Re: [patch] new spinlock variant, spinlock-2.3.30-A4

Ingo Molnar (
Tue, 30 Nov 1999 22:50:21 +0100 (CET)

On Tue, 30 Nov 1999, Davide Libenzi wrote:

> > movb $1, 3(%%esi) # spin_lock
> > movl 4(%%esi), %%edx
> > addl (%%esi), %%edx
> > cmpl $0x01000000, %%edx
> > je slow_path
> This can lead to some dead loops, due to the fact that an atomic ( locked )
> test and set is not used.
> The lock acquire is based on the assumption that only one CPU execute the
> lock code.

The spinlock is correct with an arbitrary number of CPUs.

> In theory if there are more than one CPUs executing the code, they can
> spend a lot of loops without getting the N bytes at zero. This
> probability is higher greater is the number of CPUs. Using a test and
> set instruction, even if N CPUs execute the same code at the same
> time, only one gets the lock value.

yes, true. Keep in mind that this only affects the slow path. The above
spinlock would be implemented as a function anyway (we do not want to
inline it), and in that case the slow path can eg. use queued spinlocks
(spinlock chains through on-stack variables) or exponential backoff, or
whatever technique.

-- mingo

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