ll_rw_block() changes buffer_heads to NULL?

> I'm still trying to read physical sectors, and have made progress.  Thanks
> for the pointers on set_blocksize(), that seems to do the trick.
> 
> However, now I've got another problem.  When I read blocks "too quickly",
> I guess the elevator algorithm in ll_rw_block() kicks in and re-organizes
> my buffer_head structures?  I'm trying to read 1 sector (which succeeds
> just fine), then 32 sectors, one at a time, which fails.
> ReadLBA(2) - OK
> ReadLBA(3) - OK
> ReadLBA(4) - fails, kernel dereferences NULL, 
> 
> All calls to getblk() succeed, and none of my buffer heads are NULL.  I
> pass this array to ll_rw_block(), which rewrites my array, leaving most of
> my buffer_head pointers as NULL.  So, I thought, that for each buffer head
> that isn't NULL, maybe my next buffer head is in b_reqnext, but nope.
> It's NULL too.
> 
FWIW, this is the second version of my function.  Originally, I just looped,
calling bread(dev, lba++, blocksize), which oops in the same way.  Both
IA-32 and IA-64, kernel 2.4.0-test9.

> TIA for your help.
> -Matt
> 
> struct buffer_head ** my_bread(kdev_t dev, u64 lba, u64 blockstoread)
> {
>   struct buffer_head **bh, *thisbh;
>         
>   unsigned int blocksize = get_hardblocksize(dev);
>   unsigned int i;
>   if (!blocksize) blocksize = 512;
>   printk(KERN_INFO "about to getblk %d blocks\n", blockstoread);
>   
>   bh = kmalloc(GFP_KERNEL, blockstoread * sizeof(struct buffer_head *));
>   if (!bh) return NULL;
> 
>   for (i=0; i<blockstoread; i++) {
>     bh[i] = getblk(dev, lba+i, blocksize);
>     if (!bh[i]) {
>       printk(KERN_WARNING, "getblk(lba=%x) failed.\n", lba);
>     }
>   }
> 
>  /* at this point, none of the bh[i]s are NULL. */
> 
>     ll_rw_block(READ, blockstoread, bh);
> 
>   /* now, all but the first 3 bh[i]s are NULL, and all bh[i]->b_reqnext ==
> NULL */
> 
>   /* Walk the chain */
>   for (i=0; i<blockstoread; i++) {
>     if (!bh[i]) continue;
> 
>     thisbh = bh[i];
>     do {
>       wait_on_buffer(thisbh);
>       if (!buffer_uptodate(thisbh)) {
>         printk(KERN_WARNING "bh is not uptodate after waiting!\n");
>       }
>       thisbh = thisbh->b_reqnext;
>       if (thisbh) printk(KERN_INFO "my_bread walking the chain...\n");
>     } while (thisbh);
>   }
> 
>   return bh;
> }
>