linux-kernel - Re: [PATCH v23 13/22] vfs: Cache richacl in struct inode

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Date:	Thu, 07 Jul 2016 15:14:13 +0100
From:	David Howells <dhowells@...hat.com>
To:	Jeff Layton <jlayton@...hat.com>,
	Andreas Gruenbacher <agruenba@...hat.com>,
	Alexander Viro <viro@...iv.linux.org.uk>
Cc:	dhowells@...hat.com, Christoph Hellwig <hch@...radead.org>,
	"Theodore Ts'o" <tytso@....edu>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Trond Myklebust <trond.myklebust@...marydata.com>,
	Anna Schumaker <anna.schumaker@...app.com>,
	Dave Chinner <david@...morbit.com>, linux-ext4@...r.kernel.org,
	xfs@....sgi.com, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-nfs@...r.kernel.org,
	linux-cifs@...r.kernel.org, linux-api@...r.kernel.org
Subject: Re: [PATCH v23 13/22] vfs: Cache richacl in struct inode

Jeff Layton <jlayton@...hat.com> wrote:

> > +	if (cmpxchg(&inode->i_acl, ACL_NOT_CACHED, sentinel) != ACL_NOT_CACHED)
> > +		/* fall through */ ;
> > +
> 
> So you do the same thing regardless of the outcome of the above? Why
> bother with the if at all here? Just do the cmpxchg and toss out the
> result.

gcc might complain if you don't check the result.

However, this does look like it's subject to a thundering herd problem.  If
30000 processes all look at the ACL at the same time on a network fs, could
that cause 30000 RPC calls to be transmitted for the same thing?

David