UBSAN: Undefined behaviour in kernel/rcu/tree_plugin.h in 4.20-rc1

[Christophe LEROY]

Paul,

I get the following UBSAN reports in 4.20-rc1 on an MPC8321E 
(powerpc/book3s/32)

I bisected it to 3e31009898699dfc ("rcu: Defer reporting RCU-preempt 
quiescent states when disabled")

Thanks
Christophe

[    4.919995] 
================================================================================
[    4.928428] UBSAN: Undefined behaviour in kernel/rcu/tree_plugin.h:623:28
[    4.935198] signed integer overflow:
[    4.938766] 0 - -2147483648 cannot be represented in type 'int'
[    4.944678] CPU: 0 PID: 119 Comm: mkdir Not tainted 
4.19.0-rc1-s3k-dev-00005-g5a60513 #214
[    4.952908] Call Trace:
[    4.955382] [dec4fd20] [c02cb0d0] ubsan_epilogue+0x18/0x74 (unreliable)
[    4.962003] [dec4fd30] [c02cb5e0] handle_overflow+0xd0/0xe0
[    4.967588] [dec4fdb0] [c007b424] rcu_preempt_deferred_qs+0xc0/0xc8
[    4.973857] [dec4fdd0] [c007be28] rcu_note_context_switch+0x74/0x608
[    4.980217] [dec4fe10] [c064b790] __schedule+0x58/0x6e0
[    4.985448] [dec4fe50] [c064bfdc] preempt_schedule_common+0x48/0x9c
[    4.991717] [dec4fe70] [c01308c8] handle_mm_fault+0x10fc/0x1ecc
[    4.997639] [dec4fee0] [c001339c] do_page_fault+0x10c/0x760
[    5.003225] [dec4ff40] [c001234c] handle_page_fault+0x14/0x40
[    5.008968] --- interrupt: 401 at 0xff9cff8
[    5.008968]     LR = 0xfeefd78
[    5.016170] 
================================================================================
[    5.024591] 
================================================================================
[    5.033005] UBSAN: Undefined behaviour in kernel/rcu/tree_plugin.h:627:28
[    5.039775] signed integer overflow:
[    5.043342] -2147483648 + -2147483648 cannot be represented in type 'int'
[    5.050118] CPU: 0 PID: 119 Comm: mkdir Not tainted 
4.19.0-rc1-s3k-dev-00005-g5a60513 #214
[    5.058348] Call Trace:
[    5.060813] [dec4fd20] [c02cb0d0] ubsan_epilogue+0x18/0x74 (unreliable)
[    5.067433] [dec4fd30] [c02cb5e0] handle_overflow+0xd0/0xe0
[    5.073014] [dec4fdb0] [c007b408] rcu_preempt_deferred_qs+0xa4/0xc8
[    5.079283] [dec4fdd0] [c007be28] rcu_note_context_switch+0x74/0x608
[    5.085640] [dec4fe10] [c064b790] __schedule+0x58/0x6e0
[    5.090871] [dec4fe50] [c064bfdc] preempt_schedule_common+0x48/0x9c
[    5.097139] [dec4fe70] [c01308c8] handle_mm_fault+0x10fc/0x1ecc
[    5.103059] [dec4fee0] [c001339c] do_page_fault+0x10c/0x760
[    5.108642] [dec4ff40] [c001234c] handle_page_fault+0x14/0x40
[    5.114385] --- interrupt: 401 at 0xff9cff8
[    5.114385]     LR = 0xfeefd78
[    5.121588] 
================================================================================