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From: Mateusz Guzik <mguzik@redhat.com>
To: Dave Chinner <david@fromorbit.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>, Jan Kara <jack@suse.cz>,
	Andrew Morton <akpm@linux-foundation.org>,
	Al Viro <viro@zeniv.linux.org.uk>,
	Linux Kernel <linux-kernel@vger.kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	swhiteho@redhat.com, cluster-devel@redhat.com
Subject: Re: [PATCH v3] fs: record task name which froze superblock
Date: Mon, 2 Mar 2015 05:46:34 +0100	[thread overview]
Message-ID: <20150302044633.GB2516@mguzik> (raw)
In-Reply-To: <20150302043828.GA2516@mguzik>

On Mon, Mar 02, 2015 at 05:38:29AM +0100, Mateusz Guzik wrote:
> On Sun, Mar 01, 2015 at 08:31:26AM +1100, Dave Chinner wrote:
> > On Sat, Feb 28, 2015 at 05:25:57PM +0300, Alexey Dobriyan wrote:
> > > Freezing and thawing are separate system calls, task which is supposed
> > > to thaw filesystem/superblock can disappear due to crash or not thaw
> > > due to a bug. At least record task name (we can't take task_struct
> > > reference) to make support engineer's life easier.
> > > 
> > > Hopefully 16 bytes per superblock isn't much.
> > > 
> > > TASK_COMM_LEN definition (which is userspace ABI, see prctl(PR_SET_NAME)) is
> > > moved to userspace exported header to not drag sched.h into every fs.h inclusion.
> > > 
> > > Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
> > 
> > Freeze/thaw can be nested at the block level. That means the
> > sb->s_writers.freeze_comm can point at the wrong process. i.e.
> > 
> > Task A			Task B
> > freeze_bdev
> >   freeze_super
> >     freeze_comm = A
> > 			freeze_bdev
> > .....
> > thaw_bdev
> >  <device still frozen>
> > 			<crash>
> > 
> > At this point, the block device will never be unthawed, but
> > the debug field is now pointing to the wrong task. i.e. The debug
> > helper has not recorded the process that is actually causing the
> > problem, and leads us all off on a wild goose chase down the wrong
> > path.
> > 
> > IMO, debug code is only useful if it's reliable.....
> > 
> 
> It can be trivially modified to be very useful to support people.
> 
> Actually this patch clears saved task name on unfreeze, so in this
> particular scenario we would end up with no data.
> 
> Freezer and unfreezer names don't even have to match, so there is not
> much we can do here (e.g. recording all names in a linked list or
> something is a non-starter because of this).
> 
> I propose the following:
> - on freezing:
> 1. if 0->1 save the name
> 2. if 1->2 have a flag to note there is an additional freezer
> - on unfreezing
> 1. if 1->0 clear the flag
> 2. DO NOT clear the name in any case
> 

Now that I sent this e-mail I realized we could actually keep a linked
list of freezer names. Unfreezing would delete all elements when going
1->0, but would not touch it otherwise.

This would cover a less likely use case though, so I would be fine
either way FWIW.

Just my $0,03.

> This way we keep the name for possible future reference and we know
> whether something with this name was the sole freezer in this cycle.
> 
> As explained below, this one task name is already very useful and likely
> covers majority of real life use cases.
> 
> While working in support we were getting a lot of vmcores where hung task
> detector panicked the kernel because a lot of tasks were blocked
> in UN state trying to write to frozen filesystems. I presume OP has
> similar story.
> 
> Some back on forth commuication almost always revealed one process e.g.
> freezing stuff and then blocking itself trying to access it. While we
> could see it blocked, we had no presumptive evidence to pin freezing on
> it. A matching name, while still not 100% conclusive, would be ok enough
> to push the case forward and avoid a rountrip of systemap scripts
> showing freezer process tree.
> 

-- 
Mateusz Guzik

  reply	other threads:[~2015-03-02  4:46 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-02-14 18:55 [PATCH] " Alexey Dobriyan
2015-02-16  9:38 ` Jan Kara
2015-02-18  7:34   ` Alexey Dobriyan
2015-02-18  7:36     ` [PATCH v2] " Alexey Dobriyan
2015-02-18  9:13     ` [PATCH] " Jan Kara
2015-02-18 10:18       ` Steven Whitehouse
2015-02-20 11:42       ` Alexey Dobriyan
2015-02-20 12:15         ` Jan Kara
2015-02-28 14:22           ` Alexey Dobriyan
2015-02-28 14:25             ` [PATCH v3] " Alexey Dobriyan
2015-02-28 21:31               ` Dave Chinner
2015-03-02  4:38                 ` Mateusz Guzik
2015-03-02  4:46                   ` Mateusz Guzik [this message]
2015-03-09 15:14                     ` Alexey Dobriyan
2015-03-02 21:33                   ` Dave Chinner
2015-03-04 15:14                   ` Alexey Dobriyan

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