Re: [patch] increase spinlock-debug looping timeouts from 1 sec to 1 min

[Ingo Molnar]

* Andrew Morton <akpm@xxxxxxxx> wrote:

> On Mon, 19 Jun 2006 09:02:29 +0200
> Ingo Molnar <mingo@xxxxxxx> wrote:
> 
> > Subject: increase spinlock-debug looping timeouts from 1 sec to 1 
> > min
> 
> But it's broken.  In the non-debug case we subtract RW_LOCK_BIAS so we 
> know that the writer will get the lock when all readers vacate.  But 
> in the CONFIG_DEBUG_SPINLOCK case we don't do that, with the result 
> that taking a write_lock can take over a second.
> 
> A much, much better fix (which involves visiting all architectures) 
> would be to subtract RW_LOCK_BIAS and _then_ wait for a second.

no. Write-locks are unfair too, and there's no guarantee that writes are 
listened to. That's why nested read_lock() is valid, while nested 
down_read() is invalid.

Take a look at arch/i386/kernel/semaphore.c, __write_lock_failed() just 
adds back the RW_LOCK_BIAS and retries in a loop. There's no difference 
to an open-coded write_trylock loop - unless i'm missing something 
fundamental.

> OK, it's only debug code.  But RH (for one) have decided to ship 
> zillions of kernels with this debug code turned on.

yes - Fedora enables most of the transparent kernel debugging options 
(slab, lock debugging) in rawhide. The current list is:

 CONFIG_DEBUG_KERNEL=y
 CONFIG_DEBUG_SLAB=y
 CONFIG_DEBUG_SLAB_LEAK=y
 CONFIG_DEBUG_MUTEXES=y
 CONFIG_DEBUG_SPINLOCK=y
 CONFIG_DEBUG_SPINLOCK_SLEEP=y
 CONFIG_DEBUG_HIGHMEM=y
 CONFIG_DEBUG_BUGVERBOSE=y
 CONFIG_DEBUG_INFO=y
 CONFIG_DEBUG_FS=y
 CONFIG_DEBUG_VM=y
 CONFIG_DEBUG_STACKOVERFLOW=y
 CONFIG_DEBUG_STACK_USAGE=y
 CONFIG_DEBUG_RODATA=y
 CONFIG_KEYS_DEBUG_PROC_KEYS=y

	Ingo
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