linux-kernel - Re: [PATCH v3 tip/core/rcu 40/40] rcu: Make non-preemptive schedule be Tasks RCU quiescent state

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Date:   Thu, 28 Sep 2017 18:18:50 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     "Levin, Alexander (Sasha Levin)" <alexander.levin@...izon.com>,
        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>,
        Steven Rostedt <rostedt@...dmis.org>,
        "dhowells@...hat.com" <dhowells@...hat.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Fr?d?ric 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 09:05:14AM -0700, Paul E. McKenney wrote:
> >  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...

printk() _should_ not fault AFAIK. If it does, you've done something
dodgy.