quoted

Hi,

when using a signal handler as a way to context switch between
different usercontexts a reproducible exception occurs on my rpi3 in
64-bit mode. (https://gist.github.com/DanGe42/7148946)

Running the context_demo program as a 32-bit ARM executable on a
64-bit kernel is OK, running as a 32 || 64 bit executable on an x86
kernel is OK.

In the first exception the PC doesn?t look correct, and the *pmd is 0.
The 2nd exception happens after running the program again, the PC is 0x0.

A successful function trace was not possible -> complete kernel hangup
when enabling.

Is there another way to gather more information about what is happening?

I can reproduce Segmentation fault with your program on Marvell berlin SoCs
my kernel version is 4.1, I didn't tested 4.9, 4.10-rc1 etc..

Then I increased the STACKSIZE from 4096 to 8192 in context_demo.c,
everything works fine now. Maybe arm64 need a bit larger signalstack?

yes, increased STACKSIZE to 8192 helps on 4.9/4,10-rc1 but after a
while the exception still occurs, although the message is different.
The *pmd is not 0 in this case.

to trigger this scenario:
- INTERVAL set to 500 [ns]
- kernel with maxcpus=0
- start a 'find /' command in the shell in parallel of the program
- stdout, stderr > redirected to file.

[  850.581983] a.out[173]: unhandled level 3 permission fault (11) at
0x004391f0, esr 0x8200000f
[  850.591833] pgd = ffffffc039311000
[  850.596725] [004391f0] *pgd=0000000039340003
[  850.602145] , *pud=0000000039340003
[  850.608352] , *pmd=000000003922c003
[  850.611963] , *pte=00e80000359c0f53
[  850.618111]
[  850.621102]
[  850.624032] CPU: 0 PID: 173 Comm: a.out Not tainted 4.9.0-v8+ #5
[  850.631314] Hardware name: Raspberry Pi 3 Model B (DT)
[  850.637925] task: ffffffc039a13100 task.stack: ffffffc039a14000
[  850.645314] PC is at 0x4391f0
[  850.649783] LR is at 0x4391f0
[  850.654035] pc : [<00000000004391f0>] lr : [<00000000004391f0>]
pstate: 60000000
[  850.662920] sp : 0000000000420da0
[  850.667516] x29: 00000000004391f0 x28: 0000000000000000
[  850.677145] x27: 0000000000000000 x26: 0000000000000000

When I taskset the context_demo program to other cores that are
completely isolated (CONFIG_NO_HZ_FULL, isolcpus=1,2,3) it will run
continuously with the modified STACKSIZE.

regards
Bas