Messages in this thread | | | Date | Thu, 3 Nov 2011 12:50:21 +0000 | Subject | Re: [GIT PULL rcu/next] RCU commits for 3.1 | From | Stephane Eranian <> |
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Paul,
On Tue, Nov 1, 2011 at 2:37 AM, Li Zefan <lizf@cn.fujitsu.com> 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? > We don't retrieve the task cgroup twice. We retrieve the cgroup for each of the two tasks: current and prev or next.
I don't understand what you mean by 'between two calls'. Two calls of which function?
> - 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. > What do you mean by cgroup pinning?
If a task migrates from one cgroup to another, the cgroup code calls ss->attach_task which ends up in perf_cgroup_attach_task() if the task is currently running on a CPU. If so perf_cgroup_switch() is eventually called and it will update cpuctx->cgrp. If the tasks is not running anywhere, then there is nothing to do, state will be updated when the task is scheduled back in.
> --- > 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 > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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