Re: [PATCH v2] locking/semaphore: Use wake_q to wake up processes outside lock critical section

[Waiman Long <longman@redhat.com>]

On 9/22/23 15:47, Peter Zijlstra wrote:
> On Fri, Sep 22, 2023 at 02:45:04PM -0400, Waiman Long wrote:
>
>> I believe early_printk should only be used in the non-SMP boot process. The
>> use of printk() is frequently used for debugging purpose and the insertion
>> of printk at some lock critical section can cause the lockdep splat to come
>> out obscuring the debugging process.
> By default early_printk is disabled somewhere early, but it has a keep
> argument to keep it around.
>
> Anyway, printk() as it exists today is wholly unsuited for debugging.
> There are too many contexts where it will flat out not work. When you
> use early_print with keep then you can use early_printk() instead of
> printk() to debug.
>
> Also, see the patches I pointed John at. Perf would not be what it is
> without those patches.
>
> Serial lines and early printk are not optional. That is, I flat out
> refuse to develop on machines without them.

Thanks for the debugging tip.

BTW, it is not just printk() that can be problematic in some contexts. I 
believe the various WARN*() calls cause the same kind of lockdep problem 
even though these WARN() calls shouldn't be triggered that often

Cheers,
Longman