Re: [RFC PATCH] arm64/mm: hit DEBUG_LOCKS_WARN_ON

[James Morse <james.morse@arm.com>]

Hi Anders,

(CC: +linux-arm-kernel@lists.infradead.org)

On 31/10/2018 12:13, Anders Roxell wrote:
> I'm running an arm64 kernel in a qemu guest and I hit this
> DEBUG_LOCKS_WARN_ON and this patch removes the warning. The
> interrupts_enabled(reqs) should reflect the irq->enabled state of the
> kgdb test process

(I know nothing about kgdb, please forgive the stupid questions:)

Where does this 'irq->enabled state of the kgdb test process' come from?
What happens if it takes an interrupt, then you single-step the 
interrupt handler? (How can any state in memory get updated...)


> when it hit the breakpoint, and that should be the
> same as the current->hardirqs_enabled flag
> 
> [   25.682116] kgdbts:RUN singlestep test 1000 iterations
> [   25.695926] ------------[ cut here ]------------
> [   25.696410] DEBUG_LOCKS_WARN_ON(current->hardirqs_enabled)
> [   25.697407] WARNING: CPU: 0 PID: 1 at ../kernel/locking/lockdep.c:3761 check_flags+0x64/0x198
> [   25.698349] Modules linked in:
> [   25.699144] CPU: 0 PID: 1 Comm: swapper/0 Tainted: G                T 4.19.0-next-20181030+ #6
> [   25.699739] Hardware name: linux,dummy-virt (DT)
> [   25.700359] pstate: 404003c5 (nZcv DAIF +PAN -UAO)
> [   25.700901] pc : check_flags+0x64/0x198
> [   25.701381] lr : check_flags+0x64/0x198

> [   25.710104] Call trace:
> [   25.710553]  check_flags+0x64/0x198
> [   25.711023]  lock_acquire+0x60/0x248
> [   25.711477]  call_step_hook+0x5c/0x190
> [   25.711937]  single_step_handler+0x48/0xa0
> [   25.712399]  do_debug_exception+0x104/0x160
> [   25.712846]  el1_dbg+0x18/0xa8
> [   25.713252]  el1_irq+0xa8/0x1c0
> [   25.713709]  kgdbts_break_test+0x0/0x40
> [   25.714171]  kgdbts_run_tests+0x2b0/0x430
> [   25.714637]  configure_kgdbts+0x7c/0xb0
> [   25.715110]  init_kgdbts+0x70/0xac
> [   25.715551]  do_one_initcall+0x1fc/0x490
> [   25.716010]  do_initcall_level+0x458/0x49c
> [   25.716462]  do_initcalls+0x60/0xa4
> [   25.716892]  do_basic_setup+0x3c/0x68
> [   25.717343]  kernel_init_freeable+0x170/0x2b8
> [   25.717860]  kernel_init+0x20/0x130
> [   25.718302]  ret_from_fork+0x10/0x18
> [   25.718712] irq event stamp: 266621
> [   25.719209] hardirqs last  enabled at (266621): [<ffffff80081e935c>] do_debug_exception+0x13c/0x160
> [   25.719949] hardirqs last disabled at (266620): [<ffffff8008215a58>] kgdb_roundup_cpus+0x30/0x38
> [   25.720675] softirqs last  enabled at (266616): [<ffffff80081e9f24>] __do_softirq+0x4e4/0x52c
> [   25.721374] softirqs last disabled at (266609): [<ffffff8008279694>] irq_exit+0xbc/0x128
> [   25.722050] ---[ end trace ecb2b85abf3c30a2 ]---
> [   25.722537] possible reason: unannotated irqs-off.
> [   25.722960] irq event stamp: 266621
> 
> Rework code to drop the 'if (interrupts_enabled(regs))' before
> trace_hardirqs_*() in function do_debug_exception().
> 
> We don't know why current->hardirqs_enabled and interrupts_enabled(regs)
> don't match.

You took an IRQ from kgdbts_break_test(). This causes the CPU to mask 
interrupts, we tell trace_hardirqs_off() about this in the entry asm.

 From arch/arm64/kernel/entry.S:
| el1_irq:
|       kernel_entry 1
|       enable_da_f
| #ifdef CONFIG_TRACE_IRQFLAGS
|       bl     trace_hardirqs_off
| #endif
|
|       irq_handler


Could you faddr2line 'el1_irq+0xa8/0x1c0'? I bet that is the 
'enable_da_f' line which re-enables debug, just _before_ we update call 
trace_hardirqs_off.

The problem is debug exceptions can jump in this gap and see an 
inconsistent trace_hardirqs_off/regs state.


> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> index 7d9571f4ae3d..9afe5a7ba55b 100644
> --- a/arch/arm64/mm/fault.c
> +++ b/arch/arm64/mm/fault.c
> @@ -819,11 +819,9 @@ asmlinkage int __exception do_debug_exception(unsigned long addr,
>   	int rv;
>   
>   	/*
> -	 * Tell lockdep we disabled irqs in entry.S. Do nothing if they were
> -	 * already disabled to preserve the last enabled/disabled addresses.
> +	 * Tell lockdep we disabled irqs in entry.S.
>   	 */
> -	if (interrupts_enabled(regs))
> -		trace_hardirqs_off();
> +	trace_hardirqs_off();

Taking a debug-exception from irq-unmasked code will cause IRQs to be 
disabled. This code updates trace_hardirqs_off().

But, taking a debug-exception from irq-masked code doesn't change 
anything. Making this trace_hardirqs_off() unconditional means you now 
overwrite the original location where irqs were masked.


Would making sure debug exceptions can't see current->hardirqs_enabled 
and interrupts_enabled(regs) as having different states fix this problem?

If so moving the 'enable_da_f' line in el1_irq below the #ifdef block 
should fix it.


Thanks,

James