kern: add fget_procdesc()

[kib]

Summary

Convert several callers to use fget_procdesc().
Eliminate procdesc_find() and directly use fget_procdesc() in
sys_pdkill().  Previous code structure required to fdrop() procdesc
while the process is locked.

[markj]

sys/kern/sys_procdesc.c

118–119

Can we also use the new helper here?

[kib]

sys/kern/sys_procdesc.c

118–119

The current structure makes the fdrop() call under the process lock. I eliminated procdesc_find() altogether.

[kib]

Remove procdesc_find(). Use fget_procdesc() in sys_pdkill().

[kib]

Fix all places that called fdrop() under proctree_lock.
Add herald comment for fget_procdesc().

[kib]

Slightly reduce the diff size.

[markj]

sys/kern/sys_procdesc.c

692

Shouldn't it just be, "If the process return is requested, proctree_lock must be owned"?

723

You can store to *pp before acquiring the lock.

748

After sleeping, how do you know that p hasn't been recycled?

[kib]

Hold target around pddupfd() core.
Simplify the function.

Update assert about proctree_lock in fget_procdesc().

[markj]

sys/kern/sys_procdesc.c

716

So the caller may need to fdrop() even if fget_procdesc() returns an error. Why not just make fget_procdesc() call fdrop() directly in this error case? That makes callers a bit simpler.

For instance there is a bug in sys_pdkill(): if the fget_procdesc()->fget() call fails, sys_pdkill() calls fdrop(NULL).

sys/sys/procdesc.h

115–117

The indentation here looks inconsistent.

[kib]

sys/kern/sys_procdesc.c

692

I think it should be both.

716

Because the drop might need to take the proctree_lock. I updated the comment and fixed pdkill.

[kib]

Check for NULL fp in pdkill.
Update comment explaining why callers must drop *pfp.

[kib]

Convert kern_pdwait()

[kib]

Better conversion for kern_pdwait(), drop the req to lock proctree_lock when the process is not requested.

[markj]

sys/kern/sys_procdesc.c

692

I would combine the last two sentences:

"If requested to return the process, the proctree lock must be held, and the process will be returned locked."

695

766

Extra newline?

[kib]

sys/kern/sys_procdesc.c

766

It was intended. The multi-line comment describes the execve_block+p_candebug() guarantees. After the block for which the comment is applicable, there should be a blank line.