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Message-ID: <20130801223500.GH31857@quack.suse.cz> Date: Fri, 2 Aug 2013 00:35:00 +0200 From: Jan Kara <jack@...e.cz> To: Paul Gortmaker <paul.gortmaker@...driver.com> Cc: Jan Kara <jack@...e.cz>, Andrew Morton <akpm@...ux-foundation.org>, linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org Subject: Re: [PATCH 3/4] jbd: drop checkpoint mutex when waiting in __log_wait_for_space() On Thu 01-08-13 15:01:07, Paul Gortmaker wrote: > Backport of jbd2 commit 0ef54180e0187117062939202b96faf04c8673bc > ("jbd2: drop checkpoint mutex when waiting in __jbd2_log_wait_for_space()") > > Quoting that commit: > -------------- > 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: > [<ffffffff8172dc34>] schedule+0x24/0x70 > [<ffffffff81225b5d>] jbd2_log_wait_commit+0xbd/0x140 > [<ffffffff81060390>] ? __init_waitqueue_head+0x50/0x50 > [<ffffffff81223635>] jbd2_log_do_checkpoint+0xf5/0x520 > [<ffffffff81223b09>] __jbd2_log_wait_for_space+0xa9/0x1f0 > [<ffffffff8121dc40>] start_this_handle.isra.10+0x2e0/0x530 > [<ffffffff81060390>] ? __init_waitqueue_head+0x50/0x50 > [<ffffffff8121e0a3>] jbd2__journal_start+0xc3/0x110 > [<ffffffff811de7ce>] ? ext4_rmdir+0x6e/0x230 > [<ffffffff8121e0fe>] jbd2_journal_start+0xe/0x10 > [<ffffffff811f308b>] ext4_journal_start_sb+0x5b/0x160 > [<ffffffff811de7ce>] ext4_rmdir+0x6e/0x230 > [<ffffffff811435c5>] vfs_rmdir+0xd5/0x140 > [<ffffffff8114370f>] do_rmdir+0xdf/0x120 > [<ffffffff8105c6b4>] ? task_work_run+0x44/0x80 > [<ffffffff81002889>] ? do_notify_resume+0x89/0x100 > [<ffffffff817361ae>] ? int_signal+0x12/0x17 > [<ffffffff81145d85>] sys_unlinkat+0x25/0x40 > [<ffffffff81735f22>] 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. > -------------- As much as this is a sensible optimization, I don't agree with the analysis that the system can deadlock when JBD2_FLUSHED is set. We call jbd2_log_wait_commit() only if there is already committing transaction and that gets set *after* JBD2_FLUSHED is handled. So it would be good if you went back and actually checked where exactly did we hang in jbd2_journal_commit_transaction()... Honza > > Signed-off-by: Paul Gortmaker <paul.gortmaker@...driver.com> > --- > fs/jbd/checkpoint.c | 8 ++++++++ > 1 file changed, 8 insertions(+) > > diff --git a/fs/jbd/checkpoint.c b/fs/jbd/checkpoint.c > index 08c0304..8e8e0ee 100644 > --- a/fs/jbd/checkpoint.c > +++ b/fs/jbd/checkpoint.c > @@ -160,7 +160,15 @@ void __log_wait_for_space(journal_t *journal) > /* We were able to recover space; yay! */ > ; > } else if (tid) { > + /* > + * jbd_journal_commit_transaction() may want > + * to take the checkpoint_mutex if JBD_FLUSHED > + * is set. So we need to temporarily drop it. > + */ > + mutex_unlock(&journal->j_checkpoint_mutex); > log_wait_commit(journal, tid); > + spin_lock(&journal->j_state_lock); > + continue; > } else { > printk(KERN_ERR "%s: needed %d blocks and " > "only had %d space available\n", > -- > 1.8.1.2 > -- Jan Kara <jack@...e.cz> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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