Re: PROBLEM: [REGRESSION 7.1-rc4 -> 7.1-rc5] 9p: silent NUL corruption of file data on cache modes with CACHE_WRITEBACK

[David Howells <dhowells@redhat.com>]

Dominique Martinet <asmadeus@codewreck.org> wrote:

> When v9fs_issue_write() is re-entered, writing the remaining data at the
> original subreq->start offset rather than subreq->start + subreq->transferred
> would overwrite previously written data.

Retry should now be resetting ->transferred to 0, so actually I should remove
adding ->transferred from other filesystems.

> > +		spin_lock(&inode->i_lock);
> > +
> > +		/* We can read the sizes directly as we hold i_lock. */
> > +		i_size = inode->i_size;
> Does reading and updating i_size from this asynchronous writeback path race
> with concurrent truncate operations?

Supposedly, you're not allowed to change i_size unless you hold
i_lock... which we do.

> Also, reading inode->i_size directly without i_size_read() bypasses
> i_size_seqcount, which can cause torn reads on 32-bit platforms.

Which should be irrelevant as we hold i_lock.

David