Re: [PATCH v3] fs: record task name which froze superblock

[Mateusz Guzik]

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@xxxxxxxxx>
> > 
> > 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
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