Re: [GIT PULL] 9p updates for 5.1

From: Dominique Martinet
Date: Sun Mar 17 2019 - 20:04:22 EST

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.