linux-kernel - Re: [RFC PATCH] vfs: add inode lockdep assertions

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Date:   Wed, 6 Sep 2023 16:20:07 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Mateusz Guzik <mjguzik@...il.com>
Cc:     brauner@...nel.org, viro@...iv.linux.org.uk,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        bernd.schubert@...tmail.fm
Subject: Re: [RFC PATCH] vfs: add inode lockdep assertions

On Thu, Aug 31, 2023 at 05:14:14PM +0200, Mateusz Guzik wrote:
> +++ b/include/linux/fs.h
> @@ -842,6 +842,16 @@ static inline void inode_lock_shared_nested(struct inode *inode, unsigned subcla
>  	down_read_nested(&inode->i_rwsem, subclass);
>  }
>  
> +static inline void inode_assert_locked(struct inode *inode)
> +{
> +	lockdep_assert_held(&inode->i_rwsem);
> +}
> +
> +static inline void inode_assert_write_locked(struct inode *inode)
> +{
> +	lockdep_assert_held_write(&inode->i_rwsem);
> +}

This mirrors what we have in mm, but it's only going to trigger on
builds that have lockdep enabled.  Lockdep is very expensive; it
easily doubles the time it takes to run xfstests on my laptop, so
I don't generally enable it.  So what we also have in MM is:

static inline void mmap_assert_write_locked(struct mm_struct *mm)
{
        lockdep_assert_held_write(&mm->mmap_lock);
        VM_BUG_ON_MM(!rwsem_is_locked(&mm->mmap_lock), mm);
}

Now if you have lockdep enabled, you get the lockdep check which
gives you all the lovely lockdep information, but if you don't, you
at least get the cheap check that someone is holding the lock at all.

ie I would make this:

+static inline void inode_assert_write_locked(struct inode *inode)
+{
+	lockdep_assert_held_write(&inode->i_rwsem);
+	WARN_ON_ONCE(!inode_is_locked(inode));
+}

Maybe the locking people could give us a rwsem_is_write_locked()
predicate, but until then, this is the best solution we came up with.