netdev - Re: [PATCH] arch/tile: fix rwlock so would-be write lockers don't block new readers

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Message-ID: <4CEA71AD.5010606@tilera.com>
Date:	Mon, 22 Nov 2010 08:35:41 -0500
From:	Chris Metcalf <cmetcalf@...era.com>
To:	Cypher Wu <cypher.w@...il.com>
CC:	<linux-kernel@...r.kernel.org>,
	Américo Wang <xiyou.wangcong@...il.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH] arch/tile: fix rwlock so would-be write lockers don't
 block new readers

On 11/22/2010 12:39 AM, Cypher Wu wrote:
> 2010/11/15 Chris Metcalf <cmetcalf@...era.com>:
>> This avoids a deadlock in the IGMP code where one core gets a read
>> lock, another core starts trying to get a write lock (thus blocking
>> new readers), and then the first core tries to recursively re-acquire
>> the read lock.
>>
>> We still try to preserve some degree of balance by giving priority
>> to additional write lockers that come along while the lock is held
>> for write, so they can all complete quickly and return the lock to
>> the readers.
>>
>> Signed-off-by: Chris Metcalf <cmetcalf@...era.com>
>> ---
>> This should apply relatively cleanly to 2.6.26.7 source code too.
>> [...]
>
> I've finished my business trip and tested that patch for more than an
> hour and it works. The test is still running now.
>
> But it seems there still has a potential problem: we used ticket lock
> for write_lock(), and if there are so many write_lock() occurred, is
> 256 ticket enough for 64 or even more cores to avoiding overflow?
> Since is we try to write_unlock() and there's already write_lock()
> waiting we'll only adding current ticket.

This is OK, since each core can issue at most one (blocking) write_lock(),
and we have only 64 cores.  Future >256 core machines will be based on
TILE-Gx anyway, which doesn't have the 256-core limit since it doesn't use
the spinlock_32.c implementation.

-- 
Chris Metcalf, Tilera Corp.
http://www.tilera.com

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