CVSROOT: /cvs
Module name: src
Changes by: [email protected] 2020/12/23 18:16:14
Modified files:
sys/kern : kern_synch.c
sys/sys : systm.h
Log message:
tsleep(9): add global "nowake" channel for threads avoiding wakeup(9)
It would be convenient if there were a channel a thread could sleep on
to indicate they do not want any wakeup(9) broadcasts. The easiest way
to do this is to add an "int nowake" to kern_synch.c and extern it in
sys/systm.h. You use it like this:
#include <sys/systm.h>
tsleep_nsec(&nowait, ...);
There is now no need to handroll a local dead channel, e.g.
int chan;
tsleep_nsec(&chan, ...);
which expands the stack. Local dead channels will be replaced with
&nowake in later patches.
One possible problem with this "one global channel" approach is sleep
queue congestion. If you have lots of threads sleeping on &nowake you
might slow down a wakeup(9) on a different channel that hashes into
the same queue. Unsure how much of problem this actually is, if at all.
NetBSD and FreeBSD have a "pause" interface in the kernel that chooses
a suitable channel automatically. To keep things simple and avoid
adding a new interface we will start with this global channel.
Discussed with mpi@, claudio@, kettenis@, and deraadt@.
Basically designed by kettenis@, who vetoed my other proposals.
Bugs caught by deraadt@, tb@, and patrick@.
