Messages in this thread | | | Subject | Re: new warning on sysrq kernel crash trigger | From | Rik van Riel <> | Date | Fri, 11 Dec 2015 17:10:43 -0500 |
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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 :)
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