Re: AIM7 40% regression with 2.6.26-rc1

[Matthew Wilcox <matthew@wil.cx>]

On Tue, May 06, 2008 at 11:07:52AM -0700, Andrew Morton wrote:
> Yeah, the early bootup code.  The kernel does accidental lock_kernel()s in
> various places and if that renables interrupts then powerpc goeth crunch.
> 
> Matthew, that seemingly-unneeded irqsave in lib/semaphore.c is a prime site
> for /* one of these things */, no?

I was just reviewing the code and I came across one of these:

/*
 * Some notes on the implementation:
 *
 * The spinlock controls access to the other members of the semaphore.
 * down_trylock() and up() can be called from interrupt context, so we
 * have to disable interrupts when taking the lock.  It turns out various
 * parts of the kernel expect to be able to use down() on a semaphore in
 * interrupt context when they know it will succeed, so we have to use
 * irqsave variants for down(), down_interruptible() and down_killable()
 * too.

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