Messages in this thread | | | Subject | Re: autofs4 hang in 2.6.37-rc1 | From | Ian Kent <> | Date | Mon, 15 Nov 2010 21:22:21 +0800 |
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On Mon, 2010-11-15 at 09:54 +0100, Arnd Bergmann wrote: > On Monday 15 November 2010 02:45:33 Ian Kent wrote: > > > You can't hold an exclusive mutex during an autofs expire because the > > daemon will start by calling the ioctl to check for a dentry to expire > > then call back to the daemon to perform the umount and wait for a status > > return (also an ioctl). > > Ok, I see. So it's my fault for not realizing that there are long blocking > ioctls. I was under the assumption that all of these ioctl commands were > simple non-blocking commands.
This isn't anyone's fault (except maybe mine) because I'm the one most likely to realize it was a problem and didn't notice it. I've even been caught by this deadlock (when holding a singular lock) before when I tried to use .. ummm .. netlink (I think, not even sure what it's called any more) instead of an ioctl interface for the new autofs control interface.
> > > >From memory the expire is the only ioctl that is sensitive to this > > deadlock. > > > > So, either the mutex must be released while waiting for the status > > return or get rid of the autofs4_ioctl_mutex altogether. > > Right. As I said with the original patch, I don't think the mutex > is really needed, but using it seemed to be the safer alternative. > It was in the sense that it guaranteed the breakage to be obvious > rather than silent... > > Ian, if you can prove that the lock is not needed, I think we shold > just remove it.
I don't think I can prove it but I will have a long look at the code. I don't think it is needed and I expect I'll recommend it be removed.
Oh and btw ... please excuse this off-topic question.
In your recent commit 6e9624b8caec290d28b4c6d9ec75749df6372b87 regarding BKL removal you implied that blkdev_{get,put} shouldn't need the BLK. I'm working on a btrfs problem and one of the issues is a deadlock caused by the out of order acquisition of the BLK and the bdev->bd_mutex between these two functions. Clearly this isn't a problem from 2.6.36 but do you think it would be safe just to apply the hunks for blkdev_{get,put} from your commit to fix my problem for older an older kernel, say 2.6.35?
Ian
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