Add a way to distinguish between forking and thread creation in schedtail.

[ed]

Summary

For CloudABI we need to initialize the registers of new threads
differently based on whether the thread got created through a fork or
through simple thread creation.

Add a flag, TDP_FORKING, that is set by do_fork() and cleared by
fork_exit(). This can be tested against in schedtail.

Test Plan

This seems to work all right for CloudABI.

[kib]

You might conisder adding assert to the syscallret() that TDP_FORKING is not seen there. The assert cannot be added to userret(), unfortunately.

sys/kern/kern_fork.c

474 ↗

(On Diff #9601)

td2->td_pflags is modified at line 589, you may or TDP_FORKING there as well.

[ed]

In D3973#82745, @kib wrote:

You might conisder adding assert to the syscallret() that TDP_FORKING is not seen there. The assert cannot be added to userret(), unfortunately.

That sounds like a good idea!

Out of curiosity, why isn't this possible to do in userret()? Is it because traps may occur while the process is being forked?

[ed]

Add an assertion to make sure TDP_FORKING is never left set.

[kib]

In D3973#82755, @ed wrote:

Out of curiosity, why isn't this possible to do in userret()? Is it because traps may occur while the process is being forked?

The call chain is fork_exit()->callout == fork_return()->userret(), with the TDP_FORKING flag still set.