Messages in this thread | | | Date | Wed, 3 Apr 2019 18:33:04 +0100 | From | Will Deacon <> | Subject | Re: [PATCH] locking/lockdep: Zap lock classes even with lock debugging disabled |
| |
On Wed, Apr 03, 2019 at 08:48:02AM -0700, Bart Van Assche wrote: > On Wed, 2019-04-03 at 13:44 +0100, Will Deacon wrote: > > On Tue, Mar 26, 2019 at 12:59:12PM -0700, Bart Van Assche wrote: > > > Commit a0b0fd53e1e6 ("locking/lockdep: Free lock classes that are no longer > > > in use") changed the behavior of lockdep_free_key_range() from > > > unconditionally zapping lock classes into only zapping lock classes if > > > debug_lock == true. Since the new behavior can cause cat /proc/lockdep to > > > crash due to a NULL pointer dereference, restore the pre-v5.1 behavior. > > > > Can you elaborate on this NULL dereference please, and why this patch fixes > > it? > > Not zapping lock classes if debug_lock == false leaves dangling pointers in > several lockdep datastructures, e.g. lock_class::name in the all_lock_classes > list. The shell command "cat /proc/lockdep" causes the kernel to iterate the > all_lock_classes list. Hence the "unable to handle kernel paging request" > issue that Shenghui encountered by running cat /proc/lockdep. Please let me > know if you would like me to repost this patch with a more detailed > description.
That would help me, at least (maybe with the crash log). Thanks.
> > > Cc: Thomas Gleixner <tglx@linutronix.de> > > > Cc: Will Deacon <will.deacon@arm.com> > > > Cc: Waiman Long <longman@redhat.com> > > > Cc: shenghui <shhuiw@foxmail.com> > > > Reported-by: shenghui <shhuiw@foxmail.com> > > > Fixes: a0b0fd53e1e6 ("locking/lockdep: Free lock classes that are no longer in use") # v5.1-rc1. > > > Signed-off-by: Bart Van Assche <bvanassche@acm.org> > > > --- > > > kernel/locking/lockdep.c | 23 ++++++----------------- > > > 1 file changed, 6 insertions(+), 17 deletions(-) > > > > > > diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c > > > index 34cdcbedda49..70480e4f8f5d 100644 > > > --- a/kernel/locking/lockdep.c > > > +++ b/kernel/locking/lockdep.c > > > @@ -4689,8 +4689,7 @@ static void free_zapped_rcu(struct rcu_head *ch) > > > return; > > > > > > raw_local_irq_save(flags); > > > - if (!graph_lock()) > > > - goto out_irq; > > > + arch_spin_lock(&lockdep_lock); > > > > This also throws out the recursion counting. Is that ok? > > I think that that's OK. My understanding is that lockdep keeps track of > recursion to avoid that lockdep_lock is locked recursively. However, none > of the functions modified by this patch are called with that lock held.
Might be worth adding a comment to that effect, so people don't change that in future.
Will
| |