Add comments explaining what hold/unhold do
They act as a simple one-deep semaphore to keep open/close/probe from running at the same time to avoid races that creates.
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svn2git
2020-12-20 02:59:44 +00:00
svn path=/head/; revision=340024
@ -470,6 +470,12 @@ cam_periph_release(struct cam_periph *periph)
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mtx_unlock(mtx);
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}
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/*
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* hold/unhold act as mutual exclusion for sections of the code that
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* need to sleep and want to make sure that other sections that
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* will interfere are held off. This only protects exclusive sections
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* from each other.
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*/
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int
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cam_periph_hold(struct cam_periph *periph, int priority)
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{
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@ -2697,9 +2697,10 @@ daregister(struct cam_periph *periph, void *arg)
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TASK_INIT(&softc->sysctl_task, 0, dasysctlinit, periph);
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/*
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* Take an exclusive refcount on the periph while dastart is called
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* to finish the probe. The reference will be dropped in dadone at
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* the end of probe.
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* Take an exclusive section lock qon the periph while dastart is called
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* to finish the probe. The lock will be dropped in dadone at the end
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* of probe. This locks out daopen and daclose from racing with the
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* probe.
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*
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* XXX if cam_periph_hold returns an error, we don't hold a refcount.
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*/
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