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    SubjectRe: [PATCH 1/2] BKL: Remove BKL from default_llseek()
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    On Wednesday 18 November 2009, Jamie Lokier wrote:
    > Alan Cox wrote:
    > > > Using the BKL in llseek() does not protect the inode's i_size from
    > > > modification since the i_size is protected by a seqlock nowadays. Since
    > > > default_llseek() is already using the i_size_read() wrapper it is not the
    > > > BKL which is serializing the access here.
    > > > The access to file->f_pos is not protected by the BKL either since its
    > > > access in vfs_write()/vfs_read() is not protected by any lock. If the BKL
    > > > is not protecting anything here it can clearly get removed.
    > >
    > > No. Your logic is flawed
    > >
    > > The BKL is protected something here - it protects the change of offset
    > > with respect to other BKL users within drivers. The question is what if
    > > anything in any other driver code depends upon the BKL and uses it to
    > > protect f_pos. Probably very little if anything but a grep for f_pos
    > > through the drivers might not be a bad idea before assuming this. Very
    > > few touch f_pos except in their own llseek method.
    >
    > Of course, drivers shouldn't be using f_pos outside their llseek
    > method, as they should all behave the same with pread/pwrite as with
    > llseek+read/write.
    >
    > Is that mistaken?

    There are drivers touching f_pos in ioctl() methods, which is vaguely
    reasonable. There are also driver touching it in their read()/write()
    methods, which has no effect whatsoever.

    I started grepping through the kernel trying to find any instances
    of the first case that uses the BKL, but I only found three instances
    of the second case and got heavily demotivated by that.

    Arnd <><


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