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

[Paul E. McKenney]

On Thu, Sep 28, 2017 at 06:18:50PM +0200, Peter Zijlstra wrote:
> 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.

And we really do have a page fault there.  Perhaps the fault of the caller
of printk(), printing some memory that is, as you say, dodgy?

								Thanx, Paul