Re: [PATCH] perf powerpc: Don't call perf_event_disable from atomic context

[Peter Zijlstra]

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.