On Fri, Oct 27, 2017 at 5:33 PM, Peter Zijlstra <pet...@infradead.org> wrote:
> On Fri, Oct 27, 2017 at 05:11:37PM +0200, Peter Zijlstra wrote:
>> On Fri, Oct 27, 2017 at 01:30:30AM -0700, syzbot wrote:
>>
>> > ======================================================
>> > WARNING: possible circular locking dependency detected
>> > 4.13.0-next-20170911+ #19 Not tainted
>> > ------------------------------------------------------
>> > syz-executor2/12380 is trying to acquire lock:
>> >  (&ctx->mutex){+.+.}, at: [<ffffffff8180923c>]
>> > perf_event_ctx_lock_nested+0x1dc/0x3c0 kernel/events/core.c:1210
>> >
>> > but task is already holding lock:
>> >  (&pipe->mutex/1){+.+.}, at: [<ffffffff81ac0fa6>] pipe_lock_nested
>> > fs/pipe.c:66 [inline]
>> >  (&pipe->mutex/1){+.+.}, at: [<ffffffff81ac0fa6>] pipe_lock+0x56/0x70
>> > fs/pipe.c:74
>> >
>> > which lock already depends on the new lock.
>>
>>
>> ARRGH!!
>>
>> that translates like the below, which is an absolute maze and requires
>> at least 5 concurrent callstacks, possibly more.
>>
>> We already had a lot of fun with hotplug-perf-ftrace, but the below
>> contains more. Let me try and page that previous crap back.
>>
>>
>>
>>       perf_ioctl()
>> #0      perf_event_ctx_lock()         [ctx->mutex]
>>         perf_event_set_filter
>> #1        ftrace_profile_set_filter   [event_mutex]
>>
>>
>>
>>
>>       sys_perf_event_open
>>         ...
>>         perf_trace_init
>> #1        mutex_lock [event_mutex]
>>           trace_event_reg
>>             tracepoint_probe_register
>> #2            mutex_lock() [tracepoints_mutex]
>>               tracepoint_add_func()
>> #3              static_key_slow_inc() [cpuhotplug_lock]
>>
>>
>>
>>
>>
>>       cpuhp_setup_state_nocalls
>> #3      cpus_read_lock        [cpuhotplug_lock]
>>           __cpuhp_setup_state_cpuslocked
>> #4          mutex_lock        [cpuhp_state_mutex]
>>             cpuhp_issue_call
>> #5            cpuhp_invoke_ap_callback()      [cpuhp_state]
>>
>>
>> #5    cpuhp_invoke_callback [cpuhp_state]
>>         ...
>>           devtmpfs_create_node
>> #6          wait_for_completion()     [&req.done]
>>
>>                                               devtmpfsd
>>                                                 handle_create
>> #7                                                filename_create 
>> [sb_writers]
>> #6                                              complete [&req.done]
>>
>>
>>
>>
>>
>>
>>       do_splice
>> #7      file_start_write() [sb_writers]
>>         do_splice_from
>>           iter_file_splice_write
>> #8          pipe_lock [pipe->mutex]
>>
>>
>>
>>
>>
>>       do_splice
>> #8      pipe_lock     [pipe->mutex]
>>         do_splice_to
>>           ...
>> #0          perf_read() [ctx->mutex]
>>
>
> So arguably that last op, splice_read from a perf fd is fairly
> pointless and we could dis-allow that. How about something like the
> below?
>
> ---
>  kernel/events/core.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 04989fb769f0..fd03f3082ee3 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -5468,6 +5468,13 @@ static int perf_fasync(int fd, struct file *filp, int 
> on)
>         return 0;
>  }
>
> +static ssize_t perf_splice_read(struct file *file, loff_t *ppos,
> +                               struct pipe_inode_info *pope, size_t len,
> +                               unsigned int flags)
> +{
> +       return -EOPNOTSUPP;
> +}
> +
>  static const struct file_operations perf_fops = {
>         .llseek                 = no_llseek,
>         .release                = perf_release,
> @@ -5477,6 +5484,7 @@ static int perf_fasync(int fd, struct file *filp, int 
> on)
>         .compat_ioctl           = perf_compat_ioctl,
>         .mmap                   = perf_mmap,
>         .fasync                 = perf_fasync,
> +       .splice_read            = perf_splice_read,
>  };


Another perf deadlock that involves splice:
https://groups.google.com/d/msg/syzkaller-bugs/vVy6Zj3wPxo/6oj5U6WiAwAJ

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