From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mateusz Guzik Subject: Re: [PATCH v3] fs: record task name which froze superblock Date: Mon, 2 Mar 2015 05:46:34 +0100 Message-ID: <20150302044633.GB2516@mguzik> References: <20150214185524.GA16579@p183.telecom.by> <20150216093852.GB4749@quack.suse.cz> <20150218073455.GA1752@p183.telecom.by> <20150218091323.GA4614@quack.suse.cz> <20150220121522.GC6293@quack.suse.cz> <20150228142235.GA19552@p183.telecom.by> <20150228142557.GB19552@p183.telecom.by> <20150228213126.GM4251@dastard> <20150302043828.GA2516@mguzik> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Cc: Alexey Dobriyan , Jan Kara , Andrew Morton , Al Viro , Linux Kernel , linux-fsdevel , swhiteho@redhat.com, cluster-devel@redhat.com To: Dave Chinner Return-path: Content-Disposition: inline In-Reply-To: <20150302043828.GA2516@mguzik> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org 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 > > > > 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 > > > > > > > > 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