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Date:   Fri, 6 Apr 2018 18:50:27 +0100
From:   Mark Rutland <mark.rutland@arm.com>
To:     Yury Norov <ynorov@caviumnetworks.com>
Cc:     James Morse <james.morse@arm.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Will Deacon <will.deacon@arm.com>,
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        Christopher Lameter <cl@linux.com>,
        Russell King - ARM Linux <linux@armlinux.org.uk>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
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        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
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Subject: Re: [PATCH 1/5] arm64: entry: isb in el1_irq
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On Fri, Apr 06, 2018 at 06:22:11PM +0100, Mark Rutland wrote:
> Digging a bit, I also thing that our ct_user_exit and ct_user_enter
> usage is on dodgy ground today.
> 
> For example, in el0_dbg we call do_debug_exception() *before* calling
> ct_user_exit. Which I believe means we'd use RCU while supposedly in an
> extended quiescent period, which would be bad.

It seems this is the case. I can trigger the following by having GDB
place a SW breakpoint:

[   51.217947] =============================
[   51.217953] WARNING: suspicious RCU usage
[   51.217961] 4.16.0 #4 Not tainted
[   51.217966] -----------------------------
[   51.217974] ./include/linux/rcupdate.h:632 rcu_read_lock() used illegally while idle!
[   51.217980]
[   51.217980] other info that might help us debug this:
[   51.217980]
[   51.217987]
[   51.217987] RCU used illegally from idle CPU!
[   51.217987] rcu_scheduler_active = 2, debug_locks = 1
[   51.217992] RCU used illegally from extended quiescent state!
[   51.217999] 1 lock held by ls/2412:
[   51.218004]  #0:  (rcu_read_lock){....}, at: [<0000000092efbdd5>] brk_handler+0x0/0x198
[   51.218041]
[   51.218041] stack backtrace:
[   51.218049] CPU: 2 PID: 2412 Comm: ls Not tainted 4.16.0 #4
[   51.218055] Hardware name: ARM Juno development board (r1) (DT)
[   51.218061] Call trace:
[   51.218070]  dump_backtrace+0x0/0x1c8
[   51.218078]  show_stack+0x14/0x20
[   51.218087]  dump_stack+0xac/0xe4
[   51.218096]  lockdep_rcu_suspicious+0xcc/0x110
[   51.218103]  brk_handler+0x144/0x198
[   51.218110]  do_debug_exception+0x9c/0x190
[   51.218116]  el0_dbg+0x14/0x20

We will need to fix this before we can fiddle with kick_all_cpus_sync().

Thanks,
Mark.