Return-Path: Received: from bombadil.infradead.org ([65.50.211.133]:37253 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751430AbdEJOSX (ORCPT ); Wed, 10 May 2017 10:18:23 -0400 Date: Wed, 10 May 2017 07:18:21 -0700 From: Matthew Wilcox To: Jeff Layton Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-btrfs@vger.kernel.org, linux-ext4@vger.kernel.org, linux-cifs@vger.kernel.org, linux-nfs@vger.kernel.org, linux-mm@kvack.org, jfs-discussion@lists.sourceforge.net, linux-xfs@vger.kernel.org, cluster-devel@redhat.com, linux-f2fs-devel@lists.sourceforge.net, v9fs-developer@lists.sourceforge.net, linux-nilfs@vger.kernel.org, linux-block@vger.kernel.org, dhowells@redhat.com, akpm@linux-foundation.org, hch@infradead.org, ross.zwisler@linux.intel.com, mawilcox@microsoft.com, jack@suse.com, viro@zeniv.linux.org.uk, corbet@lwn.net, neilb@suse.de, clm@fb.com, tytso@mit.edu, axboe@kernel.dk, josef@toxicpanda.com, hubcap@omnibond.com, rpeterso@redhat.com, bo.li.liu@oracle.com Subject: Re: [PATCH v4 13/27] lib: add errseq_t type and infrastructure for handling it Message-ID: <20170510141821.GB1590@bombadil.infradead.org> References: <20170509154930.29524-1-jlayton@redhat.com> <20170509154930.29524-14-jlayton@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <20170509154930.29524-14-jlayton@redhat.com> Sender: linux-nfs-owner@vger.kernel.org List-ID: On Tue, May 09, 2017 at 11:49:16AM -0400, Jeff Layton wrote: > +++ b/lib/errseq.c > @@ -0,0 +1,199 @@ > +#include > +#include > +#include > +#include > + > +/* > + * An errseq_t is a way of recording errors in one place, and allowing any > + * number of "subscribers" to tell whether it has changed since an arbitrary > + * time of their choosing. You use the word "time" in several places in the documentation, but I think it's clearer to say "sampling point" or "sample", since you're not using jiffies or nanoseconds. For example, I'd phrase this paragraph this way: * An errseq_t is a way of recording errors in one place, and allowing any * number of "subscribers" to tell whether it has changed since they last * sampled it. > + * The general idea is for consumers to sample an errseq_t value at a > + * particular point in time. Later, that value can be used to tell whether any > + * new errors have occurred since that time. * The general idea is for consumers to sample an errseq_t value. That * value can be used to tell whether any new errors have occurred since * the last time it was sampled. > +/* The "ones" bit for the counter */ Maybe "The lowest bit of the counter"? > +/** > + * errseq_check - has an error occurred since a particular point in time? "has an error occurred since the last time it was sampled" > +/** > + * errseq_check_and_advance - check an errseq_t and advance it to the current value > + * @eseq: pointer to value being checked reported "value being checked reported"? > +int errseq_check_and_advance(errseq_t *eseq, errseq_t *since) > +{ > + int err = 0; > + errseq_t old, new; > + > + /* > + * Most callers will want to use the inline wrapper to check this, > + * so that the common case of no error is handled without needing > + * to lock. > + */ > + old = READ_ONCE(*eseq); > + if (old != *since) { > + /* > + * Set the flag and try to swap it into place if it has > + * changed. > + * > + * We don't care about the outcome of the swap here. If the > + * swap doesn't occur, then it has either been updated by a > + * writer who is bumping the seq count anyway, or another > + * reader who is just setting the "seen" flag. Either outcome > + * is OK, and we can advance "since" and return an error based > + * on what we have. > + */ > + new = old | ERRSEQ_SEEN; > + if (new != old) > + cmpxchg(eseq, old, new); > + *since = new; > + err = -(new & MAX_ERRNO); > + } I probably need to read through the patchset some more to understand this. Naively, surely "since" should be updated to the current value of 'eseq' if we failed the cmpxchg()?