From owner-freebsd-arch@FreeBSD.ORG Tue Jan 29 20:01:04 2008 Return-Path: Delivered-To: arch@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34]) by hub.freebsd.org (Postfix) with ESMTP id 3A52C16A41B for ; Tue, 29 Jan 2008 20:01:04 +0000 (UTC) (envelope-from asmrookie@gmail.com) Received: from fg-out-1718.google.com (fg-out-1718.google.com [72.14.220.155]) by mx1.freebsd.org (Postfix) with ESMTP id B7B2F13C447 for ; Tue, 29 Jan 2008 20:01:03 +0000 (UTC) (envelope-from asmrookie@gmail.com) Received: by fg-out-1718.google.com with SMTP id 16so2500914fgg.35 for ; Tue, 29 Jan 2008 12:01:02 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=gamma; h=domainkey-signature:received:received:message-id:date:from:sender:to:subject:cc:in-reply-to:mime-version:content-type:content-transfer-encoding:content-disposition:references:x-google-sender-auth; bh=xi5sEg5TwjkTMQZHd4JccOmLx7OApgRlrDjHU5yFr5Q=; b=dubfg99S3Zy8vnAHbiNIbieM2ifRInG2xsCGafccDgqqsAlmsfxbHW7tCSvcAjUq8ZEaIN8ryIR2qJL8kqp/6zZhDFG/7kAvYWJOOAsBP/FJe0rNWy0Ts+lsyb294IxceUyMnjqCApC3kjqH//JAvN63PvDwOD+cdVwskCV78GM= DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=message-id:date:from:sender:to:subject:cc:in-reply-to:mime-version:content-type:content-transfer-encoding:content-disposition:references:x-google-sender-auth; b=knBSGfEyxfKu8TGmEy8+cEzfEW4Rj13102NxtPv5xTN53xtLxjY7EppsxuDhljS3JAXA3V5vSc0Bg2uuy5LiojIiQFTMj+8nOztF9Y77ojaANZFORaDtqXl6eOXroMEYJzDIdgtdSEwu+IwOzeYmgwGwfNqxtxwGvx+93/Cj5UY= Received: by 10.86.25.17 with SMTP id 17mr6791890fgy.73.1201636861595; Tue, 29 Jan 2008 12:01:01 -0800 (PST) Received: by 10.86.28.19 with HTTP; Tue, 29 Jan 2008 12:01:01 -0800 (PST) Message-ID: <3bbf2fe10801291201n7b355205o43b648791f5ca387@mail.gmail.com> Date: Tue, 29 Jan 2008 21:01:01 +0100 From: "Attilio Rao" Sender: asmrookie@gmail.com To: "Jeff Roberson" In-Reply-To: <20080128190244.R957@desktop> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Content-Disposition: inline References: <20080128190244.R957@desktop> X-Google-Sender-Auth: 9f6c8e2e50b07103 Cc: ups@freebsd.org, arch@freebsd.org, rwatson@freebsd.org Subject: Re: rwlock patch to prefer writers and improve spinning. X-BeenThere: freebsd-arch@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Discussion related to FreeBSD architecture List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 29 Jan 2008 20:01:04 -0000 2008/1/29, Jeff Roberson : > http://people.freebsd.org/~jeff/rwlock.diff > > Attilio and I have come up with a rwlock patch that prevents writer > starvation by blocking readers whenever there are writers waiting on the > turnstile. > > To avoid deadlocks caused by recursive readers a new per-thread count of > outstanding rw read locks is maintained. When a thread is known to own > read locks it will bypass the pending writers checks. This could lead to > writer starvation in pathological cases but will not deadlock. > > In addition, I have added code to optimistically spin in the write lock > path when there are readers. The spin is limited by two counters. One > controls the number of spins while waiting for the lock state to change > and another controls the number of lock state changes we'll observe before > we give up. > > To add these two features new flags were required. I removed the > recursion flag and instead there is an extra branch in the inlined code to > check the recursion count before the atomic. Another option would be to > further increase the alignment of struct thread, however, I don't really > think that is necessary. > > This has been thoroughly tested with nokia's stack but it had to be > forward ported to current. Any rwlock users are encouraged to test before > I commit. > > Feedback is welcome. Jeff, it is not missing a kern/subr_witness.c part where some checks are axed out? Attilio -- Peace can only be achieved by understanding - A. Einstein