From mboxrd@z Thu Jan 1 00:00:00 1970 From: Chris Metcalf Subject: Re: [PATCH] arch/tile: fix rwlock so would-be write lockers don't block new readers Date: Mon, 22 Nov 2010 08:35:41 -0500 Message-ID: <4CEA71AD.5010606@tilera.com> References: <1289489007.17691.1310.camel@edumazet-laptop> <4CDF1945.8090101@tilera.com> <201011151425.oAFEPU3W005682@farm-0010.internal.tilera.com> Mime-Version: 1.0 Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: 7bit Cc: , =?ISO-8859-1?Q?Am=E9rico_Wang?= , Eric Dumazet , netdev To: Cypher Wu Return-path: In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On 11/22/2010 12:39 AM, Cypher Wu wrote: > 2010/11/15 Chris Metcalf : >> 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 >> --- >> 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