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Date:	Sat, 5 Apr 2014 14:43:35 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Sasha Levin <sasha.levin@...cle.com>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alexander Lam <azl@...gle.com>
Subject: Re: [for-next][PATCH 14/14] tracing: Get trace_array ref counts
 when accessing trace files

On Sat, 05 Apr 2014 10:59:10 -0400
Sasha Levin <sasha.levin@...cle.com> wrote:


> [ 5644.290783] Chain exists of:
> trace_types_lock --> &pipe->mutex/1 --> &sig->cred_guard_mutex
> 
> [ 5644.290783]  Possible unsafe locking scenario:
> [ 5644.290783]
> [ 5644.290783]        CPU0                    CPU1
> [ 5644.290783]        ----                    ----
> [ 5644.290783]   lock(&sig->cred_guard_mutex);
> [ 5644.290783]                                lock(&pipe->mutex/1);
> [ 5644.290783]                                lock(&sig->cred_guard_mutex);
> [ 5644.290783]   lock(trace_types_lock);


Or I haven't done enough to trigger both paths in a single boot :-/

Anyway, I'm questioning the trace_types_lock being held throughout the
entire path of tracing_buffers_splice_read(). I'll have to look deeper
into this on Monday. If we can fine grain that lock in that function it
may get us out of this potential deadlock.

Thanks for reporting.

-- Steve

> [ 5644.290783]
> [ 5644.290783]  *** DEADLOCK ***
> [ 5644.290783]
> [ 5644.290783] 1 lock held by trinity-c17/19105:
> [ 5644.290783] #0: (&sig->cred_guard_mutex){+.+.+.}, at: prepare_bprm_creds (fs/exec.c:1165)
> [ 5644.290783]
> [ 5644.290783] stack backtrace:
> [ 5644.290783] CPU: 10 PID: 19105 Comm: trinity-c17 Not tainted 3.14.0-next-20140403-sasha-00019-g7474aa9-dirty #376
> [ 5644.290783]  ffffffffb4a1a1e0 ffff88071a7738f8 ffffffffb14bfb2f 0000000000000000
> [ 5644.290783]  ffffffffb49a9dd0 ffff88071a773948 ffffffffb14b2527 0000000000000001
> [ 5644.290783]  ffff88071a7739d8 ffff88071a773948 ffff8805d98cbcf0 ffff8805d98cbd28
> [ 5644.290783] Call Trace:
> [ 5644.290783] dump_stack (lib/dump_stack.c:52)
> [ 5644.290783] print_circular_bug (kernel/locking/lockdep.c:1214)
> [ 5644.290783] __lock_acquire (kernel/locking/lockdep.c:1840 kernel/locking/lockdep.c:1945 kernel/locking/lockdep.c:2131 kernel/locking/lockdep.c:3182)
> [ 5644.290783] ? _raw_spin_unlock_irqrestore (arch/x86/include/asm/paravirt.h:809 include/linux/spinlock_api_smp.h:160 kernel/locking/spinlock.c:191)
> [ 5644.290783] ? preempt_count_sub (kernel/sched/core.c:2527)
> [ 5644.290783] ? __slab_free (mm/slub.c:2598)
> [ 5644.290783] lock_acquire (arch/x86/include/asm/current.h:14 kernel/locking/lockdep.c:3602)
> [ 5644.290783] ? trace_array_get (kernel/trace/trace.c:225)
> [ 5644.290783] mutex_lock_nested (kernel/locking/mutex.c:486 kernel/locking/mutex.c:587)
> [ 5644.290783] ? trace_array_get (kernel/trace/trace.c:225)
> [ 5644.290783] ? locks_free_lock (fs/locks.c:244)
> [ 5644.290783] ? trace_array_get (kernel/trace/trace.c:225)
> [ 5644.290783] ? preempt_count_sub (kernel/sched/core.c:2527)
> [ 5644.290783] trace_array_get (kernel/trace/trace.c:225)
> [ 5644.290783] tracing_open_generic_tr (kernel/trace/trace.c:3053)
> [ 5644.290783] do_dentry_open (fs/open.c:753)
> [ 5644.290783] ? tracing_open_pipe (kernel/trace/trace.c:3047)
> [ 5644.290783] finish_open (fs/open.c:818)
> [ 5644.290783] do_last (fs/namei.c:3040)
> [ 5644.290783] ? link_path_walk (fs/namei.c:1473 fs/namei.c:1744)
> [ 5644.290783] ? __this_cpu_preempt_check (lib/smp_processor_id.c:63)
> [ 5644.290783] ? trace_hardirqs_on_caller (kernel/locking/lockdep.c:2557 kernel/locking/lockdep.c:2599)
> [ 5644.290783] path_openat (fs/namei.c:3182)
> [ 5644.290783] ? __lock_acquire (kernel/locking/lockdep.c:3189)
> [ 5644.290783] do_filp_open (fs/namei.c:3231)
> [ 5644.290783] ? put_lock_stats.isra.12 (arch/x86/include/asm/preempt.h:98 kernel/locking/lockdep.c:254)
> [ 5644.290783] ? do_execve_common.isra.19 (fs/exec.c:1489)
> [ 5644.290783] ? get_parent_ip (kernel/sched/core.c:2472)
> [ 5644.290783] do_open_exec (fs/exec.c:766)
> [ 5644.290783] do_execve_common.isra.19 (fs/exec.c:1491)
> [ 5644.290783] ? do_execve_common.isra.19 (include/linux/spinlock.h:303 fs/exec.c:1258 fs/exec.c:1486)
> [ 5644.290783] compat_SyS_execve (fs/exec.c:1627)
> [ 5644.290783] ia32_ptregs_common (arch/x86/ia32/ia32entry.S:495)
> 
> 
> Thanks,
> Sasha

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