CVS commit: src/sys/kern

["Masatake Daimon" <pho%netbsd.org@localhost>]

Module Name:    src
Committed By:   pho
Date:           Tue Jun 27 01:15:22 UTC 2023

Modified Files:
        src/sys/kern: kern_timeout.c

Log Message:
callout(9): Fix panic() in callout_destroy() (kern/57226)

The culprit was callout_halt(). "(c->c_flags & CALLOUT_FIRED) != 0" wasn't
the correct way to check if a callout is running. It failed to wait for a
running callout to finish in the following scenario:

1. cpu0 initializes a callout and schedules it.
2. cpu0 invokes callout_softlock() and fires the callout, setting the flag
   CALLOUT_FIRED.
3. The callout invokes callout_schedule() to re-schedule itself.
4. callout_schedule_locked() clears the flag CALLOUT_FIRED, and releases
   the lock.
5. Before the lock is re-acquired by callout_softlock(), cpu1 decides to
   destroy the callout. It first invokes callout_halt() to make sure the
   callout finishes running.
6. But since CALLOUT_FIRED has been cleared, callout_halt() thinks it's not
   running and therefore returns without invoking callout_wait().
7. cpu1 proceeds to invoke callout_destroy() while it's still running on
   cpu0. callout_destroy() detects that and panics.


To generate a diff of this commit:
cvs rdiff -u -r1.73 -r1.74 src/sys/kern/kern_timeout.c

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