Messages in this thread | | | Date | Tue, 28 Mar 2023 17:10:12 +0200 | From | Petr Mladek <> | Subject | Re: locking API: was: [PATCH printk v1 00/18] serial: 8250: implement non-BKL console |
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On Tue 2023-03-28 16:03:36, John Ogness wrote: > On 2023-03-28, Petr Mladek <pmladek@suse.com> wrote: > >> + if (!__serial8250_clear_IER(up, wctxt, &ier)) > >> + return false; > >> + > >> + if (console_exit_unsafe(wctxt)) { > >> + can_print = atomic_print_line(up, wctxt); > >> + if (!can_print) > >> + atomic_console_reacquire(wctxt, &wctxt_init); > > > > I am trying to review the 9th patch adding console_can_proceed(), > > console_enter_unsafe(), console_exit_unsafe() API. And I wanted > > to see how the struct cons_write_context was actually used. > > First off, I need to post the latest version of the 8250-POC patch. It > is not officially part of this series and is still going through changes > for the PREEMPT_RT tree. I will post the latest version directly after > answering this email.
Sure. I know that it is just a kind of POC.
> > I am confused now. I do not understand the motivation for the extra > > @wctxt_init copy and atomic_console_reacquire(). > > If an atomic context loses ownership while doing certain activities, it > may need to re-acquire ownership in order to finish or cleanup what it > started.
This sounds suspicious. If a console/writer context has lost the lock then all shared/locked resources might already be used by the new owner.
I would expect that the context could touch only non-shared resources after loosing the lock.
If it re-acquires the lock then the shared resource might be in another state. So, doing any further changes might be dangerous.
I could imagine that incrementing/decrementing some counter might make sense but setting some value sounds strange.
> > Why do we need a copy? > > When ownership is lost, the context is cleared. In order to re-acquire, > an original copy of the context is needed. There is no technical reason > to clear the context, so maybe the context should not be cleared after a > takeover. Otherwise, many drivers will need to implement the "backup > copy" solution.
It might make sense to clear values that are not longer valid, e.g. some state values or .len of the buffer. But I would keep the values that might still be needed to re-acquire the lock. It might be needed when the context want to re-start the entire operation,
I guess that you wanted to clean the structure to catch potential misuse. It makes some sense but the copying is really weird.
I think that we might/should add some paranoid checks into all functions manipulating the shared state instead.
> > And why we need to reacquire it? > > In this particular case the context has disabled interrupts. No other > context will re-enable interrupts because the driver is implemented such > that the one who disables is the one who enables. So this context must > re-acquire ownership in order to re-enable interrupts.
My understanding is that the driver might lose the lock only during hostile takeover. Is it safe to re-enable interrupts in this case?
Well, it actually might make sense if the interrupts should be enabled when the port is unused.
Well, I guess that they will get enabled by the other hostile owner. It should leave the serial port in a good state when it releases the lock a normal way.
Anyway, thanks a lot for the info. I still have to scratch my head around this.
Best Regards, Petr
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