm, it's not really a move though -- we'll end up calling this thing twice > > afaict. It would be better to make sure we've called notify_cpu_starting() > > early enough. Can we do that instead? > > Paul mentioned that it is fine to call rcu_cpu_starting() multiple times, and > Peter mentioned that CPU bringup is complicated. Thus, I thought about doing > something safe here. > > I tested a bit of patch below which seems fine, but I can't tell for sure if it > is safe. Any suggestion?
No, you're right -- this does look dodgy as I think we'll end up kicking the CPU notifiers before things like CPU errata have been figured out. So I'll pick up your original patch with Paul's ack. Thanks!
Will
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