Re: [RFC][PATCH 3/4] (Refcount + Waitqueue) implementation for cpu_hotplug "locking"

[Srivatsa Vaddagiri <vatsa@in.ibm.com>]

On Thu, Aug 24, 2006 at 02:25:27PM +0200, Ingo Molnar wrote:
> no. The writer first sets the global write_active flag, and _then_ goes 
> on to wait for all readers (if any) to get out of their critical 
> sections. (That's the purpose of the per-cpu waitqueue that readers use 
> to wake up a writer waiting for the refcount to go to 0.)
> 
> can you still see problems with this scheme?

This can cause a deadlock sometimes, when a thread tries to take the
read_lock() recursively, with a writer having come in between the two
recursive reads:

	Reader1 on CPU0			Writer1 on CPU1

	read_lock() - success

					write_lock() - blocks on Reader1
						  (writer_active = 1)


	read_lock() - blocks on Writer1

The only way to avoid this deadlock is to either keep track of
cpu_hp_lock_count per-task (like the preemption count kept per-task)
or allow read_lock() to succeed if reader_count > 1 (even if
writer_active = 1). The later makes the lock unduely biased towards
readers.


-- 
Regards,
vatsa