Re: [PATCH V3] firmware: google: Test spinlock on panic path to avoid lockups

Guilherme G. Piccoli
2022-09-01T18:46:54Z

On 01/09/2022 15:28, Greg KH wrote:
> [...]
>> I honestly didn't understand exactly what you're suggesting Greg...
>> Mind clarifying?
>
> Something like this totally untested code:
>
> diff --git a/drivers/firmware/google/gsmi.c b/drivers/firmware/google/gsmi.c
> index adaa492c3d2d..6ad41b22671c 100644
> --- a/drivers/firmware/google/gsmi.c
> +++ b/drivers/firmware/google/gsmi.c
> @@ -19,6 +19,7 @@
> #include <linux/dma-mapping.h>
> #include <linux/fs.h>
> #include <linux/slab.h>
> +#include <linux/panic.h>
> #include <linux/panic_notifier.h>
> #include <linux/ioctl.h>
> #include <linux/acpi.h>
> @@ -611,6 +612,11 @@ static const struct attribute *gsmi_attrs[] = {
> NULL,
> };
>
> +static bool panic_in_progress(void)
> +{
> + return unlikely(atomic_read(&panic_cpu) != PANIC_CPU_INVALID);
> +}
> +
> static int gsmi_shutdown_reason(int reason)
> {
> struct gsmi_log_entry_type_1 entry = {
> @@ -629,7 +635,8 @@ static int gsmi_shutdown_reason(int reason)
> if (saved_reason & (1 << reason))
> return 0;
>
> - spin_lock_irqsave(&gsmi_dev.lock, flags);
> + if (!panic_in_progress())
> + spin_lock_irqsave(&gsmi_dev.lock, flags);
>
> saved_reason |= (1 << reason);
>
> @@ -644,7 +651,8 @@ static int gsmi_shutdown_reason(int reason)
>
> rc = gsmi_exec(GSMI_CALLBACK, GSMI_CMD_SET_EVENT_LOG);
>
> - spin_unlock_irqrestore(&gsmi_dev.lock, flags);
> + if (!panic_in_progress())
> + spin_unlock_irqrestore(&gsmi_dev.lock, flags);
>
> if (rc < 0)
> printk(KERN_ERR "gsmi: Log Shutdown Reason failed\n");
>
>
>

Thanks! Personally, I feel the approach a bit more complex than mine,
and...racy!
Imagine CPU0 runs your tests, right after the if (!panic_in_progress())
is done, spinlock is taken and boom - panic on CPU1. This would cause
the same issue...

My approach is zero racy, since it checks if spinlock was taken in a
moment that the machine is like a no-SMP, only a single CPU running...

> That being said, are you sure spinlocks are still held in the panic
> notifier? What about the call to bust_spinlocks() that is called in
> panic() already? Wouldn't that have already dropped whatever you were
> worried about here?

This function is very weird. Basically, the call of "bust_spinlocks(1);"
in panic effectively means "++oops_in_progress;" IIUC.
So, I still think we can have lockups in panic notifiers with locks
previously taken =)

Cheers,

Guilherme