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    SubjectRe: how to write get_block?


    On Fri, 8 Oct 1999, Manfred Spraul wrote:

    > Alexander Viro wrote:
    > >All stuff with truncate()/write() is racey right now,
    > > AFAICS. The next thing I'm going to do is a (dumb) implementation of
    > > blocking rwlocks. write() being reader and truncate() - writer.
    >
    > IMHO that's the wrong approach:
    >
    > 1) overlapping concurrent write operations violate Nfsv2.

    Yes, that is serious.

    > 2) the pipe code needs O_NONBLOCK characteristics, ie no synchonization
    > is allowed. The 2.2 code uses an ugly hack: sys_write() acquires i_sem,
    > and inside the pipe code the semaphore is dropped.

    Huh? It's not a sys_write(), it's generic_file_write(). Pipes do not come
    even close to that place. And I'm talking about 2.3 - 2.2 is _way_ too
    different.

    > 3) do not forget f_pos when you implement a synchonization: even
    > concurrent reads are not allowed if both use the same file pointer.

    Ditto. It's a separate issue.

    > What about these ideas:
    > a) rename i_sem (*), and add public functions such as
    > inode_lock_exclusive(), inode_lock_truncate(), inode_lock_write() etc.
    >
    > Currently, everyone directly accesses the VFS synchonization (nfsd,
    > drivers/char/tty_io.c; arch/*/kernel/sys*.c).
    >
    > b) the requirements of the VFS users differ completely, add flags:
    > 0: synchonize nothing, eg pipes.
    > I_FPOS_SYNC: synchonize f_pos access, nothing else
    > I_FILE_SYNC: multiple read, non-overlapping write, truncate.
    >
    > Since it will be hidden by inode_lock_??(), it will be easy to optimize
    > the implementation as needed.
    >
    > Eg I'm pretty sure that Stephen will not like it if raw-io is slowed
    > down by a multiple reader-single writer synchonization.

    Erm? Writer access is for truncate() and I don't see where the raw-io
    comes into the picture.


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