Messages in this thread Patch in this message | | | From | Anirban Sinha <> | Date | Mon, 14 Dec 2015 16:14:37 -0800 (PST) | Subject | Re: new warning on sysrq kernel crash trigger |
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On Mon, 14 Dec 2015, Rik van Riel wrote:
> On 12/14/2015 11:24 AM, Ani Sinha wrote: > > Rik, any comments? > > Another good option is to simply ignore this warning, or drop > the rcu_read_lock before doing the alt-syrsq-c action. > > After all, alt-sysrq-c is "crash the system, take a crash dump", > which is not an action the system ever returns from. >
Yea I thought about this idea previously but then discarded it thinking it would be too hacky. Here's the cooked up patch. I hope this can be approved for mainline soon (I'm on vacation and working just on this issue remotely) :
From 105ff3ffce380650b3d58b3594a9be47bd604b28 Mon Sep 17 00:00:00 2001 From: Ani Sinha <ani@arista.com> Date: Mon, 14 Dec 2015 14:55:08 -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, we release the RCU read lock before we crash.
Tested this patch on linux 3.18 by booting off one of our boards.
Fixes: 984d74a72076a1 ("sysrq: rcu-ify __handle_sysrq")
Signed-off-by: Ani Sinha <ani@arista.com> --- drivers/tty/sysrq.c | 6 ++++++ 1 file changed, 6 insertions(+)
diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c index 5381a72..08987ad 100644 --- a/drivers/tty/sysrq.c +++ b/drivers/tty/sysrq.c @@ -133,6 +133,12 @@ static void sysrq_handle_crash(int key) { char *killer = NULL; + /* we need to release the RCU read lock here, + otherwise we get an annoying + 'BUG: sleeping function called from invalid context' + complaint from the kernel before the panic. + */ + rcu_read_unlock(); panic_on_oops = 1; /* force panic */ wmb(); *killer = 1; -- 1.8.1.4
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