linux-kernel - Re: [PATCH v3 17/68] fscache: Implement simple cookie state machine

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Date:   Fri, 17 Dec 2021 19:45:26 +0000
From:   David Howells <dhowells@...hat.com>
To:     Jeff Layton <jlayton@...nel.org>
Cc:     dhowells@...hat.com, linux-cachefs@...hat.com,
        Trond Myklebust <trondmy@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Steve French <sfrench@...ba.org>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        Matthew Wilcox <willy@...radead.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Omar Sandoval <osandov@...ndov.com>,
        JeffleXu <jefflexu@...ux.alibaba.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-afs@...ts.infradead.org, linux-nfs@...r.kernel.org,
        linux-cifs@...r.kernel.org, ceph-devel@...r.kernel.org,
        v9fs-developer@...ts.sourceforge.net,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 17/68] fscache: Implement simple cookie state machine

Jeff Layton <jlayton@...nel.org> wrote:

> > +	case FSCACHE_COOKIE_STATE_RELINQUISHING:
> > +	case FSCACHE_COOKIE_STATE_WITHDRAWING:
> > +		if (cookie->cache_priv) {
> > +			spin_unlock(&cookie->lock);
> > +			cookie->volume->cache->ops->withdraw_cookie(cookie);
> > +			spin_lock(&cookie->lock);
> > +		}
> > +
> > +		switch (state) {
> > +		case FSCACHE_COOKIE_STATE_RELINQUISHING:
> > +			fscache_see_cookie(cookie, fscache_cookie_see_relinquish);
> > +			fscache_unhash_cookie(cookie);
> > +			__fscache_set_cookie_state(cookie,
> > +						   FSCACHE_COOKIE_STATE_DROPPED);
> > +			wake = true;
> > +			goto out;
> > +		case FSCACHE_COOKIE_STATE_WITHDRAWING:
> > +			fscache_see_cookie(cookie, fscache_cookie_see_withdraw);
> > +			break;
> > +		default:
> > +			BUG();
> > +		}
> > +
> 
> Ugh, the nested switch here is a bit hard to follow. It makes it seem
> like the state could change due to the withdraw_cookie and you're
> checking it again, but it doesn't do that.
> 
> This would be clearer if you just duplicated the withdraw_cookie stanza
> for both states and moved the stuff below here to a helper or to a new
> goto block.

There are actually three states, but one's added in a later patch.  It
probably does make sense to split the RELINQ state from the other two.

David