On Thu, Sep 20, 2018 at 4:31 AM Greg KH <[email protected]> wrote: > > On Wed, Sep 19, 2018 at 10:59:51AM -0700, Nick Desaulniers wrote: > > On Tue, Sep 18, 2018 at 5:32 PM Matthias Kaehlcke <[email protected]> wrote: > > > > > > 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 */ > > > > The comment probably could have stayed as is; folks will have to get > > context from git blame on the line immediately below now; while you > > added context in the patch file, it's below the line so wont be part > > of the commit message. > > > > > rcu_read_unlock(); > > > - panic_on_oops = 1; /* force panic */ > > > - wmb(); > > > - *killer = 1; > > > + > > > + panic("sysrq triggered crash\n"); > > > > Otherwise this part looks good. Maybe GKH can apply just this part > > rather than a v2 > > (if we even care about git blame on comments)?
Greg, if you don't care about the git blame of these comments, the patch is good to go. > > Reviewed-by: Nick Desaulniers <[email protected]> > > I can't pick and choose parts of a patch to apply, sorry. Please fix > this up properly and resend it in the format that it should be applied > in. > > thanks, > > greg k-h -- Thanks, ~Nick Desaulniers
