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    SubjectRe: [PATCH 1/2] BKL: Remove BKL from default_llseek()
    On Wed, Nov 18, 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.

    As I said, f_pos is changed without holding BKL in the VFS already. Therefore
    even if the driver tries to protect f_pos by holding the BKL it is racing
    against concurrent read()/write() anyway on f_pos.

    Regards,
    Jan

    --
    Jan Blunck <jblunck@suse.de>


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