svn commit: r310423 - head/sys/kern

[John Baldwin]

On Thursday, December 22, 2016 05:51:44 PM Mark Johnston wrote:
> Author: markj
> Date: Thu Dec 22 17:51:44 2016
> New Revision: 310423
> URL: https://svnweb.freebsd.org/changeset/base/310423
> 
> Log:
>   Revert part of r300109.
>   
>   The removal of TAILQ_FOREACH_SAFE introduced a small race: when the last
>   thread on a sleepqueue is awoken, it reclaims the sleepqueue and may begin
>   executing on a different CPU before sleepq_resume_thread() returns. This
>   leaves a window during which it may go back to sleep and incorrectly be
>   awoken again by the caller of sleepq_broadcast().

This is very subtle.  The issue is that the last sleepq_resume_thread transfers
ownership of 'sq' from the wait channel that the sleepq_broadcast has locked,
to the thread being resumed.  I thought about using a local TAILQ_HEAD and
using TAILQ_CONCAT to move the list of threads out of the sleep queue and then
walking that list.  However, a comment explaining this transfer of ownership
(and that we can't safely access 'sq' after the last thread is resumed) is
probably sufficient (but necessary I think).  Do you feel like adding one?

-- 
John Baldwin