From mboxrd@z Thu Jan  1 00:00:00 1970
Message-ID: <d4dae137a381e610157b08c0221df288@quintile.net>
From: "Steve Simon" <steve@quintile.net>
Date: Thu, 22 Sep 2011 12:22:21 +0100
To: 9fans@9fans.net
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Subject: [9fans] rwlocks on plan9
Topicbox-Message-UUID: 2b97036e-ead7-11e9-9d60-3106f5b1d025

Its quite possible that I'am doing somthing foolish,
but just in case its in the design...

It appears to me that when using rwlocks on plan9
I have to release the read lock before I can take the write lock
in a process - i.e. a single process cannot hold both.

This means that when updating a data structure I do this:

	rlock()
	search_structure()
	if(found){
		runlock()
		return
	}
	runlock()

	wlock()
	search_structure()	# search a seccond time
	if(found){			# lost the race
		wunlock()
		return
	}
	add_entry()			# won the race
	wlock()
	return

If I could hold both locks I wouldn't need to do the seccond
search.

is this how its susposed to work?

-Steve