On Tue, Nov 28, 2017 at 5:59 PM, Vivek Goyal <vgoyal@xxxxxxxxxx> wrote:
> Now we will have the capability to have upper inodes which might be only
> metadata copy up and data is still on lower inode. So add a new xattr
> OVL_XATTR_METACOPY to distinguish between two cases.
>
> Presence of OVL_XATTR_METACOPY reflects that file has been copied up metadata
> only and and data will be copied up later from lower origin.
> So this xattr is set when a metadata copy takes place and cleared when
> data copy takes place.
>
> We also use a bit in ovl_inode->flags to cache OVL_UPPERDATA which reflects
> whether ovl inode has data or not (as opposed to metadata only copy up).
>
> If a file is copied up metadata only and later when same file is opened
> for WRITE, then data copy up takes place. We copy up data, remove METACOPY
> xattr and then set the UPPERDATA flag in ovl_inode->flags. While all
> these operations happen with oi->lock held, read side of oi->flags can be
> lockless. That is another thread on another cpu can check if UPPERDATA
> flag is set or not.
>
> So this gives us an ordering requirement w.r.t UPPERDATA flag. That is, if
> another cpu sees UPPERDATA flag set, then it should be guaranteed that
> effects of data copy up and remove xattr operations are also visible.
>
> For example.
>
> CPU1 CPU2
> ovl_d_real() acquire(oi->lock)
> ovl_open_maybe_copy_up() ovl_copy_up_data()
> open_open_need_copy_up() vfs_removexattr()
> ovl_already_copied_up()
> ovl_dentry_needs_data_copy_up() ovl_set_flag(OVL_UPPERDATA)
> ovl_test_flag(OVL_UPPERDATA) release(oi->lock)
>
> Say CPU2 is copying up data and in the end sets UPPERDATA flag. But if
> CPU1 perceives the effects of setting UPPERDATA flag but not the effects
> of preceeding operations (ex. upper that is not fully copied up), it will be
> a problem.
>
> Hence this patch introduces smp_wmb() on setting UPPERDATA flag operation
> and smp_rmb() on UPPERDATA flag test operation.
>
> May be some other lock or barrier is already covering it. But I am not sure
> what that is and is it obvious enough that we will not break it in future.
>
> So hence trying to be safe here and introducing barriers explicitly for
> UPPERDATA flag/bit.
>
> Signed-off-by: Vivek Goyal <vgoyal@xxxxxxxxxx>
Reviewed-by: Amir Goldstein <amir73il@xxxxxxxxx>
Except for minor nits below.
...
> +static bool ovl_should_check_upperdata(struct dentry *dentry) {
You should run your patch through checkpatch.
Kernel coding style doesn't like opening brackets for function without newline.
Amir.
--
To unsubscribe from this list: send the line "unsubscribe linux-unionfs" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
