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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This archive was generated by hypermail 2b29 : Thu Sep 07 2000 - 21:00:29 EST