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+The errseq_t datatype
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+=====================
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+An errseq_t is a way of recording errors in one place, and allowing any
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+number of "subscribers" to tell whether it has changed since a previous
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+point where it was sampled.
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+
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+The initial use case for this is tracking errors for file
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+synchronization syscalls (fsync, fdatasync, msync and sync_file_range),
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+but it may be usable in other situations.
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+
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+It's implemented as an unsigned 32-bit value. The low order bits are
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+designated to hold an error code (between 1 and MAX_ERRNO). The upper bits
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+are used as a counter. This is done with atomics instead of locking so that
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+these functions can be called from any context.
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+
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+Note that there is a risk of collisions if new errors are being recorded
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+frequently, since we have so few bits to use as a counter.
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+
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+To mitigate this, the bit between the error value and counter is used as
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+a flag to tell whether the value has been sampled since a new value was
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+recorded. That allows us to avoid bumping the counter if no one has
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+sampled it since the last time an error was recorded.
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+
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+Thus we end up with a value that looks something like this::
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+
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+ bit: 31..13 12 11..0
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+ +-----------------+----+----------------+
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+ | counter | SF | errno |
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+ +-----------------+----+----------------+
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+
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+The general idea is for "watchers" to sample an errseq_t value and keep
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+it as a running cursor. That value can later be used to tell whether
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+any new errors have occurred since that sampling was done, and atomically
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+record the state at the time that it was checked. This allows us to
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+record errors in one place, and then have a number of "watchers" that
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+can tell whether the value has changed since they last checked it.
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+
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+A new errseq_t should always be zeroed out. An errseq_t value of all zeroes
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+is the special (but common) case where there has never been an error. An all
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+zero value thus serves as the "epoch" if one wishes to know whether there
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+has ever been an error set since it was first initialized.
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+
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+API usage
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+=========
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+Let me tell you a story about a worker drone. Now, he's a good worker
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+overall, but the company is a little...management heavy. He has to
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+report to 77 supervisors today, and tomorrow the "big boss" is coming in
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+from out of town and he's sure to test the poor fellow too.
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+
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+They're all handing him work to do -- so much he can't keep track of who
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+handed him what, but that's not really a big problem. The supervisors
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+just want to know when he's finished all of the work they've handed him so
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+far and whether he made any mistakes since they last asked.
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+
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+He might have made the mistake on work they didn't actually hand him,
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+but he can't keep track of things at that level of detail, all he can
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+remember is the most recent mistake that he made.
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+
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+Here's our worker_drone representation::
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+
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+ struct worker_drone {
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+ errseq_t wd_err; /* for recording errors */
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+ };
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+
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+Every day, the worker_drone starts out with a blank slate::
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+
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+ struct worker_drone wd;
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+
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+ wd.wd_err = (errseq_t)0;
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+
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+The supervisors come in and get an initial read for the day. They
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+don't care about anything that happened before their watch begins::
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+
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+ struct supervisor {
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+ errseq_t s_wd_err; /* private "cursor" for wd_err */
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+ spinlock_t s_wd_err_lock; /* protects s_wd_err */
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+ }
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+
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+ struct supervisor su;
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+
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+ su.s_wd_err = errseq_sample(&wd.wd_err);
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+ spin_lock_init(&su.s_wd_err_lock);
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+
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+Now they start handing him tasks to do. Every few minutes they ask him to
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+finish up all of the work they've handed him so far. Then they ask him
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+whether he made any mistakes on any of it::
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+
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+ spin_lock(&su.su_wd_err_lock);
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+ err = errseq_check_and_advance(&wd.wd_err, &su.s_wd_err);
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+ spin_unlock(&su.su_wd_err_lock);
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+
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+Up to this point, that just keeps returning 0.
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+
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+Now, the owners of this company are quite miserly and have given him
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+substandard equipment with which to do his job. Occasionally it
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+glitches and he makes a mistake. He sighs a heavy sigh, and marks it
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+down::
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+
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+ errseq_set(&wd.wd_err, -EIO);
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+
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+...and then gets back to work. The supervisors eventually poll again
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+and they each get the error when they next check. Subsequent calls will
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+return 0, until another error is recorded, at which point it's reported
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+to each of them once.
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+
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+Note that the supervisors can't tell how many mistakes he made, only
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+whether one was made since they last checked, and the latest value
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+recorded.
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+
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+Occasionally the big boss comes in for a spot check and asks the worker
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+to do a one-off job for him. He's not really watching the worker
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+full-time like the supervisors, but he does need to know whether a
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+mistake occurred while his job was processing.
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+
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+He can just sample the current errseq_t in the worker, and then use that
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+to tell whether an error has occurred later::
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+
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+ errseq_t since = errseq_sample(&wd.wd_err);
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+ /* submit some work and wait for it to complete */
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+ err = errseq_check(&wd.wd_err, since);
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+
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+Since he's just going to discard "since" after that point, he doesn't
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+need to advance it here. He also doesn't need any locking since it's
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+not usable by anyone else.
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+
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+Serializing errseq_t cursor updates
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+===================================
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+Note that the errseq_t API does not protect the errseq_t cursor during a
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+check_and_advance_operation. Only the canonical error code is handled
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+atomically. In a situation where more than one task might be using the
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+same errseq_t cursor at the same time, it's important to serialize
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+updates to that cursor.
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+
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+If that's not done, then it's possible for the cursor to go backward
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+in which case the same error could be reported more than once.
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+
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+Because of this, it's often advantageous to first do an errseq_check to
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+see if anything has changed, and only later do an
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+errseq_check_and_advance after taking the lock. e.g.::
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+
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+ if (errseq_check(&wd.wd_err, READ_ONCE(su.s_wd_err)) {
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+ /* su.s_wd_err is protected by s_wd_err_lock */
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+ spin_lock(&su.s_wd_err_lock);
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+ err = errseq_check_and_advance(&wd.wd_err, &su.s_wd_err);
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+ spin_unlock(&su.s_wd_err_lock);
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+ }
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+
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+That avoids the spinlock in the common case where nothing has changed
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+since the last time it was checked.
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Jeff Layton