Messages in this thread | | | Date | Wed, 6 Sep 2023 15:57:47 +0200 | From | Petr Mladek <> | Subject | Re: [PATCH printk v3 4/7] printk: nbcon: Add ownership state functions |
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On Sun 2023-09-03 17:11:36, John Ogness wrote: > From: Thomas Gleixner <tglx@linutronix.de> > > Provide functions that are related to the safe handover mechanism > and allow console drivers to dynamically specify unsafe regions: > > - nbcon_context_can_proceed() > > Invoked by a console owner to check whether a handover request > is pending or whether the console has been taken over by another > context. If a handover request is pending, this function will > also perform the handover, thus cancelling its own ownership. > > - nbcon_context_update_unsafe() > > Invoked by a console owner to denote that the driver is about > to enter or leave a critical region where a take over is unsafe. > This function is also a cancellation point where loss of > ownership can occur. > > The unsafe state is stored in the console state and allows a > new context to make informed decisions whether to attempt a > takeover of such a console. The unsafe state is also available > to the driver so that it can make informed decisions about the > required actions and possibly take a special emergency path. > > Co-developed-by: John Ogness <john.ogness@linutronix.de> > Signed-off-by: John Ogness <john.ogness@linutronix.de> > Signed-off-by: Thomas Gleixner (Intel) <tglx@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
See one comment below.
> --- a/kernel/printk/nbcon.c > +++ b/kernel/printk/nbcon.c > @@ -518,6 +517,112 @@ static void nbcon_context_release(struct nbcon_context *ctxt) > ctxt->pbufs = NULL; > } > > +/** > + * nbcon_context_can_proceed - Check whether ownership can proceed > + * @ctxt: The nbcon context from nbcon_context_try_acquire() > + * @cur: The current console state > + * > + * Return: True if this context still owns the console. False if > + * ownership was handed over or taken. > + * > + * Must be invoked after the record was dumped into the assigned buffer > + * and at appropriate safe places in the driver. > + * > + * When this function returns false then the calling context no longer owns > + * the console and is no longer allowed to go forward. In this case it must > + * back out immediately and carefully. The buffer content is also no longer > + * trusted since it no longer belongs to the calling context. > + */ > +static bool nbcon_context_can_proceed(struct nbcon_context *ctxt, struct nbcon_state *cur) > +{ > + unsigned int cpu = smp_processor_id(); > + > + /* Make sure this context still owns the console. */ > + if (!nbcon_owner_matches(cur, cpu, ctxt->prio)) > + return false; > + > + /* The console owner can proceed if there is no waiter. */ > + if (cur->req_prio == NBCON_PRIO_NONE) > + return true; > + > + /* > + * A console owner within an unsafe region is always allowed to > + * proceed, even if there are waiters. It can perform a handover > + * when exiting the unsafe region. Otherwise the waiter will > + * need to perform an unsafe hostile takeover. > + */ > + if (cur->unsafe) > + return true; > + > + /* Waiters always have higher priorities than owners. */ > + WARN_ON_ONCE(cur->req_prio <= cur->prio); > + > + /* > + * Having a safe point for take over and eventually a few > + * duplicated characters or a full line is way better than a > + * hostile takeover. Post processing can take care of the garbage. > + * Release and hand over. > + */ > + nbcon_context_release(ctxt); > + > + /* > + * It is not known whether the handover succeeded. The outermost
It was not immediately clear to me what exactly "handover succeeded" did mean. I would write:
* It is not clear whether the waiter really took the lock * and re-printed the record. The outermost calsite...
> + * callsite has to make the final decision whether printing > + * should proceed or not (via reacquire, possibly hostile). The > + * console is now unlocked so go back all the way instead of > + * trying to implement heuristics in tons of places. > + */ > + return false; > +}
Best Regards, Petr
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