Messages in this thread | | | Subject | Re: [PATCH] locking/lockdep: Zap lock classes even with lock debugging disabled | From | Bart Van Assche <> | Date | Wed, 03 Apr 2019 08:48:02 -0700 |
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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?
Hi Will,
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.
> > 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.
Thanks,
Bart.
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