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Re: [Xen-devel] xl hangs instead of failing more graciously when the fs is read-only

Thursday, June 12, 2014, 9:59:37 AM, you wrote:

> On Thu, 2014-06-12 at 09:47 +0200, Sander Eikelenboom wrote:
>> Hi Ian,
>> At the moment Iâm having the privilege of thrashing my box and root-fs 
>> frequently while testing kernels. This causes the root-fs to be mounted 
>> read-only. But init continues to do it's job any way .. so we get to 
>> xendomains,
>> which in turn uses 'xl'. But 'xl' needs a writable FS and hangs when it's 
>> not,
>> couldn't and shouldn't this fail more graciously ?

> Your logs seem to be showing reads/writes to a xenbus device, which has
> nothing to do with the writeablility of your rootfs afaik.

> I'm pretty sure xl will correctly error out if it fails to write to a
> file on a readonly fs, or at least I see no evidence here that it is
> not.

[  734.993832]  [<ffffffff811f71e2>] vfs_write+0xc2/0x1e0
[  735.000239]  [<ffffffff811f76f2>] SyS_write+0x52/0xc0
[  735.019374]  #0:  (sb_writers#10){.+.+..}, at: [<ffffffff811f72e3>] 

These let me think there was also some direct fs writing done, but i'm not that 
good in interpreting stacktraces to tell which is the actually blocking part.

> Maybe the issue is that xenstored is blocked or by the rootfs ro-ness
> (or not even started due to it) and so attempts to communicate with it
> fail?

> I think the right answer is to always have a writeable rootfs rather
> than glossing over whatever error led to this situation.

I'm aiming for that :-) .. i'm not expecting it to function, but there is a 
subtle difference between hanging and failing. 

> But I suppose as a fallback xendomains could try and probe for a usable
> xenstored before proceeding, similar to how xencommons does (ideally
> with a shared scriptlet somewhere).

Is it the right place to fix places that use xl, rather than xl it self and let 
it fail when it can't use xenstored then ?

> Ian.

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