On Wed, Jul 10, 2019 at 09:15:49PM +0200, Sebastian Andrzej Siewior wrote: > On 2019-07-10 17:50:54 [+0100], Will Deacon wrote: > > On Wed, Jul 03, 2019 at 11:21:26AM +0200, Sebastian Andrzej Siewior wrote: > > > An uninitialized/ zeroed mutex will go unnoticed because there is no > > > check for it. There is a magic check in the unlock's slowpath path which > > > might go unnoticed if the unlock happens in the fastpath. > > > > > > Add a ->magic check early in the mutex_lock() and mutex_trylock() path. > > > > > > Signed-off-by: Sebastian Andrzej Siewior <bige...@linutronix.de> > > > --- > > > Nothing screamed during uninitialized usage of init_mm's context->lock > > > https://git.kernel.org/tip/32232b350d7cd93cdc65fe5a453e6a40b539e9f9 > > > > > > kernel/locking/mutex.c | 11 ++++++++++- > > > 1 file changed, 10 insertions(+), 1 deletion(-) > > > > > > diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c > > > index 0c601ae072b3f..fb1f6f1e1cc61 100644 > > > --- a/kernel/locking/mutex.c > > > +++ b/kernel/locking/mutex.c > > > @@ -908,6 +908,10 @@ __mutex_lock_common(struct mutex *lock, long state, > > > unsigned int subclass, > > > > > > might_sleep(); > > > > > > +#ifdef CONFIG_DEBUG_MUTEXES > > > + DEBUG_LOCKS_WARN_ON(lock->magic != lock); > > > +#endif > > > > Why do we need to check this so early, or could we move it into > > debug_mutex_lock_common() instead? > > debug_mutex_lock_common() is too late. A few lines later, before > "preempt_disable()" would be possible. After that, there is > __mutex_trylock() which would succeed so you don't catch the > uninitialized case. By the time you get to debug_mutex_lock_common() you > need contention and then acquire ->wait_lock which should complain about > missing magic.
Right you are; thanks for the explanation. I don't see a better approach than what you've done, so: Acked-by: Will Deacon <w...@kernel.org> Will
