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Date: Wed, 2 Nov 2011 12:23:12 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Li Zefan <lizf@...fujitsu.com>
Cc: Ingo Molnar <mingo@...e.hu>, eric.dumazet@...il.com,
shaohua.li@...el.com, ak@...ux.intel.com, mhocko@...e.cz,
alex.shi@...el.com, efault@....de, linux-kernel@...r.kernel.org,
Peter Zijlstra <peterz@...radead.org>,
Paul Turner <pjt@...gle.com>,
Stephane Eranian <eranian@...gle.com>
Subject: Re: [GIT PULL rcu/next] RCU commits for 3.1
On Tue, Nov 01, 2011 at 10:37:28AM +0800, Li Zefan wrote:
> (I shoud have cced Stephane Eranian instead of Turner..)
>
> Paul E. McKenney wrote:
> > On Mon, Oct 31, 2011 at 04:09:19PM +0800, Li Zefan wrote:
> >> (Let's cc Peter and Paul Turner for this perf cgroup issue.)
> >>
> >>> Thank you for the analysis. Does the following patch fix this problem?
> >>>
> >>> Thanx, Paul
> >>>
> >>> ------------------------------------------------------------------------
> >>>
> >>> fs: Add RCU protection in set_task_comm()
> >>>
> >>> Running "perf stat true" results in the following RCU-lockdep splat:
> >>>
> >>> ===============================
> >>> [ INFO: suspicious RCU usage. ]
> >>> -------------------------------
> >>> include/linux/cgroup.h:548 suspicious rcu_dereference_check() usage!
> >>>
> >>> other info that might help us debug this:
> >>>
> >>> rcu_scheduler_active = 1, debug_locks = 0
> >>> 1 lock held by true/655:
> >>> #0: (&sig->cred_guard_mutex){+.+.+.}, at: [<810d1bd7>] prepare_bprm_creds+0x27/0x70
> >>>
> >>> stack backtrace:
> >>> Pid: 655, comm: true Not tainted 3.1.0-tip-01868-g1271bd2-dirty #161079
> >>> Call Trace:
> >>> [<81abe239>] ? printk+0x18/0x1a
> >>> [<81064920>] lockdep_rcu_suspicious+0xc0/0xd0
> >>> [<8108aa02>] perf_event_enable_on_exec+0x1d2/0x1e0
> >>> [<81063764>] ? __lock_release+0x54/0xb0
> >>> [<8108cca8>] perf_event_comm+0x18/0x60
> >>> [<810d1abd>] ? set_task_comm+0x5d/0x80
> >>> [<81af622d>] ? _raw_spin_unlock+0x1d/0x40
> >>> [<810d1ac4>] set_task_comm+0x64/0x80
> >>> [<810d25fd>] setup_new_exec+0xbd/0x1d0
> >>> [<810d1b61>] ? flush_old_exec+0x81/0xa0
> >>> [<8110753e>] load_elf_binary+0x28e/0xa00
> >>> [<810d2101>] ? search_binary_handler+0xd1/0x1d0
> >>> [<81063764>] ? __lock_release+0x54/0xb0
> >>> [<811072b0>] ? load_elf_library+0x260/0x260
> >>> [<810d2108>] search_binary_handler+0xd8/0x1d0
> >>> [<810d2060>] ? search_binary_handler+0x30/0x1d0
> >>> [<810d242f>] do_execve_common+0x22f/0x2a0
> >>> [<810d24b2>] do_execve+0x12/0x20
> >>> [<81009592>] sys_execve+0x32/0x70
> >>> [<81af7752>] ptregs_execve+0x12/0x20
> >>> [<81af76d4>] ? sysenter_do_call+0x12/0x36
> >>>
> >>> Li Zefan noted that this is due to set_task_comm() dropping the task
> >>> lock before invoking perf_event_comm(), which could in fact result in
> >>> the task being freed up before perf_event_comm() completed tracing in
> >>> the case where one task invokes set_task_comm() on another task -- which
> >>> actually does occur via comm_write(), which can be invoked via /proc.
> >>>
> >>
> >> This is not true. The caller should ensure @tsk is valid during
> >> set_task_comm().
> >>
> >> The warning comes from perf_cgroup_from_task(). We can trigger this warning
> >> in some other cases where perf cgroup is used, for example:
> >
> > I must defer to your greater knowledge of this situation. What patch
> > would you propose?
> >
>
> With the following patch, we should see no rcu warning from perf, but as I
> don't know the internel of perf, I guess we have to defer to Peter and
> Stephane. ;)
>
> I have two doubts:
>
> - in perf_cgroup_sched_out/in(), we retrieve the task's cgroup twice in the function
> and it's callee perf_cgroup_switch(), but the task can move to another cgroup between
> two calls, so they might return two different cgroup pointers. Does it matter?
>
> - in perf_cgroup_switch():
>
> cpuctx->cgrp = perf_cgroup_from_task(task);
>
> but seems the cgroup is not pinned, so cpuctx->cgrp can be invalid in later use.
Looks sane to me, for whatever that might be worth.
Thanx, Paul
> ---
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index d1a1bee..f5e05ce 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -302,7 +302,10 @@ static inline void update_cgrp_time_from_event(struct perf_event *event)
> if (!is_cgroup_event(event))
> return;
>
> + rcu_read_lock();
> cgrp = perf_cgroup_from_task(current);
> + rcu_read_unlock();
> +
> /*
> * Do not update time when cgroup is not active
> */
> @@ -325,9 +328,11 @@ perf_cgroup_set_timestamp(struct task_struct *task,
> if (!task || !ctx->nr_cgroups)
> return;
>
> + rcu_read_lock();
> cgrp = perf_cgroup_from_task(task);
> info = this_cpu_ptr(cgrp->info);
> info->timestamp = ctx->timestamp;
> + rcu_read_unlock();
> }
>
> #define PERF_CGROUP_SWOUT 0x1 /* cgroup switch out every event */
> @@ -406,6 +411,8 @@ static inline void perf_cgroup_sched_out(struct task_struct *task,
> struct perf_cgroup *cgrp1;
> struct perf_cgroup *cgrp2 = NULL;
>
> + rcu_read_lock();
> +
> /*
> * we come here when we know perf_cgroup_events > 0
> */
> @@ -418,6 +425,8 @@ static inline void perf_cgroup_sched_out(struct task_struct *task,
> if (next)
> cgrp2 = perf_cgroup_from_task(next);
>
> + rcu_read_unlock();
> +
> /*
> * only schedule out current cgroup events if we know
> * that we are switching to a different cgroup. Otherwise,
> @@ -433,6 +442,8 @@ static inline void perf_cgroup_sched_in(struct task_struct *prev,
> struct perf_cgroup *cgrp1;
> struct perf_cgroup *cgrp2 = NULL;
>
> + rcu_read_lock();
> +
> /*
> * we come here when we know perf_cgroup_events > 0
> */
> @@ -441,6 +452,8 @@ static inline void perf_cgroup_sched_in(struct task_struct *prev,
> /* prev can never be NULL */
> cgrp2 = perf_cgroup_from_task(prev);
>
> + rcu_read_unlock();
> +
> /*
> * only need to schedule in cgroup events if we are changing
> * cgroup during ctxsw. Cgroup events were not scheduled
>
--
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