Re: [PATCH 1/19] MUTEX: Introduce simple mutex implementation

[David Howells]

Luck, Tony <tony.luck@xxxxxxxxx> wrote:

> There was a USENIX paper a couple of decades ago that described how
> to do a fast s/w disable of interrupts on machines where really disabling
> interrupts was expensive.  The rough gist was that the spl[1-7]()
> functions would just set a flag in memory to hold the desired interrupt
> mask.

Cute. The slow bit on FRV is any time you access the PSR register (read or
write). It seems to be something on the order of 60 clock cycles a pop - in
which time the CPU could have executed 120 instructions under ideal
circumstances.

I do something like this to implement "atomic" operations, playing on the
FRV's ability to pack two instructions atomically together and to have
conditionally executed instructions:

	Documentation/fujitsu/frv/atomic-ops.txt.

Trading off against the memory speed might just do it - though you have to do
a write and a read (the latter of which should hopefully be cached). I could
always steal another register (I have 31-ish to play with, plus a bunch of
single-bit condition values).

It'd make the exception prologue even more "interesting" though...:-)

Hmmm...

David
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