tree 94d77844f4c2b5732386590fa29ed8d75f8cf9eb
parent 6b3690bc6ae44c60677d55c6a82b459f76b91e30
author Willy Tarreau <w@1wt.eu> 1548607287 +0100
committer Willy Tarreau <w@1wt.eu> 1548684184 +0100
encoding latin1

BUG/MINOR: task: fix possibly missed event in inter-thread wakeups

There's a very small but existing uncertainty window when waking another
thread up where it is possible for task_wakeup() not to wake the other
task up because it's still running while this once is in the process of
finishing and loses its TASK_RUNNING flag. In this case the wakeup will
be missed.

The problem is that we have a single flag to store 3 states, since the
transition from running to sleeping isn't atomic. Thus we need to have
another flag to cover this part. This patch introduces TASK_QUEUED to
mention that the task is already in the run queue, running or not. This
bit will be removed while TASK_RUNNING is kept once dequeued, and will
be used when removing TASK_RUNNING to check if the task has been requeued.

It might be possible to slightly improve this but the occurrence rate
is quite low and we don't really need to complexify the scheduler to
optimize for a rare case.

The impact with the current code is very low since we have few inter-
thread wakeups. Most of them are caused by checks killing sessions.

This must be backported to 1.9.