Messages in this thread | | | Date | Mon, 25 Apr 2022 17:18:47 +0200 | From | Petr Mladek <> | Subject | Re: [PATCH printk v3 14/15] printk: extend console_lock for proper kthread support |
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On Fri 2022-04-22 23:31:11, John Ogness wrote: > 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().
Yeah. I did not mention it explicitely but I meant it as a problem with lockdep.
> >> 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.
I am not sure how to distinguish intentional and non-intentional ordering change.
> > 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.
I agree that it sounds like the easiest solution for now. If you prepare v5 with this change then I push it into linux-next instead of v4.
Well, I think that we need to make con->lock safe to use in the long term. The above workaround in printk_kthread_func() is good enough for now because this is the only location where con->lock is taken without console_sem. But I am sure that we/people will want to do more console-specific operations without console_sem in the future.
IMHO, the only sane approach is to follow the proposed rules:
+ console_lock() will synchronize both global and per-console stuff.
+ con->lock will synchronize per-console stuff.
+ con->lock could not be taken alone when the big console_lock() is taken.
I currently know only about two solutions:
1. The nested locking. console_lock() will take console_sem and all con->lock's and will keep them locked.
It is rather trivial in principle. The problem is lockdep and possible ABBA deadlocks caused by unstable ordering.
2. Create the wrappers around con->lock that will check whether console_sem is taken (con->locked flag).
It will require additional per-console waitqueue. But all the magic will be hidden in the wrappers.
I personally prefer 2nd approach for the long term solution. It might look more complicated but it will not break lockdep.
Best Regards, Petr
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