linux-kernel - [PATCH 1/1] Fix 'sleeping function called from invalid context' warning in sysrq generated crash.

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Message-Id: <1450401310-29117-1-git-send-email-ani@arista.com>
Date:	Thu, 17 Dec 2015 17:15:10 -0800
From:	Ani Sinha <ani@...sta.com>
To:	Ani Sinha <ani@...rban.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.com>, linux-kernel@...r.kernel.org,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Rik van Riel <riel@...hat.com>,
	Ivan Delalande <colona@...sta.com>,
	Francesco Ruggeri <fruggeri@...sta.com>
Cc:	Ani Sinha <ani@...sta.com>
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@...sta.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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