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    SubjectRe: [GIT, RFC] Killing the Big Kernel Lock
    On Sunday 28 March 2010, Frederic Weisbecker wrote:
    > On Sun, Mar 28, 2010 at 09:05:50PM +0100, Arnd Bergmann wrote:
    > > > General thoughts:
    > > >
    > > > ".llseek = NULL," so far meant "do the Right Thing on lseek() and
    > > > friends, as far as the fs core can tell". Shouldn't we keep it that
    > > > way? It's as close to other ".method = NULL," as it can get, which
    > > > either mean "silently skip this method if it doesn't matter" (e.g.
    > > > .flush) or "fail attempts to use this method with a fitting errno" (e.g.
    > > > .write).
    > >
    > > My series changes the default from 'default_llseek' to 'generic_file_llseek',
    > > which is almost identical, except for taking the inode mutex instead of the
    > > BKL.
    > What if another file operation changes the file pointer while holding the bkl?
    > You're not protected anymore in this case.

    Exactly, that's why I changed all the drivers to set default_llseek explicitly.
    Even this is very likely not needed in more than a handful of drivers (if any),
    for a number of reasons:

    - sys_read/sys_write *never* hold any locks while calling file_pos_write(),
    which is the only place they get updated for regular files.
    - concurrent llseek plus other file operations on the same file descriptor
    usually already have an undefined outcome.
    - when I started inspecting drivers that look at file->f_pos themselves (not
    the read/write operation arguments), I found that practically all of them
    are doing this in a totally broken way!
    - The only think we'd probably ever want to lock against in llseek
    is readdir, which is not used in any drivers, but only in file systems.


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