netdev - Re: [GIT PULL] 9p updates for 5.1

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Message-ID: <20190318000402.GA24768@nautica>
Date:   Mon, 18 Mar 2019 01:04:02 +0100
From:   Dominique Martinet <asmadeus@...ewreck.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     v9fs-developer@...ts.sourceforge.net,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>
Subject: Re: [GIT PULL] 9p updates for 5.1

Linus Torvalds wrote on Sun, Mar 17, 2019:
> Hmm. I wonder what makes it valid to have concurrent updates to
> i_size? Yes, yes, you added that spinlock to make the update itself
> atomic on 32-bit, but it sounds a bit odd in the first place to have
> two things possibly changing the size of a file at the same time...

If the inode attributes are currently invalid (for example after
v9fs_invalidate_inode_attr()) then two concurrent user getattr requests
for the same inode will send two network requests which can both update
the i_size.

With cache=fscache or loose a write could also be concurrent with such
an update.

I plan on improving the first case with some "being revalidated" logic
now this pattern got reported but I don't think the second one can be
avoided, so that fix is still necessary in the long run afaict.

Thanks,
-- 
Dominique