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 -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
