Subject: Re: linux-next: Tree for Feb 4

From: "Paul E. McKenney" <[email-masked]>
Date: Wed, 4 Feb 2015 16:10:19 -0800

On Wed, Feb 04, 2015 at 03:51:15PM -0800, Paul E. McKenney wrote:
> On Wed, Feb 04, 2015 at 11:59:31PM +0100, Rafael J. Wysocki wrote:
> > On Wednesday, February 04, 2015 01:53:58 PM Paul E. McKenney wrote:
> > > On Wed, Feb 04, 2015 at 10:54:07PM +0100, Rafael J. Wysocki wrote:
> > > > On Wednesday, February 04, 2015 09:18:03 PM Sedat Dilek wrote:

[ . . . ]

> > > > > [ 1144.482666] Disabling non-boot CPUs ...
> > > > > [ 1144.483000] intel_pstate CPU 1 exiting
> > > > > [ 1144.486064]
> > > > > [ 1144.486065] ===============================
> > > > > [ 1144.486067] smpboot: CPU 1 didn't die...
> > > > > [ 1144.486067] [ INFO: suspicious RCU usage. ]
> > > > > [ 1144.486069] 3.19.0-rc7-next-20150204.1-iniza-small #1 Not tainted
> > > > > [ 1144.486070] -------------------------------
> > > > > [ 1144.486072] include/trace/events/tlb.h:35 suspicious
> > > > > rcu_dereference_check() usage!
> > > > > [ 1144.486073]
> > > > > [ 1144.486073] other info that might help us debug this:
> > > > > [ 1144.486073]
> > > > > [ 1144.486074]
> > > > > [ 1144.486074] RCU used illegally from offline CPU!
> > > > > [ 1144.486074] rcu_scheduler_active = 1, debug_locks = 0
> > > > > [ 1144.486076] no locks held by swapper/1/0.
> > > > > [ 1144.486076]
> > > > > [ 1144.486076] stack backtrace:
> > > > > [ 1144.486079] CPU: 1 PID: 0 Comm: swapper/1 Not tainted
> > > > > 3.19.0-rc7-next-20150204.1-iniza-small #1
> > > > > [ 1144.486080] Hardware name: SAMSUNG ELECTRONICS CO., LTD.
> > > > > 530U3BI/530U4BI/530U4BH/530U3BI/530U4BI/530U4BH, BIOS 13XK 03/28/2013
> > > > > [ 1144.486085]  0000000000000001 ffff88011a44fe18 ffffffff817e370d
> > > > > 0000000000000011
> > > > > [ 1144.486088]  ffff88011a448290 ffff88011a44fe48 ffffffff810d6847
> > > > > ffff8800c66b9600
> > > > > [ 1144.486091]  0000000000000001 ffff88011a44c000 ffffffff81cb3900
> > > > > ffff88011a44fe78
> > > > > [ 1144.486092] Call Trace:
> > > > > [ 1144.486099]  [<ffffffff817e370d>] dump_stack+0x4c/0x65
> > > > > [ 1144.486104]  [<ffffffff810d6847>] lockdep_rcu_suspicious+0xe7/0x120
> > > 
> > > As near as I can tell, idle_task_exit() is running on an offline CPU,
> > > then calling switch_mm() which contains trace_tlb_flush(), which uses RCU.
> > > And RCU is objecting to being used from a CPU that it is ignoring.
> > > 
> > > One approach would be to push RCU's idea of when the CPU goes offline
> > > down into arch code in this case, using some Kconfig symbol and
> > > the usual conditional compilation.  Another approach would be to
> > > invoke the trace calls under cpu_online(), for example, for the
> > > first such call in switch_mm():
> > > 
> > > 	if (cpu_online(smp_processor_id()))
> > > 		trace_tlb_flush(TLB_FLUSH_ON_TASK_SWITCH, TLB_FLUSH_ALL);
> > > 
> > > The compiler would discard this if tracing was disabled.
> > 
> > That looks like less intrusive to me.
> 
> One possible concern is increased context-switch path length, but that
> would only be the case where tracing is enabled by default.

Nevertheless, here is an untested patch.  Does it help?

							Thanx, Paul

------------------------------------------------------------------------

x86: Omit switch_mm() tracing for offline CPUs

The architecture-specific switch_mm() function can be called by offline
CPUs, but includes event tracing, which cannot be legally carried out
on offline CPUs.  This results in a lockdep-RCU splat.  This commit fixes
this splat by omitting the tracing when the CPU is offline.

Reported-by: Sedat Dilek <[email-masked]>
Signed-off-by: Paul E. McKenney <[email-masked]>

diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
index 40269a2bf6f9..7e7f2445fbc9 100644
--- a/arch/x86/include/asm/mmu_context.h
+++ b/arch/x86/include/asm/mmu_context.h
@@ -47,7 +47,8 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
 
 		/* Re-load page tables */
 		load_cr3(next->pgd);
-		trace_tlb_flush(TLB_FLUSH_ON_TASK_SWITCH, TLB_FLUSH_ALL);
+		if (cpu_online(smp_processor_id()))
+			trace_tlb_flush(TLB_FLUSH_ON_TASK_SWITCH, TLB_FLUSH_ALL);
 
 		/* Stop flush ipis for the previous mm */
 		cpumask_clear_cpu(cpu, mm_cpumask(prev));
@@ -84,7 +85,8 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
 			 * to make sure to use no freed page tables.
 			 */
 			load_cr3(next->pgd);
-			trace_tlb_flush(TLB_FLUSH_ON_TASK_SWITCH, TLB_FLUSH_ALL);
+			if (cpu_online(smp_processor_id()))
+				trace_tlb_flush(TLB_FLUSH_ON_TASK_SWITCH, TLB_FLUSH_ALL);
 			load_LDT_nolock(&next->context);
 		}
 	}