CVS commit: src/sys/kern

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

Module Name:    src
Committed By:   riastradh
Date:           Sun Mar 20 00:19:11 UTC 2022

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

Log Message:
entropy(9): Avoid reentrance to per-CPU state from sleeping on lock.

Changing the global entropy lock from IPL_VM to IPL_SOFTSERIAL meant
it went from being a spin lock, which blocks preemption, to being an
adaptive lock, which might sleep -- and allow other threads to run
concurrently with the softint, even if those threads have softints
blocked with splsoftserial.

This manifested as KASSERT(!ec->ec_locked) triggering in
entropy_consolidate_xc -- presumably entropy_softintr slept on the
global entropy lock while holding the per-CPU state locked with
ec->ec_locked, and then entropy_consolidate_xc ran.

Instead, to protect access to the per-CPU state without taking a
global lock, defer entropy_account_cpu until after ec->ec_locked is
cleared.  This way, we never sleep while holding ec->ec_locked, nor
do we incur any contention on shared memory when entering entropy
unless we're about to distribute it.  To verify this, sprinkle in
assertions that curlwp->l_ncsw hasn't changed by the time we release
ec->ec_locked.


To generate a diff of this commit:
cvs rdiff -u -r1.41 -r1.42 src/sys/kern/kern_entropy.c

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