Hi Kees,

Here is a recursive lock possibility:

        ptrace_may_access()
=>        task_lock(task);
            yama_ptrace_access_check()
              get_task_comm()
=>              task_lock(task);

[   60.230444] =============================================
[   60.232078] [ INFO: possible recursive locking detected ]
[   60.232078] 3.5.0+ #281 Not tainted
[   60.232078] ---------------------------------------------
[   60.232078] trinity-child0/17019 is trying to acquire lock:
[   60.232078]  (&(&p->alloc_lock)->rlock){+.+...}, at: [<c1176ffa>] get_task_comm+0x4a/0xf0
[   60.232078] 
[   60.232078] but task is already holding lock:
[   60.232078]  (&(&p->alloc_lock)->rlock){+.+...}, at: [<c10653fa>] ptrace_may_access+0x4a/0xf0
[   60.232078] 
[   60.232078] other info that might help us debug this:
[   60.232078]  Possible unsafe locking scenario:
[   60.232078] 
[   60.232078]        CPU0
[   60.232078]        ----
[   60.232078]   lock(&(&p->alloc_lock)->rlock);
[   60.232078]   lock(&(&p->alloc_lock)->rlock);
[   60.232078]
[   60.232078]  *** DEADLOCK ***
[   60.232078]
[   60.232078]  May be due to missing lock nesting notation
[   60.232078]
[   60.232078] 3 locks held by trinity-child0/17019:
[   60.232078]  #0:  (&p->lock){+.+.+.}, at: [<c11a9683>] seq_read+0x33/0x6b0
[   60.232078]  #1:  (&sig->cred_guard_mutex){+.+.+.}, at: [<c11ff8ae>] lock_trace+0x2e/0xb0
[   60.232078]  #2:  (&(&p->alloc_lock)->rlock){+.+...}, at: [<c10653fa>] ptrace_may_access+0x4a/0xf0
[   60.232078]
[   60.232078] stack backtrace:
[   60.232078] Pid: 17019, comm: trinity-child0 Not tainted 3.5.0+ #281
[   60.232078] Call Trace:
[   60.232078]  [<c10c6238>] __lock_acquire+0x1498/0x14f0
[   60.232078]  [<c10be7e7>] ? trace_hardirqs_off+0x27/0x40
[   60.232078]  [<c10c6360>] lock_acquire+0xd0/0x110       
[   60.232078]  [<c1176ffa>] ? get_task_comm+0x4a/0xf0
[   60.232078]  [<c1422290>] _raw_spin_lock+0x60/0x110
[   60.232078]  [<c1176ffa>] ? get_task_comm+0x4a/0xf0
[   60.232078]  [<c1176ffa>] get_task_comm+0x4a/0xf0
[   60.232078]  [<c1246798>] yama_ptrace_access_check+0x468/0x4a0
[   60.232078]  [<c124648f>] ? yama_ptrace_access_check+0x15f/0x4a0
[   60.232078]  [<c124209a>] security_ptrace_access_check+0x1a/0x30
[   60.232078]  [<c1065229>] __ptrace_may_access+0x189/0x310
[   60.232078]  [<c10650cc>] ? __ptrace_may_access+0x2c/0x310
[   60.232078]  [<c106542d>] ptrace_may_access+0x7d/0xf0
[   60.232078]  [<c11ff8ea>] lock_trace+0x6a/0xb0
[   60.232078]  [<c11ffa46>] proc_pid_stack+0x76/0x170
[   60.232078]  [<c1201064>] proc_single_show+0x74/0x100
[   60.232078]  [<c11a97b3>] seq_read+0x163/0x6b0
[   60.232078]  [<c105bf70>] ? do_setitimer+0x220/0x330
[   60.232078]  [<c11a9650>] ? seq_lseek+0x1f0/0x1f0
[   60.232078]  [<c116b55a>] vfs_read+0xca/0x280    
[   60.232078]  [<c11a9650>] ? seq_lseek+0x1f0/0x1f0
[   60.232078]  [<c116b776>] sys_read+0x66/0xe0
[   60.232078]  [<c1423d9d>] syscall_call+0x7/0xb
[   60.232078]  [<c1420000>] ? __schedule+0x2a0/0xc80

Thanks,
Fengguang