On Fri, Nov 21, 2008 at 10:14:33PM +0530, Aneesh Kumar K.V wrote:
> We need to make sure we update the block bitmap and clear
> EXT4_BG_BLOCK_UNINIT flag with sb_bgl_lock held. We look
> at EXT4_BG_BLOCK_UNINIT and reinit the block bitmap each
> time in ext4_read_block_bitmap (introduced by
> c806e68f5647109350ec546fee5b526962970fd2 )
You are changing mb_clear_bits() and and mb_set_bits() so they take
the spinlock over the entire operaiton, instead of over each
particular bit. These function are used in a largish number of
places, not just for updating the block bitmap, but also the mb buddy
bitmaps, etc. So there may be a scalability impact here, although
taking the spinlock once instead of multiple times is probably a win.
My bigger concern is given that we are playing games like *this*:
if ((cur & 31) == 0 && (len - cur) >= 32) {
/* fast path: set whole word at once */
addr = bm + (cur >> 3);
*addr = 0xffffffff;
cur += 32;
continue;
}
without taking a lock, I'm a little surprised we haven't been
seriously burned by other race conditions. What's the point of
calling mb_set_bit_atomic() and passing in a spinlock if we are doing
this kind of check without the protection of the same spinlock?!?
Andreas, if you are using mb_clear_bits() and mb_set_bits() in
Lustre's mballoc.c with this in production, you may want to take a
look at this patch.
- Ted
--
To unsubscribe from this list: send the line "unsubscribe linux-ext4" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
