On 07/21/2010 02:02 PM, Jan Kara 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. > Thanks for debugging this! I was thinking hard about how it could happen that > wake_up_bit doesn't wake up the waiter but I haven't found any explanation. All > the waitqueue work seems to be properly wrapped inside the j_list_lock so > even the waitqueue_active check in wake_up_bit should be fine. > I'd really like to understand what in my mind-model of spinlocks etc. is > wrong. So could you maybe run a test with the attached debug patch and > dump 'wait.seen' value in the hung task? > And one more question - if you remove 'waitqueue_active' check from > kernel/wait.c:__wake_up_bit > is the problem still present? Thanks a lot in advance. I'll see about getting one of our systems loaded up with this change and see what happens. 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
