Re: possible deadlock in send_sigio

[Matthew Wilcox <willy@infradead.org>]

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!