Feature #12478
closed
installctx needs kpreempt_disable protection
100%
Description
Upstreaming OS-7096 for bhyve:
Prior to OS-7034, updates to the ctxop list associated with a kthread were simple: The new item would update its next pointer to the value of t_ctx, then it would update t_ctx so it pointed at the new value. If a preemption occurred any time before the t_ctx update, the new item would not be called for save or restore. If it occurred after the t_ctx update, both savectx and restorectx would act upon the new time.
With the change to a doubly-linked list for ctxops, it is no longer safe to do this without kpreempt_disable protection. There is a short window between the prev field of the head item is updated and when t_ctx itself is pointed to the new item that a preemption would result in the savectx operation being skipped for the new item but the restorectx operation executing on it.
A simple application of kpreempt_disable/kpreempt_enable should be all that's needed to protect that window from danger.
I only caught this as I was reviewing ctxop-related code. The problematic behavior described is not something I've seen in the wild.
Related issues
Updated by Dan McDonald over 3 years ago
See #12477 for testing done to confirm this as well.
Updated by Patrick Mooney over 3 years ago
- Related to Feature #12477: ctxops should use stack ordering for save/restore added
Updated by Electric Monk over 3 years ago
- Status changed from In Progress to Closed
- % Done changed from 0 to 100
git commit 6e2e67256d436ef900becfa771aee283e7e55430
commit 6e2e67256d436ef900becfa771aee283e7e55430
Author: Patrick Mooney <pmooney@pfmooney.com>
Date: 2020-04-16T19:20:32.000Z
12477 ctxops should use stack ordering for save/restore
12478 installctx needs kpreempt_disable protection
Reviewed by: John Levon <john.levon@joyent.com>
Reviewed by: Robert Mustacchi <rm@joyent.com>
Reviewed by: Jerry Jelinek <jerry.jelinek@joyent.com>
Reviewed by: Andy Fiddaman <andy@omniosce.org>
Approved by: Dan McDonald <danmcd@joyent.com>
Updated by Dan McDonald over 2 years ago
- Related to Bug #13902: Fix for 13717 may break 8-disk raidz2 added
Updated by Joshua M. Clulow over 2 years ago
- Related to Bug #13908: disable kernel FPU by default until it is stable added
Updated by Dan McDonald over 2 years ago
- Related to Bug #13915: installctx() blocking allocate causes problems added