Messages in this thread |  | | Date | Wed, 5 Oct 2016 12:37:17 +0200 | From | Petr Mladek <> | Subject | Re: [RFC 0/5] printk: Implement WARN_*DEFERRED() |
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On Fri 2016-09-30 09:48:32, Sergey Senozhatsky wrote: > On (09/29/16 13:28), Petr Mladek wrote: > > On Wed 2016-09-28 10:18:45, Sergey Senozhatsky wrote: > > > On (09/27/16 18:02), Petr Mladek wrote: > > > > The main trick is that we replace the per-CPU function pointer > > > > by a preempt_count-like variable that could track the printk context. > > > > > > > > I know that Sergey has another ideas in this area. But I wanted to see > > > > how this approach would look like. > > > > > > well, yes. I was looking at WARN_*_DEFERRED [1] for some time, and, I > > > think, the maintenance cost of that solution is just too high: > > > > > > a) every existing WARN_* in sched/timekeeping/who knows where else > > > must be evaluated to ensure that in can't be called from printk() > > > path. if `false' - then the corresponding macro must be replaced > > > with _DEFERRED flavor. > > > > > > b) any patch that adds new WARN_* usages must be additionally checked > > > to ensure that each of new WARN_* macros cannot be called from printk > > > path. if `false' -- the corresponding macro must be replaced with > > > _DEFERRED flavor. > > > > > > c) any patch that refactors the code or moves some function calls around > > > etc. must be additionally checked for any accidental WARN_* from printk > > > path. even though if none of the patches added any new WARN_* to the code. > > > > > > b) apart from WARN_* there can be `accidental' pr_err/pr_debug/etc. not > > > necessarily newly added (see 'c'). > > > > > > > > > that's too much. > > > > > > it takes a lot of additional effort, because both reviewer and contributor > > > must consider printk() internals. and, what's worse, if something goes > > > unnoticed we end up having a printk() deadlock again. > > > > > > so I decided to address some of printk() issues in printk.c, not in > > > kernel/time/timekeeping.c or kernel/sched/core.c or anywhere else.
I do not longer see how this might be achieved. If a printk()/WARN() in the scheduler/timekeeping code can be reached from printk() then it might too be reached outside printk. In this case, printk() will not know about it and will happily call the scheduler/timekeeping code recursively. This might still cause deadlock.
> > I see the point. > > well, just my 5 cents. > > > Your approach (alt buffer) adds some complexity to the printk code > > it does. > the other thing is that there are several ways to deadlock printk(). > alt_printk is addressing deadlocks that were caused by printk() > recursion only. > > printk() > acquire_lock(&foo) > printk() > acquire_lock(&foo)
This looks theoretical. The recursion in printk() is not easily possible at the moment. It is prevented by logbuf_cpu check when logbug_lock is taken. It is prevented by console_trylock() when console_sem is taken.
> which is a sub-set of all of the printk() deadlock scenarios. all of > the locks that printk() acquires can be taken outside of printk() path. > > for example, cat /proc/console locks the console_lock() for seq output. > thus we can have something like > > console_unlock() // lock &sem->lock > up() > activate_task() > WARN_ON() > printk() > console_trylock() // lock &sem->lock
The WARN_ON() here is called under &p->pi_lock that is taken by try_to_wake_up(). This WARN_ON() can be triggered also outside printk()/console_unlock(). Therefore it needs to get replaced by WARN_DEFERRED() anyway.
> DEFERRED_WARN is a good thing; it's just quite hard to keep everything > working, given that any of those "9 patches per hour" can break something > with just one WARN_ON(). > > > I assume that doing something like this > > #define WARN_ON(condition, format...) ({ \ > printk_deferred_enter(); \ > WARN(condition, ##format); \ > printk_deferred_exit(); \ > }) > > is less than exciting because WARN_ON from irq won't immediately print > the backtrace anymore.
Yup, we might need WARN_ON_DEFERRED() variant.
Best Regards, Petr
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