https://bugzilla.kernel.org/show_bug.cgi?id=15906





--- Comment #27 from Eric Sandeen <sandeen@redhat.com>  2010-05-21 02:07:01 ---
static int pin_sb_for_writeback(struct writeback_control *wbc,
                                struct inode *inode, struct super_block **psb)
{
        struct super_block *sb = inode->i_sb;

        /*
         * If this sb is already pinned, nothing more to do. If not and
         * *psb is non-NULL, unpin the old one first
         */
        if (sb == *psb)
                return 0;
        else if (*psb)
                unpin_sb_for_writeback(psb);

        /*
         * Caller must already hold the ref for this
         */
        if (wbc->sync_mode == WB_SYNC_ALL || wbc->sb_pinned) {
                WARN_ON(!rwsem_is_locked(&sb->s_umount));  <<----- here
                return 0;
        }    

http://git.kernel.org/?p=linux/kernel/git/axboe/linux-2.6-block.git;a=commitdiff;h=e91...

is what Jens had upstream, and I think 

http://git.kernel.org/?p=linux/kernel/git/axboe/linux-2.6-block.git;a=commitdiff;h=30f...

is the fix for the issue you see here...

quoted text
> Even if the writeout itself isn't a data integrity operation, we need
> to ensure that the caller doesn't drop the sb umount sem before we
> have actually done the writeback.

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