On Mon, Aug 25, 2025 at 02:32:38PM +0200, Christian Brauner wrote:
> On Mon, Aug 25, 2025 at 05:43:05AM +0100, Al Viro wrote:
> > mount_writer: write_seqlock; that's an equivalent of {un,}lock_mount_hash()
> > mount_locked_reader: read_seqlock_excl; these tend to be open-coded.
>
> Do we really need the "locked" midfix in there? Doesn't seem to buy any
> clarity. I'd drop it so the naming is nicely consistent.
It's a seqlock. "Readers" is this context are lockless ones - sample/retry under
rcu_read_lock() kind. The only difference between writer and locked reader is
that locked reader does not disrupt those sample/retry loops.
Note that for something that is never traversed locklessly (expiry lists,
lists of children, etc.) locked reader is fine for all accesses, including
modifications.
If you have better suggestions re terminology, I'd love to hear those, but
simply "writer"/"reader" is misleadingly similar to rw-semaphors/links/whatnot.
Basically, there are 3 kinds of contexts here:
1) lockless, must be under RCU, fairly limited in which pointers they
can traverse, read-only access to structures in question. Must sample
the seqcount side of mount_lock first, then verifying that it has not changed
after everything.
2) hold the spinlock side of mount_lock, _without_ bumping the seqcount
one. Can be used for reads and writes, as long as the stuff being modified
is not among the things that is traversed locklessly. Do not disrupt the previous
class, have full exclusion with calles 2 and 3
3) hold the spinlock side of mount_lock, and bump the seqcount one on
entry and leave. Any reads and writes. Full exclusion with classes 2 and 3,
invalidates the checks for class 1 (i.e. will push it into retries/fallbacks/
whatnot).
I'm used to "lockless reader" for 1, "writer" for 3. "locked reader" kinda
works for 2 - that's what it is wrt things that can be accessed by lockless
readers, but for the things that are *not* traversed without a lock it
can be actually used as a less disruptive form of 3. Is used that way in
mount locking for some of the data structures.
