Is this telling me that I'm no longer allowed to take a read_lock() in
a callback for an HRTIMER_MODE_ABS_HARD timer? Is that intentional?

If I must, I can probably cope with this by using read_trylock()
instead. The object being locked is a cache, and we opportunistically
try to use it from the fast path but fall back to a slow path in
process context which will revalidate and try again. So if someone
*has* taken the write lock, it's a fairly safe bet that the cache is
going to be invalidated and we were going to take the slow path anyway.

[   62.336965] =============================
[   62.336973] [ BUG: Invalid wait context ]
[   62.336992] 6.7.0-rc1+ #1437 Tainted: G          I       
[   62.337001] -----------------------------
[   62.337008] qemu-system-x86/1935 is trying to lock:
[   62.337017] ffffc900018fecc0 (&gpc->lock){....}-{3:3}, at: kvm_xen_set_evtchn_fast+0xe7/0x460 [kvm]
[   62.337133] other info that might help us debug this:
[   62.337142] context-{2:2}
[   62.337148] 2 locks held by qemu-system-x86/1935:
[   62.337156]  #0: ffff888108f780b0 (&vcpu->mutex){+.+.}-{4:4}, at: kvm_vcpu_ioctl+0x7f/0x730 [kvm]
[   62.337239]  #1: ffffc900018ff2d8 (&kvm->srcu){.?.+}-{0:0}, at: kvm_xen_set_evtchn_fast+0xcd/0x460 [kvm]
[   62.337339] stack backtrace:
[   62.337346] CPU: 7 PID: 1935 Comm: qemu-system-x86 Tainted: G          I        6.7.0-rc1+ #1437
[   62.337370] Hardware name: Intel Corporation S2600CW/S2600CW, BIOS SE5C610.86B.01.01.0008.021120151325 02/11/2015
[   62.337384] Call Trace:
[   62.337390]  <IRQ>
[   62.337395]  dump_stack_lvl+0x57/0x90
[   62.337407]  __lock_acquire+0x7bb/0xbb0
[   62.337416]  ? __lock_acquire+0x4f0/0xbb0
[   62.337425]  lock_acquire.part.0+0xad/0x240
[   62.337433]  ? kvm_xen_set_evtchn_fast+0xe7/0x460 [kvm]
[   62.337512]  ? rcu_is_watching+0xd/0x40
[   62.337520]  ? lock_acquire+0xf2/0x110
[   62.337529]  __raw_read_lock_irqsave+0x4e/0xa0
[   62.337538]  ? kvm_xen_set_evtchn_fast+0xe7/0x460 [kvm]
[   62.337604]  kvm_xen_set_evtchn_fast+0xe7/0x460 [kvm]
[   62.337669]  ? kvm_xen_set_evtchn_fast+0xcd/0x460 [kvm]
[   62.337734]  xen_timer_callback+0x86/0xc0 [kvm]
[   62.337799]  ? __pfx_xen_timer_callback+0x10/0x10 [kvm]
[   62.337863]  __hrtimer_run_queues+0x1b2/0x380
[   62.337874]  hrtimer_interrupt+0xfe/0x240
[   62.337883]  __sysvec_apic_timer_interrupt+0x85/0x1d0
[   62.337893]  sysvec_apic_timer_interrupt+0x99/0xc0
[   62.337905]  </IRQ>
[   62.337910]  <TASK>
[   62.337914]  asm_sysvec_apic_timer_interrupt+0x16/0x20
[   62.337925] RIP: 0010:vcpu_enter_guest.constprop.0+0x9c5/0x1240 [kvm]
[   62.337994] Code: 00 00 48 89 df e8 9b e2 12 00 48 83 bb 38 08 00 00 00 0f 85 ac 01 00 00 c6 83 32 0b 00 00 01 e8 c1 e9 8a e0 fb 0f 1f 44 00 00 <48> 83 83 c0 1c 00 00 01 9c 58 0f 1f 40 00 48 89 c2 fa 0f 1f 44 00
[   62.338241] RSP: 0018:ffffc90001923d70 EFLAGS: 00000206
[   62.338465] RAX: 000000000002ce1d RBX: ffff888108f78000 RCX: 0000000000000040
[   62.338692] RDX: 0000000000000000 RSI: ffffffff82569339 RDI: ffffffff824f5def
[   62.338919] RBP: ffffc90001923de0 R08: 0000000000000001 R09: 0000000000000000
[   62.339172] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
[   62.339396] R13: 0000000000000001 R14: 0000000000000000 R15: ffff888108f78000
[   62.339617]  ? vcpu_run+0x40/0x430 [kvm]
[   62.339888]  vcpu_run+0x40/0x430 [kvm]
[   62.340173]  kvm_arch_vcpu_ioctl_run+0x1da/0x680 [kvm]
[   62.340443]  kvm_vcpu_ioctl+0x1a1/0x730 [kvm]
[   62.340702]  ? __fget_files+0xbc/0x190
[   62.340915]  __x64_sys_ioctl+0x93/0xd0
[   62.341125]  do_syscall_64+0x36/0xb0
[   62.341335]  entry_SYSCALL_64_after_hwframe+0x6e/0x76
[   62.341544] RIP: 0033:0x7f180b200ecd
[   62.341745] Code: 04 25 28 00 00 00 48 89 45 c8 31 c0 48 8d 45 10 c7 45 b0 10 00 00 00 48 89 45 b8 48 8d 45 d0 48 89 45 c0 b8 10 00 00 00 0f 05 <89> c2 3d 00 f0 ff ff 77 1a 48 8b 45 c8 64 48 2b 04 25 28 00 00 00
[   62.342178] RSP: 002b:00007f1802bb1560 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
[   62.342398] RAX: ffffffffffffffda RBX: 00005601bc1b8530 RCX: 00007f180b200ecd
[   62.342616] RDX: 0000000000000000 RSI: 000000000000ae80 RDI: 000000000000000c
[   62.342832] RBP: 00007f1802bb15b0 R08: 00005601ba5a27d0 R09: 0000000000000000
[   62.343046] R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
[   62.343256] R13: 0000000000000022 R14: 0000000000000007 R15: 00007ffeac3c08a0
[   62.343463]  </TASK>