On Mon, Oct 03, 2016 at 03:29:32PM +0200, Jiri Olsa wrote:
> On Fri, Sep 23, 2016 at 06:37:47PM +0200, Peter Zijlstra wrote:
> > On Wed, Sep 21, 2016 at 03:55:34PM +0200, Jiri Olsa wrote:
> > > The trinity syscall fuzzer triggered following WARN on powerpc:
> > > WARNING: CPU: 9 PID: 2998 at arch/powerpc/kernel/hw_breakpoint.c:278
> > > ...
> > > NIP [c00000000093aedc] .hw_breakpoint_handler+0x28c/0x2b0
> > > LR [c00000000093aed8] .hw_breakpoint_handler+0x288/0x2b0
> > > Call Trace:
> > > [c0000002f7933580] [c00000000093aed8]
> > > .hw_breakpoint_handler+0x288/0x2b0 (unreliable)
> > > [c0000002f7933630] [c0000000000f671c] .notifier_call_chain+0x7c/0xf0
> > > [c0000002f79336d0] [c0000000000f6abc]
> > > .__atomic_notifier_call_chain+0xbc/0x1c0
> > > [c0000002f7933780] [c0000000000f6c40] .notify_die+0x70/0xd0
> > > [c0000002f7933820] [c00000000001a74c] .do_break+0x4c/0x100
> > > [c0000002f7933920] [c0000000000089fc] handle_dabr_fault+0x14/0x48
> > >
> > > Followed by lockdep warning:
> > > ===============================
> > > [ INFO: suspicious RCU usage. ]
> > > 4.8.0-rc5+ #7 Tainted: G W
> > > -------------------------------
> > > ./include/linux/rcupdate.h:556 Illegal context switch in RCU read-side
> > > critical section!
> > >
> > > other info that might help us debug this:
> > >
> > > rcu_scheduler_active = 1, debug_locks = 0
> > > 2 locks held by ls/2998:
> > > #0: (rcu_read_lock){......}, at: [<c0000000000f6a00>]
> > > .__atomic_notifier_call_chain+0x0/0x1c0
> > > #1: (rcu_read_lock){......}, at: [<c00000000093ac50>]
> > > .hw_breakpoint_handler+0x0/0x2b0
> > >
> > > stack backtrace:
> > > CPU: 9 PID: 2998 Comm: ls Tainted: G W 4.8.0-rc5+ #7
> > > Call Trace:
> > > [c0000002f7933150] [c00000000094b1f8] .dump_stack+0xe0/0x14c
> > > (unreliable)
> > > [c0000002f79331e0] [c00000000013c468]
> > > .lockdep_rcu_suspicious+0x138/0x180
> > > [c0000002f7933270] [c0000000001005d8] .___might_sleep+0x278/0x2e0
> > > [c0000002f7933300] [c000000000935584] .mutex_lock_nested+0x64/0x5a0
> > > [c0000002f7933410] [c00000000023084c]
> > > .perf_event_ctx_lock_nested+0x16c/0x380
> > > [c0000002f7933500] [c000000000230a80] .perf_event_disable+0x20/0x60
> > > [c0000002f7933580] [c00000000093aeec] .hw_breakpoint_handler+0x29c/0x2b0
> > > [c0000002f7933630] [c0000000000f671c] .notifier_call_chain+0x7c/0xf0
> > > [c0000002f79336d0] [c0000000000f6abc]
> > > .__atomic_notifier_call_chain+0xbc/0x1c0
> > > [c0000002f7933780] [c0000000000f6c40] .notify_die+0x70/0xd0
> > > [c0000002f7933820] [c00000000001a74c] .do_break+0x4c/0x100
> > > [c0000002f7933920] [c0000000000089fc] handle_dabr_fault+0x14/0x48
> > >
> >
> > Well, that lockdep warning only says you should not be taking sleeping
> > locks while holding rcu_read_lock(), which is true. It does not say the
> > context you're doing this is cannot sleep.
> >
> > I'm not familiar enough with the PPC stuff to tell if the DIE_DABR_MATCH
> > trap context is atomic or not and this Changelog doesn't tell me.
> >
> > Anybody?
>
> ping
So I think all the DIE notifiers are atomic, which means this would
indeed be the thing to do. That said, I didn't see anything similar on
other BP implementations.
So it would be good to also explain why PPC needs this in the first
place.
