vn_lock_pair(): only assert on lkflags1/lkflags2 vp1/vp2 is non-NULL

[jah]

Summary

This slightly simplifies some upcoming unionfs work.

[kib]

May I ask why? This allows to pass any flags for null vnodes.

[jah]

In D44046#1004867, @kib wrote:

May I ask why? This allows to pass any flags for null vnodes.

I have some forthcoming unionfs work that (at least in its current implementation) uses vn_lock_pair() in a helper function that takes the locking flags as parameters, and in which the second vnode may be NULL. In general it seems odd that, even when a vnode is NULL, the caller still needs to pass some contrived lock flags for it to appease the assert.

Specifically, 0, which would seem like a sensible default if there is no corresponding vnode to lock, will not work due to the SHARED^EXCLUSIVE assert.

[kib]

So might be just allow zero flags if corresponding vp is NULL?

[jah]

In D44046#1004890, @kib wrote:

So might be just allow zero flags if corresponding vp is NULL?

Sure, we could do that, but I'm curious: is there some reason why we should care what the lockflags are if there is no vnode to lock? What I have here seems more straightforward than making specific allowances for NULL vnodes.

[kib]

In D44046#1004891, @jah wrote:

In D44046#1004890, @kib wrote:

So might be just allow zero flags if corresponding vp is NULL?

Sure, we could do that, but I'm curious: is there some reason why we should care what the lockflags are if there is no vnode to lock? What I have here seems more straightforward than making specific allowances for NULL vnodes.

I am about the reliable checking for the API contracts. Assume that some function calls vn_lock_pair() with externally-specified flags, and corresponding vp could be NULL sometimes. I want such calls to always have correct flags, esp. if vp != NULL is rare or could not be easily checked by normal testing.

[jah]

In D44046#1004894, @kib wrote:

In D44046#1004891, @jah wrote:

In D44046#1004890, @kib wrote:

So might be just allow zero flags if corresponding vp is NULL?

Sure, we could do that, but I'm curious: is there some reason why we should care what the lockflags are if there is no vnode to lock? What I have here seems more straightforward than making specific allowances for NULL vnodes.

I am about the reliable checking for the API contracts. Assume that some function calls vn_lock_pair() with externally-specified flags, and corresponding vp could be NULL sometimes. I want such calls to always have correct flags, esp. if vp != NULL is rare or could not be easily checked by normal testing.

Ok, I can buy that.

[jah]

Only allow lkflags to be 0 when the corresponding vnode is NULL