Re: possible deadlock in send_sigio

[Waiman Long]

On 6/15/20 12:49 PM, Matthew Wilcox wrote:
> On Fri, Jun 12, 2020 at 03:01:01PM +0800, Boqun Feng wrote:
> > On the archs using QUEUED_RWLOCKS, read_lock() is not always a recursive
> > read lock, actually it's only recursive if in_interrupt() is true. So
> > change the annotation accordingly to catch more deadlocks.
> [...]
>
> > +#ifdef CONFIG_LOCKDEP
> > +/*
> > + * read_lock() is recursive if:
> > + * 1. We force lockdep think this way in selftests or
> > + * 2. The implementation is not queued read/write lock or
> > + * 3. The locker is at an in_interrupt() context.
> > + */
> > +static inline bool read_lock_is_recursive(void)
> > +{
> > +	return force_read_lock_recursive ||
> > +	       !IS_ENABLED(CONFIG_QUEUED_RWLOCKS) ||
> > +	       in_interrupt();
> > +}
> I'm a bit uncomfortable with having the _lockdep_ definition of whether
> a read lock is recursive depend on what the _implementation_ is.
> The locking semantics should be the same, no matter which architecture
> you're running on.  If we rely on read locks being recursive in common
> code then we have a locking bug on architectures which don't use queued
> rwlocks.
>
> I don't know whether we should just tell the people who aren't using
> queued rwlocks that they have a new requirement or whether we should
> say that read locks are never recursive, but having this inconsistency
> is not a good idea!

Actually, qrwlock is more restrictive. It is possible that systems with 
qrwlock may hit deadlock which doesn't happens in other systems that use 
recursive rwlock. However, the current lockdep code doesn't detect those 
cases.

Changing lockdep to only use qrwlock semantics can be problematic as the 
code hunk in locking selftest is due to the fact that it assumes 
recursive lock. So we need to change that. Anyway, this patch can allow 
us to see if current qrwlock semantics may have potential deadlock 
problem in the current code. I actually have bug report about deadlock 
due to qrwlock semantics in RHEL7. So I would certainly like to see if 
the current upstream code may have also this kind of problem.

Cheers,
Longman