Re: [fuse-devel] Unpleasant race condition with fuse-2.7.4

[Miklos S. <mi...@sz...>]

On Thu, 21 May 2009, John Haxby wrote:
> Hello all,
> 
> I couldn't find anything about this problem but please accept my 
> apologies if it's already known about.
> 
> We have some people who are using fuse-2.7.4 on RHEL4 for which, of 
> course, they need to compile and install the kernel module.  (Actually, 
> we also have some people using RHEL5 and they still build fuse.ko 
> because it's not configured in the RHEL5 kernel.)
> 
> There's an unpleasant race condition that crops up when creating a 
> file.  If you interrupt a process that's creating files (eg tar) at just 
> the right time then you'll get the fuse filesystem process dying with
> 
>     fuse internal error: node 0 not found
> 
> The circumstances under which this can happen are relatively unusual and 
> the window in which you can get the failure is quite narrow which is 
> probably why this hasn't happened before.
> 
> The main requirement for getting the error is that you have a non-zero 
> negative_timeout.  If that's the case then an interrupt while the fuse 
> filesystem process is in fuse_lib_lookup() when the file it's looking 
> for doesn't exist will cause e.ino and err to be set to zero:
> 
>         err = lookup_path(f, parent, name, path, &e, NULL);
>         if (err == -ENOENT && f->conf.negative_timeout != 0.0) {
>             e.ino = 0;
>             e.entry_timeout = f->conf.negative_timeout;
>             err = 0;
>         }
> 
> When fuse_lib_lookup() calls reply_entry() to finish off the LOOKUP 
> request it will get an ENOENT from the kernel because the operation has 
> been aborted and then reply_entry() will call forget_node() with that 
> zero nodeid and you'll get the abort error.
> 
> This error is relatively easy to achieve if you are untarring an archive 
> into a fuse filesystem, just keep hitting ^C to kill the tar and sooner 
> or later you'll get the error and your filesystem process abort().
> 
> Fortunately, there's an easy fix: 
> http://git.kernel.org/?p=linux/kernel/git/stable/linux-2.6.24.y.git;a=commitdiff;h=a131de0a482ac95e6469f56981c7b063593fdc5d
> (or http://tinyurl.com/opmuh2)
> 
> This stops interrupts aborting outstanding operations and so this 
> particular race condition never happens.
> 
> It would be nice if this patch could be rolled into the 2.7 tree so that 
> others don't trip over the same problem.

Sure, do you have a tested patch against the 2.7 tree?  I'd happily
apply it and release a 2.7.5 with the fix.

Thanks,
Miklos