On Mon, Sep 18, 2006 at 06:38:05PM -0500, Dave Kleikamp wrote: > On Mon, 2006-09-18 at 15:36 -0500, Dave Kleikamp wrote: > > EXT2: Remove superblock lock contention in ext2_statfs > > > > Fix a performance degradation introduced in 2.6.17. (30% degradation running > > dbench with 16 threads) > > > > Patch 21730eed11de42f22afcbd43f450a1872a0b5ea1, which claims to make > > EXT2_DEBUG work again, moves the taking of the kernel lock out of debug-only > > code in ext2_count_free_inodes and ext2_count_free_blocks and into > > ext2_statfs. This patch reverses that part of the patch. > > > > Signed-off-by: Dave Kleikamp <shaggy@xxxxxxxxxxxxxx> > > Eric Sandeen pointed out to me that taking the superblock lock in > ext2_count_free_* will cause a deadlock when EXT2FS_DEBUG is enabled, > since the superblock is locked in write_super(). > > We found that the same problem was fixed in ext3 with this patch > (forgive the long link): > http://git.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=5b11687924e40790deb0d5f959247ade82196665;hp=2384f55f8aa520172c995965bd2f8a9740d53095 > > The patch below just removes the use of the superblock lock in the debug > code. (Sorry for the delay; been on vacation.) Heh, I ran into the same lock nesting issues as you when I first tried to fix this; the lock debugging code found it for me. I asked for feedback on the locking issue when I submitted the patch, but no one had any opinions then, so I chose consistency over possible contention. Al Viro snorted at the idea of consistency in the results of statfs (I paraphrase his IRC remarks), and thinking about it further, I realized the debug code should not be doing these checks in statfs anyway; only on mount and unmount. This is because it appears that the block group accounting and the overall fs accounting are done non-atomically - see group_reserve_blocks() for example - and part of what the code does is reconcile these two numbers. It is legal for a valid fs to have the block group summaries and the fs-wide summaries out of sync, so the debug code could erroneously report an error, leading some poor soul on a wild goose chase. Removing this code from statfs also happens to fix the locking issues nicely. Rewriting this has been on my todo list for about 6 months now - anyone interested in grabbing it? I'm on #linuxfs on irc.oftc.net if anyone wants to chat about it. -VAL - 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
