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

[Alexey Dobriyan <adobriyan@gmail.com>]

On Mon, Mar 2, 2015 at 7:46 AM, Mateusz Guzik <mguzik@redhat.com> wrote:
> 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.

If you do linked list two processes constantly keeping superblock frozen
will allocate all the memory:

F
  F
  T
  F
  T
 ...

After all valid objections and comments I think the way to proceed is
a) record freeze_comm, never clear it (maybe record thaw_comm)
b) record "i am not the only one" flag
c) introduce debug_freeze which will "record" everything in dmesg
for situations when crashdump is not used but serial console is.

    Alexey