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Subject[PATCH 4.4 44/87] sysrq: Fix warning in sysrq generated crash.
Date
4.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Ani Sinha <ani@arista.com>

commit 984cf355aeaa8f2eda3861b50d0e8d3e3f77e83b upstream.

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>
Reviewed-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/tty/sysrq.c | 6 ++++++
1 file changed, 6 insertions(+)

--- 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;

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