Re: [PATCH 1/1] Fix 'sleeping function called from invalid context' warning in sysrq generated crash.

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Thu, Dec 17, 2015 at 05:15:10PM -0800, Ani Sinha wrote:
> 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>

I queued this with Rik's Signed-off-by, and fixed some checkpatch.pl
errors.  Please run scripts/checkpatch.pl on your patches in the future.

Please see below for the result.

Rik, did you test this as well?  If so, may I also have your Tested-by?

							Thanx, Paul

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

commit c95a158356397844a5a6deb0bd58758084f891df
Author: Ani Sinha <ani@arista.com>
Date:   Thu Dec 17 17:15:10 2015 -0800

    sysrq: Fix 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>
    Reviewed-by: Rik van Riel <riel@redhat.com>
    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
index 5381a728d23e..e5139402e7f8 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;