On Tue 09-05-17 11:49:16, Jeff Layton wrote:

An errseq_t is a way of recording errors in one place, and allowing any
number of "subscribers" to tell whether an error has been set again
since a previous time.

It's implemented as an unsigned 32-bit value that is managed with atomic
operations. The low order bits are designated to hold an error code
(max size of MAX_ERRNO). The upper bits are used as a counter.

The API works with consumers sampling an errseq_t value at a particular
point in time. Later, that value can be used to tell whether new errors
have been set since that time.

Note that there is a 1 in 512k risk of collisions here if new errors
are being recorded frequently, since we have so few bits to use as a
counter. To mitigate this, one bit is used as a flag to tell whether the
value has been sampled since a new value was recorded. That allows
us to avoid bumping the counter if no one has sampled it since it
was last bumped.

Later patches will build on this infrastructure to change how writeback
errors are tracked in the kernel.

Signed-off-by: Jeff Layton <redacted>

The patch looks good to me. Feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

Just two nits below:
...

+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.
+	 */

I'm not sure which locking you are speaking about here. Is the comment
stale?

+	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

"bumping the seq count anyway" part is not quite true. Writer may see
ERRSEQ_SEEN not set and so just update the error code and leave seq count
as is. But since you compare full errseq_t for equality, this works out as
designed...

+		 * 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);
+	}
+	return err;
+}
+EXPORT_SYMBOL(errseq_check_and_advance);

								Honza
-- 
Jan Kara [off-list ref]
SUSE Labs, CR