[Bug 296291] cuse(3): cuse_server_free( ) busy-waits unkillably (pause("W")) when a c lient ref is not released on virtual_oss te ardown — process stuck in D, cuse.ko pinned , reboot required

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=296291

            Bug ID: 296291
           Summary: cuse(3): cuse_server_free() busy-waits unkillably
                    (pause("W")) when a client ref   is not released on
                    virtual_oss teardown — process stuck in D, cuse.ko
                    pinned, reboot required
           Product: Base System
           Version: 15.1-RELEASE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs@FreeBSD.org
          Reporter: delleceste@gmail.com

Every time virtual_oss(8) is stopped, its exit path busy-waits in the kernel
for
  its cuse clients to drop their references — a routine multi-second stall.
  INTERMITTENTLY a client reference is never released, so the wait never ends:
the
  exiting process is left wedged forever in an uninterruptible, SIGKILL-immune
  state, cuse.ko becomes pinned (cannot be unloaded), and no new virtual_oss
can
  recreate the devices. Only a reboot recovers the machine. The proximate
severity
  is that the kernel wait is unbounded and uninterruptible (a plain
pause("W")), so
  any reference leak becomes permanent.


ENVIRONMENT
    FreeBSD 15.1-RELEASE releng/15.1-n283562-96841ea08dcf GENERIC amd64
    Base components: sys/fs/cuse/cuse.c (cuse.ko) and usr.sbin/virtual_oss.
    Setup: a loopback audio chain.
      virtual_oss -i 8 -C 2 -c 2 -b 32 -s 200ms -f /dev/null -a 0 \
                  -d dsp.play -L dsp.loop
    MPD opens /dev/dsp.play (cuse client); BruteFIR opens /dev/dsp.loop (cuse
    client) and writes the USB DAC.

  OBSERVED
      $ ps -o pid,ppid,stat,wchan,mwchan,command -p 5992
       PID PPID STAT WCHAN MWCHAN COMMAND
      5992 5988 D<E   W     W      virtual_oss ... -d dsp.play -L dsp.loop
    - D = uninterruptible kernel sleep, < = realtime prio (-i 8), E = exiting.
    - kill -TERM and kill -9 have NO effect (thread is past signal delivery).
    - cuse.ko is pinned; "kldunload cuse" then also hangs in D.
    - A fresh virtual_oss dies with "Could not create CUSE DSP device".
    - No userland process holds /dev/dsp.play or /dev/dsp.loop open at this
point
      (procstat -f sweep) — the leaked reference is purely kernel-side.
    - Only a reboot clears it; every virtual_oss stop/restart is a reboot risk.

  ROOT CAUSE (kernel)
    MWCHAN "W" pinpoints cuse_server_free(), the cdevpriv destructor for the
    daemon's /dev/cuse fd, run when the daemon exits (sys/fs/cuse/cuse.c:765):

      static void
      cuse_server_free(void *arg)
      {
          struct cuse_server *pcs = arg;
          while (cuse_server_do_close(pcs) != 1)   /* returns pcs->refs */
              pause("W", hz);                        /* <-- MWCHAN "W" */
          cuse_server_unref(pcs);
      }

    The loop busy-waits (1s ticks) until pcs->refs == 1, i.e. until every
client
    handle is released. pcs->refs is ++'d per client open (cuse.c:1506,
    cuse_client_open) and dropped in the client cdevpriv destructor
    cuse_client_free() -> cuse_server_unref() (cuse.c:1480). If a client ref is
    never dropped, this loop never terminates and is not interruptible (plain
    pause(), no signal/timeout escape) -> the exiting process is stuck in D
forever.

    A mitigation exists but is escaped: cuse_server_do_close() sets
pcs->is_closing
    and wakes all clients (cuse.c:752-755), and
cuse_client_receive_command_locked()
    returns CUSE_ERR_OTHER when is_closing is set (cuse.c:631). The comment
above
    the loop ("the client cdevpriv destructor, which cannot destroy itself")
points
    at the self-referential case, exactly the loopback (-L/-l) device, where
    virtual_oss is both server and client of the same cuse server.

  EMPIRICAL EVIDENCE (instrumented cuse.ko + DTrace)
    A diagnostic cuse.ko logging pcs->refs inside the loop, over many
start/stop
    cycles of the real chain (MPD + BruteFIR clients):

    1. EVERY teardown enters the loop with refs > 1 (it is the normal stop
path):
          cuse: server pid 11027 exit stuck: refs=3 (want 1) after 1 s
          cuse: server pid 11036 exit stuck: refs=2 (want 1) after 1 s
       refs=3 = server + BruteFIR(dsp.loop) + MPD(dsp.play); refs=2 = server +
1.

    2. DTrace lifecycle (fbt:cuse:cuse_client_open/free, cuse_server_free, by
       execname/pid):
          musicpd[2278]    OPEN
          brutefir[9705]   OPEN
          virtual_oss[9697] SERVER_FREE begin (teardown wait)
          ... loop spins ...
          brutefir[9705]   FREE   (cuse_client_free -> cuse_server_unref)
          musicpd[2278]    FREE
          (refs reaches 1, server exits)

    3. USUAL outcome: clients close within ~1-6 s and the server RECOVERS — a
       routine multi-second busy-wait on every stop (e.g. pid 11036 logged
       "after 1 s" then "after 6 s" before recovering).

    4. SEVERE/INTERMITTENT outcome: a client ref never drops, the loop spins
       forever, virtual_oss is stuck D<E (SIGKILL-immune), cuse.ko pinned,
       reboot-only (pid 5992).

    So is_closing DOES release clients in the common case; the permanent hang
is an
    unlucky-timing race where a client close cannot be serviced/completed. This
is
    consistent with virtual_oss's teardown: usr.sbin/virtual_oss/.../main.c
    destroy_threads() (main.c:2498) pthread_cancel()s the cuse worker threads
    without pthread_join(), the workers block in cuse_wait_and_process() (an
    ioctl(2), NOT a POSIX cancellation point, main.c:1990), and only the CTL
device
    is cuse_dev_destroy()'d on exit (main.c:2630) — the DSP/WAV/loopback
devices
    (created at main.c:1918/1935) are never destroyed. On unlucky timing the
close
    command for a client thus has no server thread left to process it, and the
    unbounded uninterruptible pause("W") turns that transient leak into a
    reboot-only hang.

  REPRODUCE
    1. virtual_oss -i 8 -C 2 -c 2 -b 32 -s 200ms -f /dev/null -a 0 \
                   -d dsp.play -L dsp.loop
    2. Open clients on the devices (BruteFIR reading /dev/dsp.loop S32_LE 2ch,
       and/or MPD playing to /dev/dsp.play).
    3. Stop virtual_oss (kill, killall, or stop consumers then virtual_oss).
    4. Intermittently it never exits: ps shows "D<E  W"; kill -9 no effect;
cuse.ko
       cannot be unloaded; a fresh virtual_oss cannot recreate the devices.
    Frequency rises with rapid start/stop churn and a client attached at stop
time.

  IMPACT
    Unkillable process; pinned kernel module; reboot is the only recovery. Any
    virtual_oss-based audio routing can be bricked until reboot by a normal
    stop/restart (e.g. a sample-rate change).

  SUGGESTED FIX
    Kernel (sys/fs/cuse/cuse.c): cuse_server_free() must not busy-wait
    unboundedly/uninterruptibly. Bound the wait (timeout + forced reclamation),
or
    restructure teardown so a closing server forcibly invalidates client refs
that
    can no longer be serviced (the is_closing wakeup does not cover the stuck
case,
    notably the loopback/self-client).

    virtual_oss (usr.sbin/virtual_oss/.../main.c): on exit, cuse_dev_destroy()
ALL
    created devices (DSP, WAV, loopback), not just CTL, before tearing down
threads;
    and pthread_join() the workers instead of pthread_cancel()+free(). A
candidate
    userland patch doing this (store each cuse_dev handle in the profile;
destroy
    all devices before exit) has been written and compile-tested — it makes the
    client refs drop before process exit so cuse_server_free() finds refs==1
and
    never busy-waits. That fixes the trigger; the kernel pause("W") hardening
is
    still wanted so a future ref leak cannot become an unrecoverable hang.

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