Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S264099AbTEOQZw (ORCPT ); Thu, 15 May 2003 12:25:52 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S264101AbTEOQZw (ORCPT ); Thu, 15 May 2003 12:25:52 -0400 Received: from air-2.osdl.org ([65.172.181.6]:50062 "EHLO mail.osdl.org") by vger.kernel.org with ESMTP id S264099AbTEOQZu (ORCPT ); Thu, 15 May 2003 12:25:50 -0400 Subject: Re: Race between vmtruncate and mapped areas? From: Daniel McNeil To: Andrea Arcangeli Cc: Andrew Morton , dmccr@us.ibm.com, mika.penttila@kolumbus.fi, linux-mm@kvack.org, Linux Kernel Mailing List In-Reply-To: <20030515094041.GA1429@dualathlon.random> References: <154080000.1052858685@baldur.austin.ibm.com> <20030513181018.4cbff906.akpm@digeo.com> <18240000.1052924530@baldur.austin.ibm.com> <20030514103421.197f177a.akpm@digeo.com> <82240000.1052934152@baldur.austin.ibm.com> <20030515004915.GR1429@dualathlon.random> <20030515013245.58bcaf8f.akpm@digeo.com> <20030515085519.GV1429@dualathlon.random> <20030515022000.0eb9db29.akpm@digeo.com> <20030515094041.GA1429@dualathlon.random> Content-Type: multipart/mixed; boundary="=-FFfYtZuZ9pM5fMlSmUEX" Organization: Message-Id: <1053016706.2693.10.camel@ibm-c.pdx.osdl.net> Mime-Version: 1.0 X-Mailer: Ximian Evolution 1.2.1 Date: 15 May 2003 09:38:26 -0700 Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3096 Lines: 104 --=-FFfYtZuZ9pM5fMlSmUEX Content-Type: text/plain Content-Transfer-Encoding: 7bit On Thu, 2003-05-15 at 02:40, Andrea Arcangeli wrote: > On Thu, May 15, 2003 at 02:20:00AM -0700, Andrew Morton wrote: > > Andrea Arcangeli wrote: > > > > > > and it's still racy > > > > damn, and it just booted ;) > > > > I'm just a little bit concerned over the ever-expanding inode. Do you > > think the dual sequence numbers can be replaced by a single generation > > counter? > > yes, I wrote it as a single counter first, but was unreadable and it had > more branches, so I added the other sequence number to make it cleaner. > I don't mind another 4 bytes, that cacheline should be hot anyways. You could use the seqlock.h sequence locking. It only uses 1 sequence counter. The 2.5 isize patch 1 has a sequence lock without the spinlock so it only uses 4 bytes and it is somewhat more readable. I don't think it has more branches. I've attached the isize seqlock.h patch. -- Daniel McNeil --=-FFfYtZuZ9pM5fMlSmUEX Content-Disposition: attachment; filename=patch-2.5.69-isize.1 Content-Type: text/x-patch; name=patch-2.5.69-isize.1; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit diff -rupN -X /home/daniel/dontdiff linux-2.5.69/include/linux/seqlock.h linux-2.5.69-isize/include/linux/seqlock.h --- linux-2.5.69/include/linux/seqlock.h Sun May 4 16:53:14 2003 +++ linux-2.5.69-isize/include/linux/seqlock.h Wed May 7 16:00:25 2003 @@ -94,6 +94,57 @@ static inline int read_seqretry(const se return (iv & 1) | (sl->sequence ^ iv); } + +/* + * Version using sequence counter only. + * This can be used when code has its own mutex protecting the + * updating starting before the write_seqcntbeqin() and ending + * after the write_seqcntend(). + */ + +typedef struct seqcnt { + unsigned sequence; +} seqcnt_t; + +#define SEQCNT_ZERO { 0 } +#define seqcnt_init(x) do { *(x) = (seqcnt_t) SEQCNT_ZERO; } while (0) + +/* Start of read using pointer to a sequence counter only. */ +static inline unsigned read_seqcntbegin(const seqcnt_t *s) +{ + unsigned ret = s->sequence; + smp_rmb(); + return ret; +} + +/* Test if reader processed invalid data. + * Equivalent to: iv is odd or sequence number has changed. + * (iv & 1) || (*s != iv) + * Using xor saves one conditional branch. + */ +static inline int read_seqcntretry(const seqcnt_t *s, unsigned iv) +{ + smp_rmb(); + return (iv & 1) | (s->sequence ^ iv); +} + + +/* + * Sequence counter only version assumes that callers are using their + * own mutexing. + */ +static inline void write_seqcntbegin(seqcnt_t *s) +{ + s->sequence++; + smp_wmb(); +} + +static inline void write_seqcntend(seqcnt_t *s) +{ + smp_wmb(); + s->sequence++; +} + /* * Possible sw/hw IRQ protected versions of the interfaces. */ --=-FFfYtZuZ9pM5fMlSmUEX-- - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/