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Date: Tue, 4 Jun 2013 14:09:51 +0200
From: Frederic Weisbecker <fweisbec@...il.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Peter Zijlstra <peterz@...radead.org>,
LKML <linux-kernel@...r.kernel.org>,
"Paul E. McKenney" <paulmck@...ibm.com>,
Dave Jones <davej@...hat.com>, Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH v2][RFC] tracing/context-tracking: Add
preempt_schedule_context() for tracing
On Fri, May 31, 2013 at 09:30:18PM -0400, Steven Rostedt wrote:
> Dave Jones hit the following bug report:
>
> ===============================
> [ INFO: suspicious RCU usage. ]
> 3.10.0-rc2+ #1 Not tainted
> -------------------------------
> include/linux/rcupdate.h:771 rcu_read_lock() used illegally while idle!
> other info that might help us debug this:
> RCU used illegally from idle CPU! rcu_scheduler_active = 1, debug_locks = 0
> RCU used illegally from extended quiescent state!
> 2 locks held by cc1/63645:
> #0: (&rq->lock){-.-.-.}, at: [<ffffffff816b39fd>] __schedule+0xed/0x9b0
> #1: (rcu_read_lock){.+.+..}, at: [<ffffffff8109d645>] cpuacct_charge+0x5/0x1f0
>
> CPU: 1 PID: 63645 Comm: cc1 Not tainted 3.10.0-rc2+ #1 [loadavg: 40.57 27.55 13.39 25/277 64369]
> Hardware name: Gigabyte Technology Co., Ltd. GA-MA78GM-S2H/GA-MA78GM-S2H, BIOS F12a 04/23/2010
> 0000000000000000 ffff88010f78fcf8 ffffffff816ae383 ffff88010f78fd28
> ffffffff810b698d ffff88011c092548 000000000023d073 ffff88011c092500
> 0000000000000001 ffff88010f78fd60 ffffffff8109d7c5 ffffffff8109d645
> Call Trace:
> [<ffffffff816ae383>] dump_stack+0x19/0x1b
> [<ffffffff810b698d>] lockdep_rcu_suspicious+0xfd/0x130
> [<ffffffff8109d7c5>] cpuacct_charge+0x185/0x1f0
> [<ffffffff8109d645>] ? cpuacct_charge+0x5/0x1f0
> [<ffffffff8108dffc>] update_curr+0xec/0x240
> [<ffffffff8108f528>] put_prev_task_fair+0x228/0x480
> [<ffffffff816b3a71>] __schedule+0x161/0x9b0
> [<ffffffff816b4721>] preempt_schedule+0x51/0x80
> [<ffffffff816b4800>] ? __cond_resched_softirq+0x60/0x60
> [<ffffffff816b6824>] ? retint_careful+0x12/0x2e
> [<ffffffff810ff3cc>] ftrace_ops_control_func+0x1dc/0x210
> [<ffffffff816be280>] ftrace_call+0x5/0x2f
> [<ffffffff816b681d>] ? retint_careful+0xb/0x2e
> [<ffffffff816b4805>] ? schedule_user+0x5/0x70
> [<ffffffff816b4805>] ? schedule_user+0x5/0x70
> [<ffffffff816b6824>] ? retint_careful+0x12/0x2e
> ------------[ cut here ]------------
>
> What happened was that the function tracer traced the schedule_user() code
> that tells RCU that the system is coming back from userspace, and to
> add the CPU back to the RCU monitoring.
>
> Because the function tracer does a preempt_disable/enable_notrace() calls
> the preempt_enable_notrace() checks the NEED_RESCHED flag. If it is set,
> then preempt_schedule() is called. But this is called before the user_exit()
> function can inform the kernel that the CPU is no longer in user mode and
> needs to be accounted for by RCU.
>
> The fix is to create a new preempt_schedule_context() that checks if
> the kernel is still in user mode and if so to switch it to kernel mode
> before calling schedule. It also switches back to user mode coming back
> from schedule in need be.
>
> The only user of this currently is the preempt_enable_notrace(), which is
> only used by the tracing subsystem.
>
> Link: http://lkml.kernel.org/r/1369423420.6828.226.camel@gandalf.local.home
>
> Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
> ---
> include/linux/preempt.h | 18 +++++++++++++++++-
> kernel/context_tracking.c | 38 ++++++++++++++++++++++++++++++++++++++
> 2 files changed, 55 insertions(+), 1 deletions(-)
>
> diff --git a/include/linux/preempt.h b/include/linux/preempt.h
> index 87a03c7..f5d4723 100644
> --- a/include/linux/preempt.h
> +++ b/include/linux/preempt.h
> @@ -33,9 +33,25 @@ do { \
> preempt_schedule(); \
> } while (0)
>
> +#ifdef CONFIG_CONTEXT_TRACKING
> +
> +void preempt_schedule_context(void);
> +
> +#define preempt_check_resched_context() \
> +do { \
> + if (unlikely(test_thread_flag(TIF_NEED_RESCHED))) \
> + preempt_schedule_context(); \
> +} while (0)
> +#else
> +
> +#define preempt_check_resched_context() preempt_check_resched()
> +
> +#endif /* CONFIG_CONTEXT_TRACKING */
> +
> #else /* !CONFIG_PREEMPT */
>
> #define preempt_check_resched() do { } while (0)
> +#define preempt_check_resched_context() do { } while (0)
>
> #endif /* CONFIG_PREEMPT */
>
> @@ -88,7 +104,7 @@ do { \
> do { \
> preempt_enable_no_resched_notrace(); \
> barrier(); \
> - preempt_check_resched(); \
> + preempt_check_resched_context(); \
> } while (0)
>
> #else /* !CONFIG_PREEMPT_COUNT */
> diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
> index 65349f0..15c9f2e 100644
> --- a/kernel/context_tracking.c
> +++ b/kernel/context_tracking.c
> @@ -71,6 +71,44 @@ void user_enter(void)
> local_irq_restore(flags);
> }
>
> +/**
> + * preempt_schedule_context - preempt_schedule called by tracing
> + *
> + * The tracing infrastructure uses preempt_enable_notrace to prevent
> + * recursion and tracing preempt enabling caused by the tracing
> + * infrastructure itself. But as tracing can happen in areas coming
> + * from userspace or just about to enter userspace, a preempt enable
> + * can occur before user_exit() is called. This will cause the scheduler
> + * to be called when the system is still in usermode.
> + *
> + * To prevent this, the preempt_enable_notrace will use this function
> + * instead of preempt_schedule() to exit user context if needed before
> + * calling the scheduler.
> + */
> +void __sched notrace preempt_schedule_context(void)
> +{
> + struct thread_info *ti = current_thread_info();
> + enum ctx_state prev_ctx;
> +
> + if (likely(ti->preempt_count || irqs_disabled()))
> + return;
or:
if (!preemptible())
return;
> +
> + /*
> + * Need to disable preemption in case user_exit() is traced
> + * and the tracer calls preempt_enable_notrace() causing
> + * an infinite recursion.
> + */
> + preempt_disable_notrace();
> + prev_ctx = exception_enter();
> + preempt_enable_no_resched_notrace();
> +
> + preempt_schedule();
> +
> + preempt_disable_notrace();
> + exception_exit(prev_ctx);
> + preempt_enable_notrace();
> +}
> +EXPORT_SYMBOL(preempt_schedule_context);
That's quite a tricky change but I can't think of anything better.
Acked-by: Frederic Weisbecker <fweisbec@...il.com>
> /**
> * user_exit - Inform the context tracking that the CPU is
> --
> 1.7.3.4
>
>
>
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