Messages in this thread | | | From | John Ogness <> | Subject | Re: [PATCH printk v3 14/15] printk: extend console_lock for proper kthread support | Date | Fri, 22 Apr 2022 23:31:11 +0206 |
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On 2022-04-22, Petr Mladek <pmladek@suse.com> wrote: > Another problem is that the ordering is not stable. The console > might come and go.
The console list is protected by @console_sem, so it wouldn't be an actual problem. The real issue is that lockdep would not like it. A new lockdep class would need to be setup for each register_console().
>> Anyway, I will first look into the nested locking solution. That >> seems more promising to me and it would go a long way to simplify the >> locking hierarchy. > > Please, do not spend too much time on this. The solution must be > simple in principle. If it gets complicated than it will likely > be worse than the current code.
Sure. The goal is to simplify. The only complexity will be doing in a way that allow lockdep to understand it.
> Alternative solution would be to reduce the number of variables > affected by the race. I mean: > > + replace CON_THB_BLOCKED flag with con->blocked to avoid > the needed of READ_ONCE()/WRITE_ONCE(). > > + check con->blocked right after taking con->lock in > printk_kthread_func() so that all the other accesses are > safe.
Honestly, I would prefer this to what v4 is doing. The only reason CON_THD_BLOCKED is a flag is to save space. But we are only talking about a few bytes being saved. There aren't that many consoles.
It would be a very simple change. Literally just replacing the 3 lines that set/clear CON_THD_BLOCKED and replacing/reordering the 2 lines that check the flag. Then all the READ_ONCE/WRITE_ONCE to @flags could be removed.
John
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