linux-kernel - Re: [PATCH] perf powerpc: Don't call perf_event

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Message-ID: <20160923163747.GE5012@twins.programming.kicks-ass.net>
Date:   Fri, 23 Sep 2016 18:37:47 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jiri Olsa <jolsa@...nel.org>
Cc:     lkml <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Michael Neuling <mikey@...ling.org>,
        Paul Mackerras <paulus@...ba.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jan Stancek <jstancek@...hat.com>
Subject: Re: [PATCH] perf powerpc: Don't call perf_event_disable from atomic
 context

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?