Messages in this thread | | | Date | Fri, 11 Dec 2015 16:02:47 -0800 | From | "Paul E. McKenney" <> | Subject | Re: new warning on sysrq kernel crash trigger |
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On Fri, Dec 11, 2015 at 03:41:04PM -0800, Ani Sinha wrote: > On Fri, Dec 11, 2015 at 2:27 PM, Paul E. McKenney > <paulmck@linux.vnet.ibm.com> wrote: > > On Fri, Dec 11, 2015 at 05:10:43PM -0500, Rik van Riel wrote: > >> On 12/11/2015 03:44 PM, Ani Sinha wrote: > >> > > >> > > >> > On Thu, 10 Dec 2015, Paul E. McKenney wrote: > >> > > >> >> On Thu, Dec 10, 2015 at 03:57:09PM -0800, Ani Sinha wrote: > >> >>> Hi guys > >> >>> > >> >>> I am noticing a new warning in linux 3.18 which we did not see before > >> >>> in linux 3.4 : > >> >>> > >> >>> bash-4.1# echo c > /proc/sysrq-trigger > >> >>> [ 978.807185] BUG: sleeping function called from invalid context at > >> >>> ../arch/x86/mm/fault.c:1187 > >> >>> [ 978.909816] in_atomic(): 0, irqs_disabled(): 0, pid: 4706, name: bash > >> >>> [ 978.987358] Preemption disabled at:[<ffffffff81484339>] printk+0x48/0x4a > >> >>> > >> >>> > >> >>> I have bisected this to the following change : > >> >>> > >> >>> commit 984d74a72076a12b400339973e8c98fd2fcd90e5 > >> >>> Author: Rik van Riel <riel@redhat.com> > >> >>> Date: Fri Jun 6 14:38:13 2014 -0700 > >> >>> > >> >>> sysrq: rcu-ify __handle_sysrq > >> >>> > >> >>> > >> >>> the rcu_read_lock() in handle_sysrq() bumps up > >> >>> current->rcu_read_lock_nesting. Hence, in __do_page_fault() when it > >> >>> calls might_sleep() in x86/mm/fault.c line 1191, > >> >>> preempt_count_equals(0) returns false and hence the warning is > >> >>> printed. > >> >>> > >> >>> One way to handle this would be to do something like this: > >> >>> > >> >>> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c > >> >>> index eef44d9..d4dbe22 100644 > >> >>> --- a/arch/x86/mm/fault.c > >> >>> +++ b/arch/x86/mm/fault.c > >> >>> @@ -1132,7 +1132,7 @@ __do_page_fault(struct pt_regs *regs, unsigned > >> >>> long error_code, > >> >>> * If we're in an interrupt, have no user context or are running > >> >>> * in a region with pagefaults disabled then we must not take the fault > >> >>> */ > >> >>> - if (unlikely(faulthandler_disabled() || !mm)) { > >> >>> + if (unlikely(faulthandler_disabled() || rcu_preempt_depth() || !mm)) { > >> >> > >> >> This works if CONFIG_PREEMPT=y, but if CONFIG_PREEMPT=n, then > >> >> rcu_preempt_depth() unconditionally returns zero. And if > >> >> CONFIG_PREEMPT_COUNT=y && CONFIG_PREEMPT=n, you would still see > >> >> the might_sleep() splat. > >> >> > >> >> Maybe use SRCU instead of RCU for this purpose? > >> >> > >> > > >> > From ae232ce3fb167b2ad363bfac7aab69001bc55a50 Mon Sep 17 00:00:00 2001 > >> > From: Ani Sinha <ani@arista.com> > >> > Date: Fri, 11 Dec 2015 12:07:42 -0800 > >> > Subject: [PATCH 1/1] Fix 'sleeping function called from invalid context' > >> > warning in sysrq generated crash. > >> > > >> > Commit 984d74a72076a1 ("sysrq: rcu-ify __handle_sysrq") > >> > replaced spin_lock_irqsave() calls with > >> > rcu_read_lock() calls in sysrq. Since rcu_read_lock() does not > >> > disable preemption, faulthandler_disabled() in > >> > __do_page_fault() in x86/fault.c returns false. When the code > >> > later calls might_sleep() in the pagefault handler, we get the > >> > following warning: > >> > > >> > BUG: sleeping function called from invalid context at ../arch/x86/mm/fault.c:1187 > >> > in_atomic(): 0, irqs_disabled(): 0, pid: 4706, name: bash > >> > Preemption disabled at:[<ffffffff81484339>] printk+0x48/0x4a > >> > > >> > To fix this, replace RCU call in handle_sysrq() to use SRCU. > >> > >> The sysrq code can be called from irq context. > >> > >> Trying to use SRCU from an irq context sounds like it could > >> be a bad idea, though admittedly I do not know enough about > >> SRCU to know for sure :) > > > > Indeed, not the best idea! ;-) > > > > I could imagine something like this: > > > > if (in_irq()) > > rcu_read_lock(); > > else > > idx = srcu_read_lock(&sysrq_rcu); > > > > And ditto for unlock. Then, for the update: > > > > synchronize_rcu_mult(call_rcu, call_sysrq_srcu); > > This won't work on 3.18 as this api was introduced in linux 4.3.
Then do this:
synchronize_rcu(); synchronize_srcu(&sysrq_rcu);
> > Where: > > > > static void call_sysrq_srcu(struct rcu_head *head, rcu_callback_t func) > > { > > call_srcu(&sysrq_rcu, head, func); > > } > > > > Here I presume that the page-fault code avoids the might_sleep if invoked > > from irq context. > > Quick look at the code seems to indicate that this is true.
Good! ;-)
Thanx, Paul
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