On Tue, May 09, 2017 at 11:49:16AM -0400, Jeff Layton wrote:

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()?