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Date: Tue, 01 Nov 2011 10:37:28 +0800
From: Li Zefan <lizf@...fujitsu.com>
To: paulmck@...ux.vnet.ibm.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
(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.
---
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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