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
next prev parent 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] fs: record task name which froze superblock 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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