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SubjectDAC960 SMP deadlock fix
Hi Leonard,

this night I (hopefully) finally spotted and fixed a longstanding deadlock
that was hitting us on heavily loaded server running the DAC960.

The bug is present also in the earlier 2.2.x and 2.4.x and it's _not_ been
introduced with the DAC960 updates in 2.2.17.

In 2.4.x the SMP deadlock can't trigger because of two reasons:

1) the waitqueue uses a per-queue spinlock and the driver always wakeup
the waitqueue with the io_request_lock acquired
2) the waitqueue lock in 2.4.x at the moment is a spinlock_t (thus
there isn't a __cli-less read_lock (no-irqsave) wake_up).

The bug triggers in 2.2.x because the waitqueue_lock is shard by all
waitqueues and the wake_up uses a read_lock (that can be interrupted by
irq handlers that can later spin on the io_request_lock).

The SMP locking rule that was not enforced by the driver and that was
causing the deadlock is:

"never run add_wait_queue with the io_request_lock acquired"

The reason of the rule is that the irq_request_lock can be acquired by
interrupt handlers as well.

The patch with a description of the problem is here:

I recommend adding the above fix to 2.2.18pre3.


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