Who hasn't been frustrated by some device stuck in a hardware
read or write? Maybe it's a tape drive? It's effectively dead, maybe
because the person who wrote the driver is an idiot or maybe because
the person who designed the hardware is more of an idiot, or more likely
something has just gone tragically wrong and you are stuck. Literally
stuck, because the process doing the reading or writing dove down into
the driver and it's never coming back up for air. Never. Go ahead,
send it a "kill -9" signal. The process will never see that, because
it's way down at the bottom of the ocean waiting for something that
apparently is never going to happen.
On Linux, you'd see the process state is "D for "Defunct"". OS X uses "U" for
"uninterruptible". I don't know about you, but I think "U" makes more sense. After all, we have to suppose that it is at least possible that whatever trouble
is going down, it might resolve itself. If a very large counter is ticking down,
maybe if we wait long enough the driver will finish or give up? Yeah, right: we're
going to power-cycle the box and hope the problem doesn't repeat.
What an awful thing to have to do. Reboot a perfectly good system because a tape drive is stuck?
Wouldn't it be nice if you could kill the stupid thing? Or tell the driver to give up with an ioctl? Well,
don't get too excited, but somebody agrees and is working on just that.. Apparently it was Linus Torvald who
actually suggested this back in 2002, and now it's actually there - well,
somewhat anyway. The problem is that programmers need to use
this and of course unless someone has kindly rewritten a driver for
that purpose, it doesn't. This TASK_KILLABLE: New process state in Linux article from IBM describes its use in NFS.
Update: This was still in the include files as of 3.10, but isn't to be found in my Ubuntu 12.04 include files. I still find mention of it in this fuse, TASK_KILLABLE and daemon deadlocks thread from 2013, though.
The arguments AGAINST TASK_KILLABLE are that it complicates things. I think that among driver developers there's a certain amount of misplaced arrogance too: "MY code can't get stuck sleeping!".
But of course it can, because unexpected hardware flakiness can confuse
any code - well, maybe not "any", but it sure isn't unusual to see
something get stuck like this. Yes, of course these things can be solved by better code: don't assume, don't wait forever for anything no matter how sure you
are that it can't possibly block, cover your bases, yadda, yadda. Trouble
is, all that goes against tight and fast, doesn't it?
So, the author of this thought about it and:
A while back, Matthew Wilcox realized that many of these concerns about application bugs do not really apply if the application is about to be killed anyway. It does not matter if the developer thought about the possibility of an interrupted system call if said system call is doomed to never return to user space.
Makes sense to me, but I'm no kernel programmer.
Then again: if the problem is internal hardware that has gotten itself all confused with registers contradicting one another and the hardware design lacks any way to clear to a default state without removing power, you may have to shut
everything down anyway. Still.. isn't it better to be able to at least try
If this page was useful to you, please help others find it:
More Articles by Anthony Lawrence
- Find me on Google+
Have you tried Searching this site?
Unix/Linux/Mac OS X support by phone, email or on-site:
This is a Unix/Linux resource website. It contains technical articles about Unix, Linux and general computing related subjects, opinion, news, help files, how-to's, tutorials and more. We appreciate comments and article submissions.
Publishing your articles here
Jump to Comments
Many of the products and books I review are things I purchased for my own use. Some were given to me specifically for the purpose of reviewing them. I resell or can earn commissions from the sale of some of these items. Links within these pages may be affiliate links that pay me for referring you to them. That's mostly insignificant amounts of money; whenever it is not I have made my relationship plain. I also may own stock in companies mentioned here. If you have any question, please do feel free to contact me.
I am a Kerio reseller. Articles here related to Kerio products reflect my honest opinion, but I do have an obvious interest in selling those products also.
Specific links that take you to pages that allow you to purchase the item I reviewed are very likely to pay me a commission. Many of the books I review were given to me by the publishers specifically for the purpose of writing a review. These gifts and referral fees do not affect my opinions; I often give bad reviews anyway.
We use Google third-party advertising companies to serve ads when you visit our website. These companies may use information (not including your name, address, email address, or telephone number) about your visits to this and other websites in order to provide advertisements about goods and services of interest to you. If you would like more information about this practice and to know your choices about not having this information used by these companies, click here.