linux-kernel - Re: [PATCH v4] lib/spinlock_debug.c: prevent a recursive cycle in the debug code

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Date:	Fri, 29 Jan 2016 17:13:45 +0900
From:	Byungchul Park <byungchul.park@....com>
To:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:	Peter Hurley <peter@...leysoftware.com>,
	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	akpm@...ux-foundation.org, mingo@...nel.org,
	linux-kernel@...r.kernel.org, akinobu.mita@...il.com, jack@...e.cz,
	torvalds@...ux-foundation.org
Subject: Re: [PATCH v4] lib/spinlock_debug.c: prevent a recursive cycle in
 the debug code

On Fri, Jan 29, 2016 at 04:13:30PM +0900, Sergey Senozhatsky wrote:
> On (01/29/16 15:54), Byungchul Park wrote:
> > On Fri, Jan 29, 2016 at 09:27:03AM +0900, Sergey Senozhatsky wrote:
> > > 
> > > well, the stack is surely limited, but on every
> > > spin_dump()->spin_lock() recursive call it does another
> > > round of
> > > 
> > > 	u64 loops = loops_per_jiffy * HZ;
> > > 
> > > 	for (i = 0; i < loops; i++) {
> > > 		if (arch_spin_trylock(&lock->raw_lock))
                       ^^^
		       this is a trylock.

> > > 			return;
> > > 		__delay(1);
> > > 	}
> > > 
> > > so if you have 1000 spin_dump()->spin_lock() then, well,
> > > something has been holding the lock for '1000 * loops_per_jiffy * HZ'.
> > 
> > Or the printk() is heavily called and the lock is congested.
> 
> well, isn't it the case that ticket-based locking assumes at least
> some sort of fairness? how many cpus do you have there? you can

It's true for a congestion between arch_spin_lock()s, not trylock(). And I
cannot remember how many online cpus there are, since I frequently change
the number. Sorry. But the range is from 2 to 4.

> have `num_online_cpus() - 1' tasks spinning on the spin lock and
> 1 owning the spin lock... if your lock is in correct state (no
> before/after spinlock debug errors) even most unlucky task should
> get the lock eventually...
> 
> 	-ss