linux-kernel - Re: possible deadlock in send

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Date:   Mon, 15 Jun 2020 09:49:02 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     Waiman Long <longman@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        syzbot <syzbot+a9fb1457d720a55d6dc5@...kaller.appspotmail.com>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>, allison@...utok.net,
        areber@...hat.com, aubrey.li@...ux.intel.com,
        Andrei Vagin <avagin@...il.com>,
        Bruce Fields <bfields@...ldses.org>,
        Christian Brauner <christian@...uner.io>, cyphar@...har.com,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>, guro@...com,
        Jeff Layton <jlayton@...nel.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Kees Cook <keescook@...omium.org>, linmiaohe@...wei.com,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...e.com>, Ingo Molnar <mingo@...nel.org>,
        Oleg Nesterov <oleg@...hat.com>, sargun@...gun.me,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: possible deadlock in send_sigio

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!