On Wed, Jun 14, 2017 at 11:06:58AM +0100, Will Deacon wrote:
> Apologies, I misunderstood your algorithm (I thought step (a) was on one CPU
> and step (b) was on another). Still, I don't understand the need for the
> timeout. If you instead read back the flag immediately, wouldn't it still
> work? e.g.
>
>
> lock:
> Readl_relaxed flag
> if (locked)
> goto lock;
>
> Writel_relaxed unique ID to flag
> Readl flag
> if (locked by somebody else)
> goto lock;
>
> <critical section>
>
> unlock:
> Writel unlocked value to flag
I think the delay is to counter this:
Agent 1 Agent 2
read flag
not locked
read flag
not locked
write unique ID
read back
not locked by someone else
write unique ID
read back
not locked by someone else
With the delay present, this becomes:
Agent 1 Agent 2
read flag
not locked
read flag
not locked
write unique ID
delay
write unique ID
delay
read back
locked by agent 2
read back
not locked by someone else
For this to work, the delay has to be guaranteed to be greater than
the maximum duration that any agent takes between the initial read
and the write of its unique ID. The delay doesn't even have to be
identical between each agent, it just has to satisfy that condition.
The key thing though is that the reads and writes must happen when
the program intends them to, so I don't think the _relaxed variants
should be used here. If they're buffered, then the delay doesn't
have the desired effect.
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