linux-kernel - Re: [RFC][PATCH 3/4] (Refcount + Waitqueue) implementation for cpu

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Message-ID: <20060824125813.GE25452@in.ibm.com>
Date:	Thu, 24 Aug 2006 18:28:14 +0530
From:	Srivatsa Vaddagiri <vatsa@...ibm.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Gautham R Shenoy <ego@...ibm.com>, rusty@...tcorp.com.au,
	torvalds@...l.org, akpm@...l.org, linux-kernel@...r.kernel.org,
	arjan@...ux.intel.com, davej@...hat.com, dipankar@...ibm.com,
	ashok.raj@...el.com
Subject: Re: [RFC][PATCH 3/4] (Refcount + Waitqueue) implementation for cpu_hotplug "locking"

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
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