CVS commit: src/sys

["Taylor R Campbell" <riastradh%netbsd.org@localhost>]

Module Name:    src
Committed By:   riastradh
Date:           Mon Oct  3 19:57:06 UTC 2022

Modified Files:
        src/sys/dev: cons.c
        src/sys/kern: subr_prf.c tty.c
        src/sys/sys: tty.h

Log Message:
constty(4): Make MP-safe.

Access to the global constty variable is coordinated as follows:

1. Setting constty to nonnull, with atomic_store_release, is allowed
   only under the new adaptive constty_lock in thread context.  This
   serializes TIOCCONS operations and ensures unlocked readers can
   safely use a constty pointer read with atomic_load_consume.

2. Changing constty from nonnull to null, with atomic_cas_ptr, is
   allowed in any context -- printf(9) uses this to disable a broken
   constty.

3. Reading constty under constty_lock is allowed with
   atomic_load_relaxed, because while constty_lock is held, it can
   only be made null by some other thread/CPU, never made nonnull.

4. Reading constty outside constty_lock is allowed with
   atomic_load_consume in a pserialize read section -- constty is
   only ever made nonnull with atomic_store_release, in (1).
   ttyclose will wait for all these pserialize read sections to
   complete before flushing the tty.

5. To continue to use a struct tty pointer in (4) after the
   pserialize read section has completed, caller must use tty_acquire
   during the pserialize read section and then tty_release when done.
   ttyclose will wait for all these references to drain before
   returning.

These access rules allow us to serialize TIOCCONS, and safely destroy
ttys, without putting any locks on the access paths like printf(9)
that use constty.  Once we set D_MPSAFE, operations on /dev/console
will contend only with other users of the same tty as constty, which
will be an improvement over contending with all other kernel lock
users in the system.


To generate a diff of this commit:
cvs rdiff -u -r1.82 -r1.83 src/sys/dev/cons.c
cvs rdiff -u -r1.189 -r1.190 src/sys/kern/subr_prf.c
cvs rdiff -u -r1.301 -r1.302 src/sys/kern/tty.c
cvs rdiff -u -r1.97 -r1.98 src/sys/sys/tty.h

Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.