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 16:13:30 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:	Byungchul Park <byungchul.park@....com>
Cc:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	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 (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))
> > 			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
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