160 lines
6.4 KiB
ReStructuredText
160 lines
6.4 KiB
ReStructuredText
|
=====================
|
||
|
|
The errseq_t datatype
|
||
|
|
=====================
|
||
|
|
|
||
|
|
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 a previous
|
||
|
|
point where it was sampled.
|
||
|
|
|
||
|
|
The initial use case for this is tracking errors for file
|
||
|
|
synchronization syscalls (fsync, fdatasync, msync and sync_file_range),
|
||
|
|
but it may be usable in other situations.
|
||
|
|
|
||
|
|
It's implemented as an unsigned 32-bit value. The low order bits are
|
||
|
|
designated to hold an error code (between 1 and MAX_ERRNO). The upper bits
|
||
|
|
are used as a counter. This is done with atomics instead of locking so that
|
||
|
|
these functions can be called from any context.
|
||
|
|
|
||
|
|
Note that there is a risk of collisions if new errors are being recorded
|
||
|
|
frequently, since we have so few bits to use as a counter.
|
||
|
|
|
||
|
|
To mitigate this, the bit between the error value and counter 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 the last time an error was recorded.
|
||
|
|
|
||
|
|
Thus we end up with a value that looks something like this:
|
||
|
|
|
||
|
|
+--------------------------------------+----+------------------------+
|
||
|
|
| 31..13 | 12 | 11..0 |
|
||
|
|
+--------------------------------------+----+------------------------+
|
||
|
|
| counter | SF | errno |
|
||
|
|
+--------------------------------------+----+------------------------+
|
||
|
|
|
||
|
|
The general idea is for "watchers" to sample an errseq_t value and keep
|
||
|
|
it as a running cursor. That value can later be used to tell whether
|
||
|
|
any new errors have occurred since that sampling was done, and atomically
|
||
|
|
record the state at the time that it was checked. This allows us to
|
||
|
|
record errors in one place, and then have a number of "watchers" that
|
||
|
|
can tell whether the value has changed since they last checked it.
|
||
|
|
|
||
|
|
A new errseq_t should always be zeroed out. An errseq_t value of all zeroes
|
||
|
|
is the special (but common) case where there has never been an error. An all
|
||
|
|
zero value thus serves as the "epoch" if one wishes to know whether there
|
||
|
|
has ever been an error set since it was first initialized.
|
||
|
|
|
||
|
|
API usage
|
||
|
|
=========
|
||
|
|
|
||
|
|
Let me tell you a story about a worker drone. Now, he's a good worker
|
||
|
|
overall, but the company is a little...management heavy. He has to
|
||
|
|
report to 77 supervisors today, and tomorrow the "big boss" is coming in
|
||
|
|
from out of town and he's sure to test the poor fellow too.
|
||
|
|
|
||
|
|
They're all handing him work to do -- so much he can't keep track of who
|
||
|
|
handed him what, but that's not really a big problem. The supervisors
|
||
|
|
just want to know when he's finished all of the work they've handed him so
|
||
|
|
far and whether he made any mistakes since they last asked.
|
||
|
|
|
||
|
|
He might have made the mistake on work they didn't actually hand him,
|
||
|
|
but he can't keep track of things at that level of detail, all he can
|
||
|
|
remember is the most recent mistake that he made.
|
||
|
|
|
||
|
|
Here's our worker_drone representation::
|
||
|
|
|
||
|
|
struct worker_drone {
|
||
|
|
errseq_t wd_err; /* for recording errors */
|
||
|
|
};
|
||
|
|
|
||
|
|
Every day, the worker_drone starts out with a blank slate::
|
||
|
|
|
||
|
|
struct worker_drone wd;
|
||
|
|
|
||
|
|
wd.wd_err = (errseq_t)0;
|
||
|
|
|
||
|
|
The supervisors come in and get an initial read for the day. They
|
||
|
|
don't care about anything that happened before their watch begins::
|
||
|
|
|
||
|
|
struct supervisor {
|
||
|
|
errseq_t s_wd_err; /* private "cursor" for wd_err */
|
||
|
|
spinlock_t s_wd_err_lock; /* protects s_wd_err */
|
||
|
|
}
|
||
|
|
|
||
|
|
struct supervisor su;
|
||
|
|
|
||
|
|
su.s_wd_err = errseq_sample(&wd.wd_err);
|
||
|
|
spin_lock_init(&su.s_wd_err_lock);
|
||
|
|
|
||
|
|
Now they start handing him tasks to do. Every few minutes they ask him to
|
||
|
|
finish up all of the work they've handed him so far. Then they ask him
|
||
|
|
whether he made any mistakes on any of it::
|
||
|
|
|
||
|
|
spin_lock(&su.su_wd_err_lock);
|
||
|
|
err = errseq_check_and_advance(&wd.wd_err, &su.s_wd_err);
|
||
|
|
spin_unlock(&su.su_wd_err_lock);
|
||
|
|
|
||
|
|
Up to this point, that just keeps returning 0.
|
||
|
|
|
||
|
|
Now, the owners of this company are quite miserly and have given him
|
||
|
|
substandard equipment with which to do his job. Occasionally it
|
||
|
|
glitches and he makes a mistake. He sighs a heavy sigh, and marks it
|
||
|
|
down::
|
||
|
|
|
||
|
|
errseq_set(&wd.wd_err, -EIO);
|
||
|
|
|
||
|
|
...and then gets back to work. The supervisors eventually poll again
|
||
|
|
and they each get the error when they next check. Subsequent calls will
|
||
|
|
return 0, until another error is recorded, at which point it's reported
|
||
|
|
to each of them once.
|
||
|
|
|
||
|
|
Note that the supervisors can't tell how many mistakes he made, only
|
||
|
|
whether one was made since they last checked, and the latest value
|
||
|
|
recorded.
|
||
|
|
|
||
|
|
Occasionally the big boss comes in for a spot check and asks the worker
|
||
|
|
to do a one-off job for him. He's not really watching the worker
|
||
|
|
full-time like the supervisors, but he does need to know whether a
|
||
|
|
mistake occurred while his job was processing.
|
||
|
|
|
||
|
|
He can just sample the current errseq_t in the worker, and then use that
|
||
|
|
to tell whether an error has occurred later::
|
||
|
|
|
||
|
|
errseq_t since = errseq_sample(&wd.wd_err);
|
||
|
|
/* submit some work and wait for it to complete */
|
||
|
|
err = errseq_check(&wd.wd_err, since);
|
||
|
|
|
||
|
|
Since he's just going to discard "since" after that point, he doesn't
|
||
|
|
need to advance it here. He also doesn't need any locking since it's
|
||
|
|
not usable by anyone else.
|
||
|
|
|
||
|
|
Serializing errseq_t cursor updates
|
||
|
|
===================================
|
||
|
|
|
||
|
|
Note that the errseq_t API does not protect the errseq_t cursor during a
|
||
|
|
check_and_advance_operation. Only the canonical error code is handled
|
||
|
|
atomically. In a situation where more than one task might be using the
|
||
|
|
same errseq_t cursor at the same time, it's important to serialize
|
||
|
|
updates to that cursor.
|
||
|
|
|
||
|
|
If that's not done, then it's possible for the cursor to go backward
|
||
|
|
in which case the same error could be reported more than once.
|
||
|
|
|
||
|
|
Because of this, it's often advantageous to first do an errseq_check to
|
||
|
|
see if anything has changed, and only later do an
|
||
|
|
errseq_check_and_advance after taking the lock. e.g.::
|
||
|
|
|
||
|
|
if (errseq_check(&wd.wd_err, READ_ONCE(su.s_wd_err)) {
|
||
|
|
/* su.s_wd_err is protected by s_wd_err_lock */
|
||
|
|
spin_lock(&su.s_wd_err_lock);
|
||
|
|
err = errseq_check_and_advance(&wd.wd_err, &su.s_wd_err);
|
||
|
|
spin_unlock(&su.s_wd_err_lock);
|
||
|
|
}
|
||
|
|
|
||
|
|
That avoids the spinlock in the common case where nothing has changed
|
||
|
|
since the last time it was checked.
|
||
|
|
|
||
|
|
Functions
|
||
|
|
=========
|
||
|
|
|
||
|
|
.. kernel-doc:: lib/errseq.c
|