fusefs: Fix vnode lock mismatch in fuse_internal_do_getattr()

[guest-seuros]

Summary

fuse_internal_cache_attrs() requires an exclusive vnode lock
(ASSERT_VOP_ELOCKED), but fuse_internal_do_getattr() can be called
with only a shared lock from the read path:

fuse_read_biobackend() [LK_SHARED]
 => fuse_vnode_size()
    -> fuse_internal_do_getattr()
      => fuse_internal_cache_attrs() [needs LK_EXCLUSIVE]

This caused lock assertion panics during read operations.

Test Plan

The panic was happening when i tried to write files to an usb exfat.

[asomers]

Thanks for your attention in this area. As a matter of fact, I've been working on this exact same problem. My first attempt at a solution was close to this one, but ultimately it won't work. Today I'm going to try a different solution, based on mutex protection rather than the vnode lock. I'll update Bugzilla when I have a working patch, so you can subscribe to this bug to find out.
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=283391

[guest-seuros]

Sure i can close this diff.

But can you tell me what won't work ? I use 16-Current and have INVARIANTS

after this patch, i could transfer 50GB without any issue to the usb, checked checksum and all.

The system is stable for many days now.

[asomers]

In D55145#1260918, @guest-seuros wrote:

Sure i can close this diff.

But can you tell me what won't work ? I use 16-Current and have INVARIANTS

There are other code paths that also need to modify the cached attributes, but where it isn't possible to upgrade the vnode lock to exclusive.

[asomers]

See https://reviews.freebsd.org/D55230 for a more complete solution.