kern/58167: unnecessary mutex_tryenter in tap(4)

[campbell+netbsd%mumble.net@localhost]

>Number:         58167
>Category:       kern
>Synopsis:       unnecessary mutex_tryenter in tap(4)
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Wed Apr 17 18:50:01 +0000 2024
>Originator:     Taylor R Campbell
>Release:        current, 10, 9, 8, ...
>Organization:
The TryTAP Foundation
>Environment:
>Description:
    875 	/* In the TAP_NBIO case, we have to make sure we won't be sleeping */
    876 	if ((sc->sc_flags & TAP_NBIO) != 0) {
    877 		if (!mutex_tryenter(&sc->sc_lock))
    878 			return EWOULDBLOCK;
    879 	} else
    880 		mutex_enter(&sc->sc_lock);

https://nxr.netbsd.org/xref/src/sys/net/if_tap.c#875

This lock is only used to serialize access to the packet queue, not to serialize operations that sleep for I/O or anything -- it's a spin lock anyway (though I'm not sure there's any reason for it to be).  So there is no need to make tap_dev_read flakier and harder to understand by using mutex_tryenter.
>How-To-Repeat:
code inspection
>Fix:
just use mutex_enter unconditionally