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

Byungchul Park
2016-01-29T08:14:10Z

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