Re: [PATCH next v4 1/2] lib/dump_stack: move cpu lock to printk.c

[Steven Rostedt]

On Fri, 18 Jun 2021 17:01:37 +0206
John Ogness <john.ogness@xxxxxxxxxxxxx> wrote:

> Since the cpu lock is also taken in NMI context (for example, via
> nmi_cpu_backtrace()/dump_stack()), the main concerns are:
> 
> 1. locks that are taken by a CPU that is holding the cpu lock
> 
> 2. NMI contexts that take any type of lock
> 
> (Actually, #2 is just a special case of #1 where an NMI interrupted a
> task that was holding the cpu lock.)
> 
> For early_printk() the early USB devices look to be a
> problem. early_xdbc_write() will take a spinlock. Assuming the
> early_console was also registered as a normal console (via "keep") we
> could end up in the following deadlock between the normal console and
> early_printk() writes:

My use case of earlyprintk is to avoid all the crud in printk, and
just have something to print to the serial console. USB early printk is
not my use case, as that adds even more crud.

Thus, I don't care about this being an issue with USB early printk ;-)


> 
>     CPU0                          CPU1
>     ----                          ----
>     early_printk()                console->write()
>       cpu_lock()                    spinlock()
>       early_console->write()      *NMI*
>         spinlock()                cpu_lock()
> 
> The upcoming atomic console work addresses this by implementing a new
> write_atomic() callback that is lockless (and SMP-safe) or aware of the
> cpu lock to avoid dead locks such as above.
> 
> AFAICT, the USB devices (CONFIG_EARLY_PRINTK_USB) are the only
> early_printk() candidates that use locking. So for all other
> early_printk() implementations I think your suggestion would work fine.
> 
> Although, in general, early_printk() is not SMP-safe. So I'm not sure
> how much safety we need to include at this point.
> 

Right. I say we ignore the issue with usb earlyprintk. The only time I
can see that even useful, is for issues that are happening before
printk is initialized, and all you have for a console is the usb for
early printk, and the above scenario shouldn't be an issue. But for
places that would utilize early printk for later on in the boot process
(and normal activity), any console that takes locks would be useless.

-- Steve