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List:       lustre-devel
Subject:    [lustre-devel] Possible minor bug
From:       paf () cray ! com (Patrick Farrell)
Date:       2015-05-27 21:15:36
Message-ID: DB0C8A206669AC40B1C4E52A8584A6F614C8947B () CFWEX01 ! americas ! cray ! com
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Ok, thanks.

Here's the LU:
https://jira.hpdd.intel.com/browse/LU-6656
________________________________________
From: Dilger, Andreas [andreas.dilger at intel.com]
Sent: Wednesday, May 27, 2015 3:55 PM
To: Patrick Farrell
Cc: lustre-devel at lists.lustre.org
Subject: Re: [lustre-devel] Possible minor bug

On 2015/05/27, 2:03 PM, "Patrick Farrell" <paf at cray.com<mailto:paf at cray.com>> \
wrote:

While doing some other work, I noticed something I believe is a potential problem in \
the server side quota code.

Specifically, in qmt_glimpse_lock:

While the resource lock (a spin lock) is held, it does

OBD_ALLOC_PTR(work);

Since allocations can sleep, doesn't this allocation need to be atomic?

So, following the current Lustre convention, it should be:
LIBCFS_ALLOC_ATOMIC(work, sizeof(struct
ldlm_glimpse_work));

You could also use OBD_ALLOC_GFP(work, sizeof(*work), GFP_ATOMIC), which is \
equivalent, but at least keeps the same paradigm of other allocations in this code.

I have seen no actual bugs from this, but I hit a hang while modifying the equivalent \
code in ofd_intent_policy for lock ahead, and I think the same hang is theoretically \
possible here.  My understanding is that, in general, doing allocations while holding \
a spin lock is not recommended.

Not only not recommended, but not allowed for GFP_NOFS allocations.  Probably if you \
had CONFIG_DEBUG_SPINLOCK or similar enabled, you would get a warning here due to \
__might_sleep() in the allocation path.

I'm hoping for other input before I go further - Am I right that this is something \
which needs fixing?  If so, I'll open an LU and submit a patch.

Yes, please do.

Cheers, Andreas


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