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. It only clears it i thaw_super(), which is *not called* until the last nested thaw_bdev() call is made. When the system is hung what we actually need to know is who is responsible for *thawing* the filesystem and then we can work out why that hasn't run. What this code tries to do is identify who froze the filesystem and so indicate who *might* be responsible for thawing it. If we mis-identify the agent who holds the freeze status, then we fail to identify who needs to run the thaw and hence we're still stuck not knowing WTF happened.... I understand why you want to record this - I'm not arguing that we shouldn't do this. My point is that we should *make it reliable* and not in any way ambiguous, otherwise we failed to solve the problem it was intended for. Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- 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
