2.6.23 spinlock hang in kswapd under heavy disk write loads

[Berkley Shands]

2.6.23 with CONFIG_DEBUG_SPINLOCK on does not hang under very high write loads
to either an LSI8888ELP (write rate 1.1GB/Sec) or to a highpoint RR2340 (write rate 1.0GB/Sec).
With CONFIG_DEBUG_SPINLOCK off however, the system hangs with kswapd getting
100% of the cpu and most if not all other processes are locked out. Sometimes even the
keyboard is locked out.

This is seen on a supermicro H8DM3-2 and H8DMi-2 motherboards, 16 GB RAM, 2222 SE or 2216 processors.
Not seen on a Uniwide 3546ES, or on a Supermicro H8DM8 with 8222 CPUS.

kswapd sits at (according to KDB)
__spinlock_irqsave + 0x15
prepare_to_wait + 0x15
kswapd + 0xe3
kthread + 0x47

berkley
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