sysrq_handle_crash() currently forces a crash by dereferencing a NULL pointer, which is undefined behavior in C. Just call panic() instead, which is simpler and doesn't depend on compiler specific handling of the undefined behavior.
Suggested-by: Greg Kroah-Hartman <[email protected]> Signed-off-by: Matthias Kaehlcke <[email protected]> --- Not sure if it is strictly needed to release the RCU read lock now that panic() is invoked directly (I couldn't repro the warning without rcu_read_unlock()), but since this is a forced crash it seems good practice to keep doing it. The commit that added rcu_read_unlock() and the comment is: commit 984cf355aeaa8f2eda3861b50d0e8d3e3f77e83b Author: Ani Sinha <[email protected]> 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. --- drivers/tty/sysrq.c | 13 +++---------- 1 file changed, 3 insertions(+), 10 deletions(-) diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c index 06ed20dd01ba..d779a51499a0 100644 --- a/drivers/tty/sysrq.c +++ b/drivers/tty/sysrq.c @@ -134,17 +134,10 @@ static struct sysrq_key_op sysrq_unraw_op = { 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. - */ + /* release the RCU read lock before crashing */ rcu_read_unlock(); - panic_on_oops = 1; /* force panic */ - wmb(); - *killer = 1; + + panic("sysrq triggered crash\n"); } static struct sysrq_key_op sysrq_crash_op = { .handler = sysrq_handle_crash, -- 2.19.0.397.gdd90340f6a-goog
