Messages in this thread | | | Date | Fri, 4 Nov 2011 09:02:17 +0000 | Subject | Re: [GIT PULL rcu/next] RCU commits for 3.1 | From | Stephane Eranian <> |
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On Fri, Nov 4, 2011 at 8:44 AM, Li Zefan <lizf@cn.fujitsu.com> wrote: > Stephane Eranian wrote: >> 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? >> > > perf_cgroup_sched_out(task, next) > { > cgrp1 = perf_cgroup_from_task(task); > ... > perf_cgroup_switch(task, PERF_CGROUP_SWOUT); > } > > perf_cgroup_switch(task) > { > ... > cpuctx->cgrp = perf_cgroup_from_task(task); > } > Ok, yes it may happen that we call it twice.
I tested your patch and it looks good to me. I would make the following adjustments though: - perf_cgroup_set_timestamp(), move rcu_read_unlock() before info->timestamp = as it is not needed for this statement.
> So we call perf_cgroup_from_task() twice on @task. Just want to be sure the code > is not problematic. > >>> - 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. >> > > Thanks for clarification! > -- 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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