sysrq: Fix warning in sysrq generated crash.

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: Luis Henriques <luis.henriques@canonical.com>

diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
index b5e98de489dc7559ce347e0ef83cea8cd0b1451e..0c79f95383c0c4013c089ba8b306f3091f89fb0f 100644 (file)
--- 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;