[Q] How does linux kernel lockdep record lock-class dependency?
From: Du, Changbin
Date: Thu Mar 15 2018 - 02:46:40 EST
Hello everyone,
I got a warning as below which is a AB-BA deadlock issue. But I don't understand
how the 'existing dependency' happened.
It looks like: kvm_read_guest() held (&mm->mmap_sem), then reading userspace memory
(which is not ready yet) caused page_fault() invoked, then in i915_gem_fault()
it tries to hold (&dev->struct_mutex).
But this sequence must haven't happened. Otherwise, double-lock already happed,
since intel_vgpu_create_workload() has held (&dev->struct_mutex) already:
(&dev->struct_mutex)->(&mm->mmap_sem)->(&dev->struct_mutex)
So how could lockdep find such 'existing dependency'? Thanks!
[ 163.179109] ======================================================
[ 163.185306] WARNING: possible circular locking dependency detected
[ 163.191504] 4.16.0-rc5+ #44 Tainted: G U
[ 163.196655] ------------------------------------------------------
[ 163.202854] qemu-system-x86/4514 is trying to acquire lock:
[ 163.208443] (&mm->mmap_sem){++++}, at: [<00000000bca68632>] __might_fault+0x36/0x80
[ 163.216230]
but task is already holding lock:
[ 163.222090] (&dev->struct_mutex){+.+.}, at: [<00000000a6b81a24>] copy_gma_to_hva+0xe5/0x140 [i915]
[ 163.231205]
which lock already depends on the new lock.
[ 163.239421]
the existing dependency chain (in reverse order) is:
[ 163.246925]
-> #1 (&dev->struct_mutex){+.+.}:
[ 163.252792] i915_mutex_lock_interruptible+0x66/0x170 [i915]
[ 163.259005] i915_gem_fault+0x1e0/0x630 [i915]
[ 163.263985] __do_fault+0x19/0xed
[ 163.267830] __handle_mm_fault+0x9fa/0x1140
[ 163.272550] handle_mm_fault+0x1a7/0x390
[ 163.277006] __do_page_fault+0x286/0x530
[ 163.281462] page_fault+0x45/0x50
[ 163.285307]
-> #0 (&mm->mmap_sem){++++}:
[ 163.290722] __might_fault+0x60/0x80
[ 163.294839] __kvm_read_guest_page+0x3d/0x80 [kvm]
[ 163.300173] kvm_read_guest+0x47/0x80 [kvm]
[ 163.304891] kvmgt_rw_gpa+0x9d/0x110 [kvmgt]
[ 163.309714] intel_gvt_scan_and_shadow_workload+0x1be/0x480 [i915]
[ 163.316448] intel_vgpu_create_workload+0x3d9/0x550 [i915]
[ 163.322488] intel_vgpu_submit_execlist+0xc0/0x2a0 [i915]
[ 163.328440] elsp_mmio_write+0xcb/0x140 [i915]
[ 163.333448] intel_vgpu_mmio_reg_rw+0x250/0x4f0 [i915]
[ 163.339138] intel_vgpu_emulate_mmio_write+0xaa/0x240 [i915]
[ 163.345337] intel_vgpu_rw+0x200/0x250 [kvmgt]
[ 163.350319] intel_vgpu_write+0x164/0x1f0 [kvmgt]
[ 163.355558] __vfs_write+0x33/0x170
[ 163.359580] vfs_write+0xc5/0x1c0
[ 163.363427] SyS_pwrite64+0x90/0xb0
[ 163.367447] do_syscall_64+0x70/0x1c0
[ 163.371642] entry_SYSCALL_64_after_hwframe+0x42/0xb7
[ 163.377230]
other info that might help us debug this:
[ 163.385258] Possible unsafe locking scenario:
[ 163.391196] CPU0 CPU1
[ 163.395737] ---- ----
[ 163.400280] lock(&dev->struct_mutex);
[ 163.404125] lock(&mm->mmap_sem);
[ 163.410062] lock(&dev->struct_mutex);
[ 163.416436] lock(&mm->mmap_sem);
[ 163.419846]
*** DEADLOCK ***
[ 163.425780] 3 locks held by qemu-system-x86/4514:
[ 163.430496] #0: (&gvt->lock){+.+.}, at: [<00000000d313c2c5>] intel_vgpu_emulate_mmio_write+0x64/0x240 [i915]
[ 163.440544] #1: (&dev->struct_mutex){+.+.}, at: [<00000000a6b81a24>] copy_gma_to_hva+0xe5/0x140 [i915]
[ 163.450068] #2: (&kvm->srcu){....}, at: [<00000000db73c4fc>] kvmgt_rw_gpa+0x4c/0x110 [kvmgt]
[ 163.458721]
stack backtrace:
[ 163.463097] CPU: 0 PID: 4514 Comm: qemu-system-x86 Tainted: G U 4.16.0-rc5+ #44
[ 163.471663] Hardware name: Dell Inc. OptiPlex 7040/0Y7WYT, BIOS 1.2.8 01/26/2016
[ 163.479093] Call Trace:
[ 163.481547] dump_stack+0x7c/0xbe
[ 163.484872] print_circular_bug.isra.33+0x21b/0x228
[ 163.489765] __lock_acquire+0xf7d/0x1470
[ 163.493700] ? lock_acquire+0xec/0x1e0
[ 163.497459] lock_acquire+0xec/0x1e0
[ 163.501046] ? __might_fault+0x36/0x80
[ 163.504805] __might_fault+0x60/0x80
[ 163.508389] ? __might_fault+0x36/0x80
[ 163.512155] __kvm_read_guest_page+0x3d/0x80 [kvm]
[ 163.516966] kvm_read_guest+0x47/0x80 [kvm]
[ 163.521161] kvmgt_rw_gpa+0x9d/0x110 [kvmgt]
[ 163.525459] intel_gvt_scan_and_shadow_workload+0x1be/0x480 [i915]
[ 163.531675] intel_vgpu_create_workload+0x3d9/0x550 [i915]
[ 163.537192] intel_vgpu_submit_execlist+0xc0/0x2a0 [i915]
[ 163.542621] elsp_mmio_write+0xcb/0x140 [i915]
[ 163.547093] intel_vgpu_mmio_reg_rw+0x250/0x4f0 [i915]
[ 163.552261] intel_vgpu_emulate_mmio_write+0xaa/0x240 [i915]
[ 163.557938] intel_vgpu_rw+0x200/0x250 [kvmgt]
[ 163.562396] intel_vgpu_write+0x164/0x1f0 [kvmgt]
[ 163.567114] __vfs_write+0x33/0x170
[ 163.570614] ? common_file_perm+0x68/0x250
[ 163.574723] ? security_file_permission+0x36/0xb0
[ 163.579440] vfs_write+0xc5/0x1c0
[ 163.582765] SyS_pwrite64+0x90/0xb0
[ 163.586261] ? trace_hardirqs_off_thunk+0x1a/0x1c
[ 163.590980] do_syscall_64+0x70/0x1c0
[ 163.594653] entry_SYSCALL_64_after_hwframe+0x42/0xb7
[ 163.599720] RIP: 0033:0x7f88649bcda3
[ 163.603303] RSP: 002b:00007f8854d5c6c0 EFLAGS: 00000293 ORIG_RAX: 0000000000000012
[ 163.610896] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f88649bcda3
[ 163.618051] RDX: 0000000000000004 RSI: 00007f8854d5c6f0 RDI: 0000000000000019
[ 163.625206] RBP: 00007f8854d5c710 R08: 0000000000000004 R09: 00000000ffffffff
[ 163.632363] R10: 0000000000002230 R11: 0000000000000293 R12: 0000000000000000
[ 163.639518] R13: 00007ffcc49b9a3f R14: 00007f8854d5d9c0 R15: 0000000000000000
--
Thanks,
Changbin Du