Messages in this thread Patch in this message | | | Date | Wed, 7 Nov 2012 14:22:34 -0800 | From | "Paul E. McKenney" <> | Subject | Re: Linux 3.7-rc3 |
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On Fri, Nov 02, 2012 at 04:10:34PM -0700, Linus Torvalds wrote: > On Fri, Nov 2, 2012 at 3:43 PM, Rafael J. Wysocki <rjw@sisk.pl> wrote: > > > > Well, not everything is rosy in the suspend land, though. This is a > > failure to freeze khubd during the second in a row attempt to suspend to > > RAM (your current tree): > > Ugh. So khubd is blocked in usb_start_wait_urb(), and apparently the > timeout for that block is longer than the freezing timeout. > > There's a comment about why khubd needs to be freezable, but I wonder > if that whole thing isn't doing something wrong. Causing the suspend > to fail is definitely always the wrong thing. > > Greg? > > > [ 125.780766] [ INFO: suspicious RCU usage. ] > > [ 125.780804] 3.7.0-rc3+ #988 Not tainted > > [ 125.780838] ------------------------------- > > [ 125.780875] /home/rafael/src/linux/kernel/sched/core.c:4497 suspicious rcu_dereference_check() usage! > > Heh. The RCU usage is from the debug printout from sched_show_task(), > so it's "related", but it's a totally independent issue. > > It's apparently because we've not done a "rcu_read_lock()" around that > sequence, but I seriously doubt we care. But it's technically a real > bug - even if the fix might be to just not print out the parent pid > (or to just ignore the bug and turn the rcu dereference into an > ACCESS_ONCE() or something. > > Ingo, Peter, any comments about that sched/core.c:4497 RCU usage?
Rafael, does the following patch fix that problem?
Thanx, Paul
------------------------------------------------------------------------
sched: Mark RCU reader in sched_show_task()
When sched_show_task() is invoked from try_to_freeze_tasks(), there is no RCU read-side critical section, resulting in the following splat:
[ 125.780730] =============================== [ 125.780766] [ INFO: suspicious RCU usage. ] [ 125.780804] 3.7.0-rc3+ #988 Not tainted [ 125.780838] ------------------------------- [ 125.780875] /home/rafael/src/linux/kernel/sched/core.c:4497 suspicious rcu_dereference_check() usage! [ 125.780946] [ 125.780946] other info that might help us debug this: [ 125.780946] [ 125.781031] [ 125.781031] rcu_scheduler_active = 1, debug_locks = 0 [ 125.781087] 4 locks held by s2ram/4211: [ 125.781120] #0: (&buffer->mutex){+.+.+.}, at: [<ffffffff811e2acf>] sysfs_write_file+0x3f/0x160 [ 125.781233] #1: (s_active#94){.+.+.+}, at: [<ffffffff811e2b58>] sysfs_write_file+0xc8/0x160 [ 125.781339] #2: (pm_mutex){+.+.+.}, at: [<ffffffff81090a81>] pm_suspend+0x81/0x230 [ 125.781439] #3: (tasklist_lock){.?.?..}, at: [<ffffffff8108feed>] try_to_freeze_tasks+0x2cd/0x3f0 [ 125.781543] [ 125.781543] stack backtrace: [ 125.781584] Pid: 4211, comm: s2ram Not tainted 3.7.0-rc3+ #988 [ 125.781632] Call Trace: [ 125.781662] [<ffffffff810a3c73>] lockdep_rcu_suspicious+0x103/0x140 [ 125.781719] [<ffffffff8107cf21>] sched_show_task+0x121/0x180 [ 125.781770] [<ffffffff8108ffb4>] try_to_freeze_tasks+0x394/0x3f0 [ 125.781823] [<ffffffff810903b5>] freeze_kernel_threads+0x25/0x80 [ 125.781876] [<ffffffff81090b65>] pm_suspend+0x165/0x230 [ 125.781924] [<ffffffff8108fa29>] state_store+0x99/0x100 [ 125.781975] [<ffffffff812f5867>] kobj_attr_store+0x17/0x20 [ 125.782038] [<ffffffff811e2b71>] sysfs_write_file+0xe1/0x160 [ 125.782091] [<ffffffff811667a6>] vfs_write+0xc6/0x180 [ 125.782138] [<ffffffff81166ada>] sys_write+0x5a/0xa0 [ 125.782185] [<ffffffff812ff6ae>] ? trace_hardirqs_on_thunk+0x3a/0x3f [ 125.782242] [<ffffffff81669dd2>] system_call_fastpath+0x16/0x1b
This commit therefore adds the needed RCU read-side critical section.
Reported-by: "Rafael J. Wysocki" <rjw@sisk.pl> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 6d4569e..36f2608 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -4474,6 +4474,7 @@ static const char stat_nam[] = TASK_STATE_TO_CHAR_STR; void sched_show_task(struct task_struct *p) { unsigned long free = 0; + int ppid; unsigned state; state = p->state ? __ffs(p->state) + 1 : 0; @@ -4493,8 +4494,11 @@ void sched_show_task(struct task_struct *p) #ifdef CONFIG_DEBUG_STACK_USAGE free = stack_not_used(p); #endif + rcu_read_lock(); + ppid = task_pid_nr(rcu_dereference(p->real_parent)); + rcu_read_unlock(); printk(KERN_CONT "%5lu %5d %6d 0x%08lx\n", free, - task_pid_nr(p), task_pid_nr(rcu_dereference(p->real_parent)), + task_pid_nr(p), ppid, (unsigned long)task_thread_info(p)->flags); show_stack(p, NULL);
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