linux-kernel - Re: [PATCH PREEMPT RT] rt-mutex: fix deadlock in device mapper

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Date:   Thu, 23 Nov 2017 15:42:31 +0100
From:   Sebastian Siewior <bigeasy@...utronix.de>
To:     Mike Galbraith <efault@....de>
Cc:     Mikulas Patocka <mpatocka@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-rt-users@...r.kernel.org
Subject: Re: [PATCH PREEMPT RT] rt-mutex: fix deadlock in device mapper

On 2017-11-21 22:20:51 [+0100], Mike Galbraith wrote:
> On Tue, 2017-11-21 at 14:56 -0500, Mikulas Patocka wrote:
> > 
> > If we don't have any reason why it is needed to unplug block requests when 
> > a spinlock is taken - so let's not do this.
> 
> That's perfectly fine.  I guess I shouldn't have even mentioned having
> encountered unplug at mutex being insufficient.

While at it, I intend to drop
fs-jbd2-pull-your-plug-when-waiting-for-space.patch from the -RT queue
for v4.14 which does

--- a/fs/jbd2/checkpoint.c
+++ b/fs/jbd2/checkpoint.c
@@ -116,6 +116,8 @@ void __jbd2_log_wait_for_space(journal_t
 	nblocks = jbd2_space_needed(journal);
 	while (jbd2_log_space_left(journal) < nblocks) {
 		write_unlock(&journal->j_state_lock);
+		if (current->plug)
+			io_schedule();
 		mutex_lock(&journal->j_checkpoint_mutex);
 
 		/*

and is/was probably a workaround for the missing schedule while blocking
on mutex/rwsem.

> 	-Mike

Sebastian