Messages in this thread | | | Date | Wed, 2 Nov 2011 12:23:12 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [GIT PULL rcu/next] RCU commits for 3.1 |
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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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