Messages in this thread Patch in this message | | | Date | Mon, 22 Apr 2002 14:01:27 -0700 | From | Dave Hansen <> | Subject | locking in sync_old_buffers |
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BKL hold times in sync_old_buffers aren't good. This is one of the last places that we see millisecond-scale BKL hold times. 0.09% 8.1% 4598us(9648us) 105us( 231us)(0.00%) 37 91.9% 8.1% 0% sync_old_buffers+0x1c
So, I'll ask the eternal question that plagues us all, why is the BKL held here? I saw some discussions referring to races prevented by BKL here: http://groups.google.com/groups?selm=linux.kernel.Pine.GSO.4.21.0103212354420.2632-100000%40weyl.math.psu.edu&output=gplain From Al: > Ehh... Linus, the main problem is in get_super(). Want a nice race? > sys_ustat() vs. sys_umount(). The former does get_super(), finds > struct super_block and does nothing to guarantee that it will stay. > Then it calls ->statfs(). In the meanwhile, you umount the thing > and do rmmod. Oops..
With the addition of sb_lock and refcounting on the sb, are these races still there? sync_supers() and sync_unlocked_inodes() both hold sb_lock.
If we just remove the BKL from sync_old_buffers() the long hold time goes from BKL to sb_lock. But, sb_lock is dropped and reacquired a few times in that code, so latency should be better than BKL. BKL wouldn't have been released except in those places where it had to drop the locks and sleep.
Patch to do it is attached, but I'm not holding my breath.
-- Dave Hansen haveblue@us.ibm.com --- linux-2.5.8-clean/fs/buffer.c Mon Apr 22 13:45:34 2002 +++ linux/fs/buffer.c Mon Apr 22 13:45:49 2002 @@ -2612,10 +2612,8 @@ static void sync_old_buffers(unsigned long dummy) { - lock_kernel(); sync_unlocked_inodes(); sync_supers(); - unlock_kernel(); for (;;) { struct buffer_head *bh; | |