[PATCH RFC] kexec: Freeze processes before kexec

["Joel Fernandes (Google)" <joel@joelfernandes.org>]

During kexec, it is possible for userspace to race with
device_shutdown() causing accesses to GPU after pm_runtime suspend has
already happened. Fix by freezing userspace before device_shutdown().

Such freezing is already being done if kernel supports KEXEC_JUMP and
kexec_image->preserve_context is true. However, doing it if either of
these are not true prevents crashes/races.

This fixes the following crash during kexec reboot on an ARM64 device
with adreno GPU:

[  292.534314] Kernel panic - not syncing: Asynchronous SError Interrupt
[  292.534323] Hardware name: Google Lazor (rev3 - 8) with LTE (DT)
[  292.534326] Call trace:
[  292.534328]  dump_backtrace+0x0/0x1d4
[  292.534337]  show_stack+0x20/0x2c
[  292.534342]  dump_stack_lvl+0x60/0x78
[  292.534347]  dump_stack+0x18/0x38
[  292.534352]  panic+0x148/0x3b0
[  292.534357]  nmi_panic+0x80/0x94
[  292.534364]  arm64_serror_panic+0x70/0x7c
[  292.534369]  do_serror+0x0/0x7c
[  292.534372]  do_serror+0x54/0x7c
[  292.534377]  el1h_64_error_handler+0x34/0x4c
[  292.534381]  el1h_64_error+0x7c/0x80
[  292.534386]  el1_interrupt+0x20/0x58
[  292.534389]  el1h_64_irq_handler+0x18/0x24
[  292.534395]  el1h_64_irq+0x7c/0x80
[  292.534399]  local_daif_inherit+0x10/0x18
[  292.534405]  el1h_64_sync_handler+0x48/0xb4
[  292.534410]  el1h_64_sync+0x7c/0x80
[  292.534414]  a6xx_gmu_set_oob+0xbc/0x1fc
[  292.534422]  a6xx_get_timestamp+0x40/0xb4
[  292.534426]  adreno_get_param+0x12c/0x1e0
[  292.534433]  msm_ioctl_get_param+0x64/0x70
[  292.534440]  drm_ioctl_kernel+0xe8/0x158
[  292.534448]  drm_ioctl+0x208/0x320
[  292.534453]  __arm64_sys_ioctl+0x98/0xd0
[  292.534461]  invoke_syscall+0x4c/0x118
[  292.534467]  el0_svc_common+0x98/0x104
[  292.534473]  do_el0_svc+0x30/0x80
[  292.534478]  el0_svc+0x20/0x50
[  292.534481]  el0t_64_sync_handler+0x78/0x108
[  292.534485]  el0t_64_sync+0x1a4/0x1a8
[  292.534632] Kernel Offset: 0x1a5f800000 from 0xffffffc008000000
[  292.534635] PHYS_OFFSET: 0x80000000
[  292.534638] CPU features: 0x40018541,a3300e42
[  292.534644] Memory Limit: none

Cc: rostedt@goodmis.org
Cc: ribalda@google.com
Cc: zwisler@google.com
Cc: robdclark@gmail.com
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
 kernel/kexec_core.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
index ca2743f9c634..2614a7f1f8a6 100644
--- a/kernel/kexec_core.c
+++ b/kernel/kexec_core.c
@@ -1175,6 +1175,12 @@ int kernel_kexec(void)
 	} else
 #endif
 	{
+		error = freeze_processes();
+		if (error) {
+			error = -EBUSY;
+			goto Unlock;
+		}
+
 		kexec_in_progress = true;
 		kernel_restart_prepare("kexec reboot");
 		migrate_to_reboot_cpu();
-- 
2.38.1.584.g0f3c55d4c2-goog