lkml.org 
[lkml]   [2002]   [Sep]   [4]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: Questions on semaphores
On Wed, Sep 04, 2002 at 11:29:31PM +0200, Juan M. de la Torre wrote:
>
> Hi people, I have two question regarding the i386 semaphore implementation
> in kernel 2.4.19.
>
> Please dont blame me if they are too obvius; i'm a newbie in kernel hacking
> :)
>
> The functions __down, __down_interruptible and __down_trylock (defined
> in arch/i386/kernel/semaphore.c) use the global spinlock
> 'semaphore_lock' to access some fields of the semaphore they are
> working on:
>
> 1) Is there any reason to do this?

It was easy to do.

> 2) Wouldn't it be more scalable to use a per-semaphore lock instead a
> global spinlock?
>

Yes it would be more scalable, but not as much as you would think.
The __down, __down_interruptible and __down_trylock code only gets
invoked when the semaphore is contended for.

> The function __down_trylock try to get the spinlock using
> spin_lock_irqsave, instead of using spin_lock_irq:
>
> 1) why? :)
>

The __down_trylock() code can be called with another lock held. The
spin_lock_irqsave()/spin_lock_irqrestore() interface is used to restore
the irq value for the lock that may already be held.

> Thanks in advance,
> Juanma
>
The code in the 2.5 tree was changed a while back to use the spinlock in
the wait_queue_head_t to replace the global semaphore spin lock. So, this
has been "FIXED" in 2.5.

--
Bob Miller Email: rem@osdl.org
Open Source Development Lab Phone: 503.626.2455 Ext. 17
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2005-03-22 13:28    [W:0.045 / U:0.044 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site