Re: [RFC][PATCHv6 00/12] printk: introduce printing kernel thread

Sergey Senozhatsky
2017-12-15T05:06:13Z

Hello,

On (12/14/17 22:18), Steven Rostedt wrote:
> > Steven, your approach works ONLY when we have the following preconditions:
> >
> > a) there is a CPU that is calling printk() from the 'safe' (non-atomic,
> > etc) context
> >
> > what does guarantee that? what happens if there is NO non-atomic
> > CPU or that non-atomic simplky missses the console_owner != false
> > point? we are going to conclude
> >
> > "if printk() doesn't work for you, it's because you are holding it wrong"?
> >
> >
> > what if that non-atomic CPU does not call printk(), but instead
> > it does console_lock()/console_unlock()? why there is no handoff?
> >
> > CPU0 CPU1 ~ CPU10
> > in atomic contexts [!]. ping-ponging console_sem
> > ownership to each other. while what they really
> > need to do is to simply up() and let CPU0 to
> > handle it.
> > printk
> > console_lock()
> > schedule()
> > ...
> > printk
> > printk
> > ...
> > printk
> > printk
> >
> > up()
> >
> > // woken up
> > console_unlock()
> >
> > why do we make an emphasis on fixing vprintk_printk()?
>
> Where do we do the above? And has this been proven to be an issue?

um... hundreds of cases.

deep-stack spin_lock_irqsave() lockup reports from multiple CPUs (3 cpus)
happening at the same moment + NMI backtraces from all the CPUs (more
than 3 cpus) that follows the lockups, over not-so-fast serial console.
exactly the bug report I received two days ago. so which one of the CPUs
here is a good candidate to successfully emit all of the pending logbuf
entries? none. all of them either have local IRQs disabled, or dump_stack()
from either backtrace IPI or backtrace NMI (depending on the configuration).

do we periodically do console_lock() on a running system? yes, we do.
add to console_unlock()

---

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index b9006617710f..1c811f6d94bf 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2143,6 +2143,10 @@ void console_unlock(void)
bool wake_klogd = false;
bool do_cond_resched, retry;

+ if (!(current->flags & PF_KTHREAD))
+ dump_stack();
+
+
if (console_suspended) {
up_console_sem();
return;

---

and just boot the system.

I work for a company that has several thousand engineers spread
across the globe. and people do use printk(), and issues do happen.

the scenarios that Tejun and I talk about are not theoretical. if those
scenarios are completely theoretical, as you suggest, - then, OK, what
exactly guarantees that

whenever atomic CPUs printk there is always a non-atomic
CPU to take over the printing?

> > b) non-atomic CPU sees console_owner set (which is set for a very short
> > period of time)
> >
> > again. what if that non-atomic CPU does not see console_owner?
> > "don't use printk()"?
>
> May I ask, why are we doing the printk in the first place?

this argument is really may be applied against your patch as well. I
really don't want us to have this type of "technical" discussion.

printk() is a tool for developers. but developers can't use.

> > c) the task that is looping in console_unlock() sees non-atomic CPU when
> > console_owner is set.
>
> I haven't looked at the latest code, but my last patch didn't care
> about "atomic" and "non-atomic"

I know. and I think it is sort of a problem.

lots of printk-s are happening from IRQs / softirqs and so on.
take a look at CONFIG_IP_ROUTE_VERBOSE, for example.

do_softirq() -> ip_handle_martian_source() and a bunch of other
places.
these irq->printk-s can "steal" the console_sem and go to
console_unlock().

"don't use printk() then" type of argument does not really help
to a guy who reports the lockup.

> > Steven, I thought we reached the agreement [**] that the solution we should
> > be working on is a combination of prinkt_kthread and console_sem hand
> > off. Simply because it adds the missing "there is a non-atomic CPU wishing
> > to console_unlock()" thing.
> >
> > lkml.kernel.org/r/20171108162813.GA983427@devbig577.frc2.facebook.com
> >
> > https://marc.info/?l=linux-kernel&m=151011840830776&w=2
> > https://marc.info/?l=linux-kernel&m=151015141407368&w=2
> > https://marc.info/?l=linux-kernel&m=151018900919386&w=2
> > https://marc.info/?l=linux-kernel&m=151019815721161&w=2
> > https://marc.info/?l=linux-kernel&m=151020275921953&w=2
> > ** https://marc.info/?l=linux-kernel&m=151020404622181&w=2
> > ** https://marc.info/?l=linux-kernel&m=151020565222469&w=2
>
> I'm still fine with the hybrid approach, but I want to see a problem
> first before we fix it.
>
> >
> >
> > what am I missing?
>
> The reproducer.

will that printk_test module

lkml.kernel.org/r/20171204135314.9122-2-sergey.senozhatsky@gmail.com

suffice?

-ss