Date: Mon, 14 Feb 2022 15:08:03 UTC
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=261198 --- Comment #9 from commit-hook@FreeBSD.org --- A commit in branch main references this bug: URL: https://cgit.FreeBSD.org/src/commit/?id=893be9d8ac161c4cc96e9f3f12f1260355dd123b commit 893be9d8ac161c4cc96e9f3f12f1260355dd123b Author: Mark Johnston <markj@FreeBSD.org> AuthorDate: 2022-02-14 14:38:53 +0000 Commit: Mark Johnston <markj@FreeBSD.org> CommitDate: 2022-02-14 15:06:47 +0000 sleepqueue: Address a lock order reversal After commit 74cf7cae4d22 ("softclock: Use dedicated ithreads for running callouts."), there is a lock order reversal between the per-CPU callout lock and the scheduler lock. softclock_thread() locks callout lock then the scheduler lock, when preparing to switch off-CPU, and sleepq_remove_thread() stops the timed sleep callout while potentially holding a scheduler lock. In the latter case, it's the thread itself that's locked, and if the thread is sleeping then its lock will be a sleepqueue lock, but if it's still in the process of going to sleep it'll be a scheduler lock. We could perhaps change softclock_thread() to try to acquire locks in the opposite order, but that'd require dropping and re-acquiring the callout lock, which seems expensive for an operation that will happen quite frequently. We can instead perhaps avoid stopping the td_slpcallout callout if the thread is still going to sleep, which is what this patch does. This will result in a spurious call to sleepq_timeout(), but some counters suggest that this is very rare. PR: 261198 Fixes: 74cf7cae4d22 ("softclock: Use dedicated ithreads for running callouts.") Reported and tested by: thj Reviewed by: kib Sponsored by: The FreeBSD Foundation Differential Revision: https://reviews.freebsd.org/D34204 sys/kern/subr_sleepqueue.c | 11 ++++++++++- 1 file changed, 10 insertions(+), 1 deletion(-) -- You are receiving this mail because: You are the assignee for the bug.