From 0ef54180e0187117062939202b96faf04c8673bc Mon Sep 17 00:00:00 2001 From: Paul Gortmaker Date: Wed, 12 Jun 2013 22:47:35 -0400 Subject: [PATCH] jbd2: drop checkpoint mutex when waiting in __jbd2_log_wait_for_space() While trying to debug an an issue under extreme I/O loading on preempt-rt kernels, the following backtrace was observed via SysRQ output: rm D ffff8802203afbc0 4600 4878 4748 0x00000000 ffff8802217bfb78 0000000000000082 ffff88021fc2bb80 ffff88021fc2bb80 ffff88021fc2bb80 ffff8802217bffd8 ffff8802217bffd8 ffff8802217bffd8 ffff88021f1d4c80 ffff88021fc2bb80 ffff8802217bfb88 ffff88022437b000 Call Trace: [] schedule+0x24/0x70 [] jbd2_log_wait_commit+0xbd/0x140 [] ? __init_waitqueue_head+0x50/0x50 [] jbd2_log_do_checkpoint+0xf5/0x520 [] __jbd2_log_wait_for_space+0xa9/0x1f0 [] start_this_handle.isra.10+0x2e0/0x530 [] ? __init_waitqueue_head+0x50/0x50 [] jbd2__journal_start+0xc3/0x110 [] ? ext4_rmdir+0x6e/0x230 [] jbd2_journal_start+0xe/0x10 [] ext4_journal_start_sb+0x5b/0x160 [] ext4_rmdir+0x6e/0x230 [] vfs_rmdir+0xd5/0x140 [] do_rmdir+0xdf/0x120 [] ? task_work_run+0x44/0x80 [] ? do_notify_resume+0x89/0x100 [] ? int_signal+0x12/0x17 [] sys_unlinkat+0x25/0x40 [] system_call_fastpath+0x16/0x1b What is interesting here, is that we call log_wait_commit, from within wait_for_space, but we are still holding the checkpoint_mutex as it surrounds mostly the whole of wait_for_space. And then, as we are waiting, journal_commit_transaction can run, and if the JBD2_FLUSHED bit is set, then we will also try to take the same checkpoint_mutex. It seems that we need to drop the checkpoint_mutex while sitting in jbd2_log_wait_commit, if we want to guarantee that progress can be made by jbd2_journal_commit_transaction(). There does not seem to be anything preempt-rt specific about this, other then perhaps increasing the odds of it happening. Signed-off-by: Paul Gortmaker Signed-off-by: "Theodore Ts'o" --- fs/jbd2/checkpoint.c | 8 ++++++++ 1 file changed, 8 insertions(+) diff --git a/fs/jbd2/checkpoint.c b/fs/jbd2/checkpoint.c index 75a15f371b00..7f34f4716165 100644 --- a/fs/jbd2/checkpoint.c +++ b/fs/jbd2/checkpoint.c @@ -156,7 +156,15 @@ void __jbd2_log_wait_for_space(journal_t *journal) /* We were able to recover space; yay! */ ; } else if (tid) { + /* + * jbd2_journal_commit_transaction() may want + * to take the checkpoint_mutex if JBD2_FLUSHED + * is set. So we need to temporarily drop it. + */ + mutex_unlock(&journal->j_checkpoint_mutex); jbd2_log_wait_commit(journal, tid); + write_lock(&journal->j_state_lock); + continue; } else { printk(KERN_ERR "%s: needed %d blocks and " "only had %d space available\n", -- 2.20.1