linux-kernel - Re: [PATCH] Introduce a config option that introduces a bias in favour of writers in rwlocks

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Message-ID: <1274535417.5605.23306.camel@twins>
Date:	Sat, 22 May 2010 15:36:57 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Salman <sqazi@...gle.com>
Cc:	akpm@...ux-foundation.org, mingo@...e.hu,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] Introduce a config option that introduces a bias in
 favour of writers in rwlocks

On Fri, 2010-05-21 at 19:38 -0700, Salman wrote:
> If one or more readers are holding the lock, and one or more writers
> are contending for it, then do not admit any new readers.  However,
> if a writer is holding a lock, then let readers contend for it at
> equal footing with the writers.
> 
> This fixes a pathological case (see the code below), where the
> tasklist_lock is continuously held by the readers, and the writers starve.
> 
> The change does not introduce any unexpected test failures in the locking
> self-test.  Furthermore, it makes the original problem go away.  In
> particular, after the change, the following code can run without
> causing a lockup: 

So how does this work with recursion?

rwlock_t is assumed recursive and quite a lot of code relies on that.

CPU0			CPU1

read_lock(&A)
			write_lock_irq(&A)

<IRQ>
  read_lock(&A) <-- deadlock because there's a pending writer


Also, I really think having config options for lock behaviour is utter
suckage, either a new implementation is better or its not.

If you want your waitpid() case to work better, try converting its
tasklist_lock usage to RCU, or try and break the lock into smaller
locks.

NAK on both your patch and your approach, rwlock_t should be killed off,
not 'improved'.

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