[PATCH] coresight: etm3x: convert struct etm_drvdata's spinlock to raw_spinlock

[Greg Kroah-Hartman]

On Tue, Jul 11, 2023 at 03:05:36PM +0800, quanyang.wang at windriver.com wrote:
> From: Quanyang Wang <quanyang.wang at windriver.com>
> 
> For PREEMPT_RT kernel, spinlock_t locks become sleepable. The functions
> etm_dying_cpu and etm_starting_cpu which call spin_lock/unlock run in
> an irq-disabled context, this will trigger the following calltrace:
> 
>     BUG: sleeping function called from invalid context at kernel/locking/spinlock_rt.c:46
>     in_atomic(): 1, irqs_disabled(): 128, non_block: 0, pid: 25, name: migration/1
>     preempt_count: 1, expected: 0
>     RCU nest depth: 0, expected: 0
>     1 lock held by migration/1/25:
>      #0: 82a7587c (&drvdata->spinlock){....}-{2:2}, at: etm_dying_cpu+0x28/0x54
>     Preemption disabled at:
>     [<801ec760>] cpu_stopper_thread+0x94/0x120
>     CPU: 1 PID: 25 Comm: migration/1 Not tainted 6.1.35-rt10-yocto-preempt-rt #30
>     Hardware name: Xilinx Zynq Platform
>     Stopper: multi_cpu_stop+0x0/0x174 <- __stop_cpus.constprop.0+0x48/0x88
>      unwind_backtrace from show_stack+0x18/0x1c
>      show_stack from dump_stack_lvl+0x58/0x70
>      dump_stack_lvl from __might_resched+0x14c/0x1c0
>      __might_resched from rt_spin_lock+0x4c/0x84
>      rt_spin_lock from etm_dying_cpu+0x28/0x54
>      etm_dying_cpu from cpuhp_invoke_callback+0x140/0x33c
>      cpuhp_invoke_callback from __cpuhp_invoke_callback_range+0xa4/0x104
>      __cpuhp_invoke_callback_range from take_cpu_down+0x7c/0xa8
>      take_cpu_down from multi_cpu_stop+0x15c/0x174
>      multi_cpu_stop from cpu_stopper_thread+0x9c/0x120
>      cpu_stopper_thread from smpboot_thread_fn+0x31c/0x360
>      smpboot_thread_fn from kthread+0x100/0x124
>      kthread from ret_from_fork+0x14/0x2c
> 
> Convert struct etm_drvdata's spinlock to raw_spinlock to fix it.

wait, why will a raw_spinlock fix this?  Why not fix the root problem
here, that of calling these locks inproperly in irq context?

How is changing to a raw_spinlock going to fix the above splat?

thanks,

greg k-h