linux-kernel - Re: [rfc][possible solution] RCU vfsmounts

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Message-ID: <20130929060601.GL13318@ZenIV.linux.org.uk>
Date:	Sun, 29 Sep 2013 07:06:01 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [rfc][possible solution] RCU vfsmounts

On Sat, Sep 28, 2013 at 01:43:49PM -0700, Linus Torvalds wrote:

> Sounds reasonable to to me.

Sigh...  Looks like there's a lot of fun in shrink_dcache_for_umount() -
at the very least, it needs to bump ->d_seq on everything, because with
that change we *can* walk into a filesystem in the middle of that.
We obviously don't want to slap rcu_barrier() into the final mntput() -
it's far too costly; even one in deactivate_locked_super() (in the
wrong place and gone since a while back) had been causing problems.

Moreover, any filesystem that has e.g. ->d_hash() use an object hanging
off private data of superblock and freed by its ->kill_sb() before
generic_shutdown_super() will have an additional set of PITA; there
shouldn't be many of those, though.

Oh, well - this is going to be a fun series, by the look of it...
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