On 08/27/2010 02:10 PM, Ted Ts'o wrote: > On Wed, Jul 14, 2010 at 01:58:46PM -0500, Brian King wrote: >> >> I've been debugging a hang in jbd2_journal_release_jbd_inode >> which is being seen on Power 6 systems quite a lot. When we get >> in the hung state, all I/O to the disk in question gets blocked >> where we stay indefinitely. Looking at the task list, I can see >> we are stuck in jbd2_journal_release_jbd_inode waiting on a >> wake up. I added some debug code to detect this scenario and >> dump additional data if we were stuck in jbd2_journal_release_jbd_inode >> for longer than 30 minutes. When it hit, I was able to see that >> i_flags was 0, suggesting we missed the wake up. >> >> This patch changes i_flags to be an unsigned long, uses bit operators >> to access it, and adds barriers around the accesses. Prior to applying >> this patch, we were regularly hitting this hang on numerous systems >> in our test environment. After applying the patch, the hangs no longer >> occur. Its still not clear to me why the j_list_lock doesn't protect us >> in this path. It also appears a hang very similar to this was seen >> in the past and then was no longer recreatable: > > I've been look at this patch, and I can see how converting to bitops > definitely makes sense. I can also see how adding > smp_mb__after_clear_bit() makes sense. However, it's not clear the > smp_mb() call here helps? It may not be necessary. I originally added it in order to balance the test_bit with the clear_bit. I'll check with the folks hitting this in test and see if I can get access to the failing machine. If so, I'll pull this out and see if we actually need it or not. Thanks, Brian -- Brian King Linux on Power Virtualization IBM Linux Technology Center -- 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
