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 From : Sergey Lentsov                       2:4615/71.10   02 Aug 2001  16:37:48
 To : All
 Subject : URL: http://www.lwn.net/2001/0802/kernel.php3
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    See also: [14]last week's Kernel page.
    
 Kernel development
 
    The current kernel release is still 2.4.7. The 2.4.8 prepatch is
    currently at [15]2.4.8pre3; it includes the usual collection of fixes,
    along with the single-use patch from Daniel Phillips which was covered
    [16]last week. There have been [17]complaints that the 2.4.8pre series
    is much slower on systems with large amounts of memory; the VM hackers
    are currently hot on the trail of those problems.
    
    Users of Adaptec adaptors (i.e. your editor, grumble grumble...) on
    SMP systems were unpleasantly surprised with 2.4.8pre2, which crashed
    on boot. The check that caused the crash has been removed, but there
    appears to be a strange problem that still lurks in there somewhere.
    
    Alan Cox's latest patch is [18]2.4.7ac3. It contains a great many
    architecture-specific changes; slowly the kernel trees for the various
    ports are finding their way back toward the mainline. There's also
    some enhancements for User-Mode Linux and many miscellaneous fixes.
    
    A new kernel API for completion events. It is common in kernel code to
    set some sort of process in motion, then to go to sleep and wait until
    that process completes. There are several ways of implementing the
    "wait for completion" part; which is the proper one to use depends on
    the specific situation. Until 2.4.7 came out, one technique used
    involved semaphores. The initiating process would declare a semaphore
    as a local variable (i.e. on the stack), starting out in the locked
    state; the process would do what was needed to arrange for some work
    to be done, then wait on the semaphore. The code actually doing the
    work would simply unlock the semaphore when the task was complete.
    
    On the surface, this technique is appealing because it avoids some
    obvious race conditions. If, for example, the work gets done before
    the kernel gets around to waiting on the semaphore, it notices that
    fact and simply doesn't wait. The sleep_on() and wake_up() calls can
    be much trickier to use correctly in this situation. But, as it turns
    out, there is a race condition here too, which is a result of how the
    semaphores themselves work.
    
    When a semaphore is to be unlocked, the code (1) sets the semaphore
    itself to the unlocked state, then (2) calls wake_up() to notify any
    processes that might have been waiting on the semaphore. If the waiter
    tests the semaphore between those two steps, it will never actually
    wait, and may well execute the rest of its code before the wake_up()
    call happens. That is not normally a problem, but, if the semaphore is
    sitting on a kernel stack somewhere, it could cease to exist before
    the wake_up() call, which requires data from the semaphore, runs. In
    other words, it could be working with a pointer into random memory;
    the technical term for this is "oops." This particular race is highly
    unlikely to ever actually happen, but it's still a race.
    
    The performance of this approach is also suboptimal, due to the fact
    that semaphores are optimized for the unlocked case. In this
    particular situation, the semaphore will almost always be locked.
    
    Linus chose not to change the semaphore implementation (it's "painful
    as hell"); instead, he [19]created a new interface for the handling of
    completion events. All a process need do to use this facility is to
    create and initialize a completion structure:
         struct completion event;
         init_completion(&event);
 
    Then it can set things in motion, and call:
         wait_for_completion(&event);
 
    to sleep until things are done. The task actually doing the work can
    perform a simple call to
         complete(&event);
 
    and the waiting process wakes up.
    
    It's a relatively straightforward solution, even if changing APIs in
    the middle of a stable kernel series may look a little strange. If
    nothing else, the whole affair makes it clear, once again, just how
    hard it is to avoid race conditions in kernel code.
    
    The first initramfs patch was [20]posted by Alexander Viro this week.
    This patch is the implementation of the new 2.5 boot process that was
    first discussed in the [21]July 12 kernel page. In this scheme, the
    kernel executable image carries with it a cpio archive containing the
    contents of the initial root filesystem. That archive is loaded into a
    ramdisk at boot time, at which time it can be used to continue the
    system initialization process.
    
    The hope is to move much kernel initialization code out of kernel
    space and into this ramdisk. The result is a smaller kernel and more
    flexibility in how the bootstrap process is set up. For the moment,
    the tasks that have been moved to user space include:
      * Finding and mounting the real (permanent) root filesystem. NFS
        root filesystems are handled here as well.
      * Setting up any initial ramdisk (usually for the purpose of loading
        kernel modules needed for the boot process).
      * Running the linuxrc boot script.
      * Finding the real init process and running it.
        
    There is more that can be moved into this filesystem, but that's a
    good start. The claim is that kernels running with this patch will
    function identically; no boot setups should be broken or require
    changes. Mr. Viro would, of course, like to hear from anybody with
    evidence to the contrary.
    
    Heading toward ext3 1.0. [22]ext3 2.4-0.9.5 was released by Andrew
    Morton. This version continues the work toward a truly stable ext3
    journaling filesystem release, fixing a number of bugs. Much work has
    also gone into performance improvements on a number of fronts. Among
    other things, synchronous operations happen more quickly; this should
    make people running large mail systems happy, since many mail transfer
    agents make heavy use of synchronous directory operations.
    
    Another change in 0.9.5 is the ability to use an external journal.
    External journals live on a separate device (perhaps a non-volatile
    RAM device), and, in theory, can speed up the operation of the
    filesystem. Writes to an external journal should be very quick, and
    journal operations will not contend with writes to the rest of the
    disk. The initial performance results with external journals appear to
    be mixed, however.
    
    Those interested in ext3 may also want to see [23]an older patch
    announcement from Andrew which contains a detailed explanation of the
    three journaling modes supported.
    
    Much slower routing performance in 2.4 has been reported by some
    users. The common factor in these reports is that the people involved
    are still using the 2.2 ipchains interface to set up their
    firewalling. The ipchains module in 2.4 carries full connection
    tracking along with it; most people setting up ipchains rules probably
    do not need that feature. The solution is to switch to iptables.
    
    Other patches and updates released this week include:
    
      * Daniel Phillips has posted [24]a new version of his patch for the
        handling of pages that are used only once.
      * Anton Altaparmakov has [25]released version 1.2.0 of the
        Linux-NTFS support tools.
      * Also from Anton is [26]this patch which adds support for Windows
        2000/XP dynamic disks.
      * David Schleef has posted [27]Comedi-0.7.60, a collection of data
        acquisition device drivers.
      * Alan Cox has [28]modified the kernel Makefile to add a "make rpm"
        target. The result, of course, is an RPM file containing the
        compiled kernel. A "make deb" option will likely be added in the
        near future.
      * Milan Pikula has [29]started a new mailing list for those who are
        interested in filesystem repair and crash recovery topics.
      * An [30]Mwave modem driver for 2.4.7 has been released by Paul
        Schroeder.
      * [31]devfsd v1.3.12 was released by Richard Gooch.
      * Richard also released [32]a patch that, when used with devfs,
        enables a 2.4 kernel to support up to ("approximately") 2144 SCSI
        disks. He warns that it is untested and could result in filesystem
        corruption. There have been few problem reports, but it turns out
        that, for now, limitations in other parts of the system will still
        limit the maximum number of disks to far less than 2144.
      * The Linux Test Project has released [33]ltp-20010801, the latest
        version of its kernel test suite.
      * Andreas Gruenbacher has posted [34]an access control list patch
        for 2.4.7.
      * Constantin Loizides has been working with a number of journaling
        filesystems to determine the degree to which they experience
        fragmentation under long-term, sustained use. He has [35]posted
        his findings; the results vary significantly between the various
        filesystems.
      * Keith Owens has released [36]a 2.5 kbuild release candidate.
      * Adam Goode has [37]started a project to write a driver for the
        Logitech iFeel mouse. This device is fun in that it can be used to
        provide tactile feedback to the user - little bumps as the pointer
        moves over buttons and such.
      * [38]mdctl 0.4 was released by Neil Brown; it is a utility for
        controlling RAID devices, meant to replace mkraid, raidstart, etc.
      * Version 2.2.0 of the [39]Functionally Overloaded Linux Kernel
        patch is now available; it has almost anything one could imagine,
        including several kitchen sinks. FOLK creator Jonathan Day informs
        us that the size of the patch is now 1/3 that of the standard
        kernel.
        
    Section Editor: [40]Jonathan Corbet
    August 2, 2001
    
    For other kernel news, see:
      * [41]Kernel traffic
      * [42]Kernel Newsflash
      * [43]Kernel Trap
    
    Other resources:
      * [44]Kernel Source Reference
      * [45]L-K mailing list FAQ
      * [46]Linux-MM
      * [47]Linux Scalability Project
      * [48]Kernel Newbies
    
    
    
                                                   [49]Next: Distributions
    
    [50]Eklektix, Inc. Linux powered! Copyright Л 2001 [51]Eklektix, Inc.,
    all rights reserved
    Linux (R) is a registered trademark of Linus Torvalds
 
 References
 
    1. http://lwn.net/
    2. http://ads.tucows.com/click.ng/pageid=001-012-132-000-000-003-000-000-012
    3. http://lwn.net/2001/0802/
    4. http://lwn.net/2001/0802/security.php3
    5. http://lwn.net/2001/0802/dists.php3
    6. http://lwn.net/2001/0802/desktop.php3
    7. http://lwn.net/2001/0802/devel.php3
    8. http://lwn.net/2001/0802/commerce.php3
    9. http://lwn.net/2001/0802/press.php3
   10. http://lwn.net/2001/0802/announce.php3
   11. http://lwn.net/2001/0802/history.php3
   12. http://lwn.net/2001/0802/letters.php3
   13. http://lwn.net/2001/0802/bigpage.php3
   14. http://lwn.net/2001/0726/kernel.php3
   15. http://lwn.net/2001/0802/a/2.4.8pre3.php3
   16. http://lwn.net/2001/0726/kernel.php3
   17. http://lwn.net/2001/0802/a/2.4.8-vm.php3
   18. http://lwn.net/2001/0802/a/2.4.7ac3.php3
   19. http://lwn.net/2001/0802/a/lt-completions.php3
   20. http://lwn.net/2001/0802/a/initramfs.php3
   21. http://lwn.net/2001/0712/kernel.php3
   22. http://lwn.net/2001/0802/a/ext3.php3
   23. http://lwn.net/2001/0802/a/ext3-modes.php3
   24. http://lwn.net/2001/0802/a/use-once.php3
   25. http://lwn.net/2001/0802/a/ntfs.php3
   26. http://lwn.net/2001/0802/a/dynamic-disks.php3
   27. http://lwn.net/2001/0802/a/comedi.php3
   28. http://lwn.net/2001/0802/a/make-rpm.php3
   29. http://lwn.net/2001/0802/a/fs-salvage.php3
   30. http://lwn.net/2001/0802/a/mwave.php3
   31. http://lwn.net/2001/0802/a/devfsd.php3
   32. http://lwn.net/2001/0802/a/lotsa-scsi.php3
   33. http://lwn.net/2001/0802/a/ltp.php3
   34. http://lwn.net/2001/0802/a/acl.php3
   35. http://lwn.net/2001/0802/a/fragmentation.php3
   36. http://lwn.net/2001/0802/a/kbuild.php3
   37. http://lwn.net/2001/0802/a/tactile.php3
   38. http://lwn.net/2001/0802/a/mdctl.php3
   39. http://folk.sourceforge.net/
   40. mailto:lwn@lwn.net
   41. http://kt.zork.net/
   42. http://www.atnf.csiro.au/~rgooch/linux/docs/kernel-newsflash.html
   43. http://www.kerneltrap.com/
   44. http://lksr.org/
   45. http://www.tux.org/lkml/
   46. http://www.linux.eu.org/Linux-MM/
   47. http://www.citi.umich.edu/projects/linux-scalability/
   48. http://www.kernelnewbies.org/
   49. http://lwn.net/2001/0802/dists.php3
   50. http://www.eklektix.com/
   51. http://www.eklektix.com/
 
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