Messages in this thread | | | Date | Thu, 10 Dec 2015 21:26:47 -0800 | From | "Paul E. McKenney" <> | Subject | Re: new warning on sysrq kernel crash trigger |
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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?
Thanx, Paul
> bad_area_nosemaphore(regs, error_code, address); > return; > } > > I am wondering if this would be the right approach. I have tested that > this patch does indeed suppress the warning. If you guys agree, I will > send a patch. It's true that this is a trivial issue since we are > intentionally crashing the kernel but in our case, this additional > complaint from the kernel is confusing our test scripts and they are > generating false positives. >
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