Re: [PATCH 02/14] add blockconsole version 1.1

["Jörn Engel" <joern@logfs.org>]

On Wed, 22 May 2013 15:43:22 -0700, Andrew Morton wrote:
> On Wed, 22 May 2013 17:04:16 -0400 J__rn Engel <joern@logfs.org> wrote:
> 
> > > >  Documentation/block/blockconsole.txt            |   94 ++++
> > > >  Documentation/block/blockconsole/bcon_tail      |   62 +++
> > > >  Documentation/block/blockconsole/mkblockconsole |   29 ++
> > > 
> > > We really need somewhere better to put userspace tools.
> > 
> > Agreed.  It started as "use this horrible code as a bad example" and
> > turned into a repository for userspace code, which it should have
> > been.
> > 
> > Do you have an opinion on tools/ vs. a standalong git tree for the
> > tools?
> 
> ./tools/blockconsole/ sounds nice.  I like maintaining simple userspace
> utils in the main kernel tree - it's very easy and efficient for us.

tools/blockconsole/ is shall be!

> > > > +#define BCON_MAX_ERRORS	10
> > > > +static void bcon_end_io(struct bio *bio, int err)
> > > > +{
> > > > +	struct bcon_bio *bcon_bio = container_of(bio, struct bcon_bio, bio);
> > > > +	struct blockconsole *bc = bio->bi_private;
> > > > +	unsigned long flags;
> > > > +
> > > > +	/*
> > > > +	 * We want to assume the device broken and free this console if
> > > > +	 * we accumulate too many errors.  But if errors are transient,
> > > > +	 * we also want to forget about them once writes succeed again.
> > > > +	 * Oh, and we only want to reset the counter if it hasn't reached
> > > > +	 * the limit yet, so we don't bcon_put() twice from here.
> > > > +	 */
> > > > +	spin_lock_irqsave(&bc->end_io_lock, flags);
> > > > +	if (err) {
> > > > +		if (bc->error_count++ == BCON_MAX_ERRORS) {
> > > > +			pr_info("no longer logging to %s\n", bc->devname);
> > > 
> > > pr_info() may be too low a severity level?
> > 
> > There is no distinction between "someone pulled the usb device" and
> > "broken cable".  Blockconsole will continue writing and retire the
> > device if errors accumulate.  The code is deliberately stupid, because
> > stupid code tends to be more robust and devoid of strange
> > corner-cases.
> 
> I meant should we use pr_err() or pr_emerg() rather than pr_info(). 
> Chances are that pr_info() is being hidden.

Agreed.  Trouble is that pr_info() is about right for "someone pulled
the usb device", which pr_err() or pr_emerg() is about right for
"broken hardware".  So either choice will be wrong some of the time.

I guess in the end very few people care about debugging blockconsole
failures.  Those that run blockconsole tend to care about debugging
something else already.  So if you lean one way or the other, you have
a stronger opinion than me.

> > > How does the code handle the obvious recursion concern here?
> > 
> > I think your next comment answers that.
> 
> hm, that was clever of me.  I meant more generally, if we do a printk()
> from within this code how do we prevent arbitrarily deep recursion?

I think in general you have to be careful and think twice before
adding a printk.  Which is why there are only two instances so far.
The more interesting case is in functions called from blockconsole (or
netconsole or serial console or...).  And here the small set of
functions when writing to the buffer should be safe and the much
larger and complex set of functions for writeback is in the context of
a different thread.

I have some fuzzy memory of this coming up in the past for netconsole.
Wasn't hard to fix, which explains the lack of detailed memory.

> > > > +		memcpy(bc->bio_array[i].sector +
> > > > +				__bcon_console_ofs(console_bytes), msg, n);
> > > > +		len -= n;
> > > > +		msg += n;
> > > > +		bcon_advance_console_bytes(bc, n);
> > > > +	}
> > > > +	wake_up_process(bc->writeback_thread);
> > > > +	mod_timer(&bc->pad_timer, jiffies + HZ);
> > > > +}
> > > > +
> > > > +static void bcon_init_bios(struct blockconsole *bc)
> > > 
> > > This code really needs some comments :(
> > > 
> > > Oh I see.  It's BIOs, not BIOS.  Had me fooled there.
> > 
> > But, but, but isn't this obvious to someone who has just written said
> > function?  It has been a year now, so my own definition of obvious may
> > have changed as well.
> 
> heh.  I blame Jens for calling it a "bio".

I wonder what his response is when a recruiter asks him for a recent
bio.

> > > > +void bcon_add(const char *name)
> > > > +{
> > > > +	/*
> > > > +	 * We add each name to a small static buffer and ask for a workqueue
> > > > +	 * to go pick it up asap.  Once it is picked up, the buffer is empty
> > > > +	 * again, so hopefully it will suffice for all sane users.
> > > > +	 */
> > > > +	spin_lock(&device_lock);
> > > > +	if (scanned_devices[0])
> > > > +		strncat(scanned_devices, ",", sizeof(scanned_devices));
> > > > +	strncat(scanned_devices, name, sizeof(scanned_devices));
> > > > +	spin_unlock(&device_lock);
> > > > +	schedule_work(&bcon_add_work);
> > > > +}
> > > 
> > > What's all this do?
> > 
> > Work around init ordering problems.  Quite likely there is a much
> > nicer way to this that I should know about and don't.
> 
> Well, without adequate description of the problem, nobody can help
> solve it!

Will dig through the dark corners of my memory or reproduce it,
whichever is easier.

Jörn

--
When in doubt, use brute force.
-- Ken Thompson