linux-ext4 - Re: Is ext4_dio_read_iter() broken?

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Message-ID: <20200312104239.GA13235@lst.de>
Date:   Thu, 12 Mar 2020 11:42:39 +0100
From:   Christoph Hellwig <hch@....de>
To:     David Howells <dhowells@...hat.com>
Cc:     mbobrowski@...browski.org, darrick.wong@...cle.com, jack@...e.cz,
        hch@....de, linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: Is ext4_dio_read_iter() broken? - and xfs_file_dio_aio_read()

On Thu, Mar 12, 2020 at 10:07:57AM +0000, David Howells wrote:
> David Howells <dhowells@...hat.com> wrote:
> 
> > Is ext4_dio_read_iter() broken?  It calls:
> > 
> > 	file_accessed(iocb->ki_filp);
> > 
> > at the end of the function - but surely iocb should be expected to have been
> > freed when iocb->ki_complete() was called?

The iocb is refcounted and only completed when the refcount hits zero,
and an extra reference is held until the submission has completed.
Take a look at iocb_put().

> I think it's actually worse than that.  You also can't call
> inode_unlock_shared(inode) because you no longer own a ref on the inode since
> ->ki_complete() is expected to call fput() on iocb->ki_filp.

the file reference also hold an inode reference.

> 
> Yes, you own a shared lock on it, but unless somewhere along the
> fput-dput-iput chain the inode lock is taken exclusively, the inode can be
> freed whilst you're still holding the lock.
> 
> Oh - and ext4_dax_read_iter() is also similarly broken.

In addition to that DAX never executes asynchronously.

> And xfs_file_dio_aio_read() appears to be broken as it touches the inode after
> calling iomap_dio_rw() to unlock it.

Same as above.