Messages in this thread | | | Date | Fri, 11 Dec 2015 14:27:42 -0800 | From | "Paul E. McKenney" <> | Subject | Re: new warning on sysrq kernel crash trigger |
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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);
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.
Thoughts?
Thanx, Paul
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