Messages in this thread Patch in this message | | | From | Ani Sinha <> | Date | Thu, 10 Dec 2015 15:57:09 -0800 | Subject | new warning on sysrq kernel crash trigger |
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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)) { 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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