Re: [PATCH] Avoid calling down_read and down_write during startup

[Andrew Morton]

Benjamin LaHaise <bcrl@xxxxxxxxx> wrote:
>
> On Thu, Feb 23, 2006 at 05:36:56PM -0500, Alan Stern wrote:
>  > This patch (as660) changes the registration and unregistration routines 
>  > for blocking notifier chains.  During system startup, when task switching 
>  > is illegal, the routines will avoid calling down_write().
> 
>  Why is that necessary?  The down_write() will immediately succeed as no 
>  other process can possibly be holding the lock when the system is booting, 
>  so the special casing doesn't fix anything.

down_write() unconditionally (and reasonably) does local_irq_enable() in
the uncontended case.  And enabling local interrupts early in boot can
cause crashes.
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