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Date:   Thu, 31 Aug 2023 23:49:20 -0700
From:   Christoph Hellwig <hch@infradead.org>
To:     Mateusz Guzik <mjguzik@gmail.com>
Cc:     Bernd Schubert <bschubert@ddn.com>, linux-fsdevel@vger.kernel.org,
        bernd.schubert@fastmail.fm, miklos@szeredi.hu, dsingh@ddn.com,
        Josef Bacik <josef@toxicpanda.com>,
        linux-btrfs@vger.kernel.org,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Christian Brauner <brauner@kernel.org>
Subject: Re: [PATCH 0/2] Use exclusive lock for file_remove_privs
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On Thu, Aug 31, 2023 at 12:18:24PM +0200, Mateusz Guzik wrote:
> Turns out notify_change has the following:
>         WARN_ON_ONCE(!inode_is_locked(inode));
> 
> Which expands to:
> static inline int rwsem_is_locked(struct rw_semaphore *sem)
> {
>         return atomic_long_read(&sem->count) != 0;
> }
> 
> So it does check the lock, except it passes *any* locked state,
> including just readers.
> 
> According to git blame this regressed from commit 5955102c9984
> ("wrappers for ->i_mutex access") by Al -- a bunch of mutex_is_locked
> were replaced with inode_is_locked, which unintentionally provides
> weaker guarantees.
> 
> I don't see a rwsem helper for wlock check and I don't think it is all
> that beneficial to add. Instead, how about a bunch of lockdep, like so:

Yes, that's a good idea.