On Fri, 29 Aug 2014, Trond Myklebust wrote:
On Fri, Aug 29, 2014 at 11:34 AM, Benjamin Coddington
<bcodding@xxxxxxxxxx> wrote:
On Thu, 21 Aug 2014, Trond Myklebust wrote:
On Thu, Aug 21, 2014 at 2:15 PM, David Jeffery <djeffery@xxxxxxxxxx>
wrote:
On 08/21/2014 11:50 AM, Trond Myklebust wrote:
On Thu, Aug 21, 2014 at 11:40 AM, David Jeffery <djeffery@xxxxxxxxxx>
wrote:
On 08/20/2014 08:28 PM, Trond Myklebust wrote:
What guarantees that this does not lead to silent corruption of the
file
if there are outstanding write requests?
Do you have a particular scenario in mind you are concerned about?
Right before the code the patch modifies, nfs_sync_mapping() is called.
Any writes started before the unlock operation began have already been
flushed, so we shouldn't have a corruption case of writes from before
the unlock began being sent after the unlock is complete.
Are you concerned about some other nfs4 writes being started while we
initially waited on the counter? Such a write racing with the unlock
No. I'm worried about the writes that have been started, but which are
now completing in the background while the lock is being freed.
going ahead instead of erroring out could initially fail from a wrong
state ID, but should retry with the new state. Is there something I've
overlooked?
Loss of lock atomicity due to the fact that the writes are completing
after the lock was released.
I don't think my patches break lock atomicity, unless I've completely
misunderstood nfs_sync_mapping()'s behavior.
The background writes should have already been waited for by
nfs_sync_mapping() at the beginning of do_unlk(). nfs_sync_mapping()
would call nfs_wb_all() which calls sync_inode() with WB_SYNC_ALL, so
it's going to push out any dirty data and wait for any writes to
complete. Only after the writes are complete do we go on to call
nfs_iocounter_wait().
What if nfs_wb_all() is interrupted? Currently, that doesn't matter
precisely because of the call to nfs_iocounter_wait.
Hi Trond, it looks like the intent of:
577b423 NFS: Add functionality to allow waiting on all outstanding reads..
7a8203d NFS: Ensure that NFS file unlock waits for readahead to complete..
was to wait for readahead, not to avoid releasing the lock if nfs_wb_all()
is interrupted. What was the underlying reason for waiting for readahead to
complete? I know there was one issue where the NFS server would release the
lockowner before processing the reads - was that what these were trying to
avoid? I ask because if we're only worried about outstanding writes we
could just wait on the writes, not the reads.
Would it be better to use TASK_UNINTERRUPTIBLE instead of TASK_KILLABLE in
__nfs_iocounter_wait?
Ben
Hi Ben,
The main intent of the two patches was to ensure that all I/O that
uses the lock stateid should have completed before we attempt to
release the lock. The main reason is to avoid an unnecessary
"recovery" situation where the server asks us to resend the I/O
because the lock stateid is no longer valid.
Concerning writes; if there are any outstanding then we must ensure
those complete before we unlock the file. This rule should be
unaffected by the above 2 patches (although it is just a bonus if they
help to enforce it).
If so, then David's patch shouldn't risk corruption any more than before
577b423 and 7a8203d were applied.
He's trying to avoid the problem (I think) where very aggressive readahead
behavior blocks the unlocking thread and a sysadmin decides to kill the
process, which results in the abandoned lock. That can make a big mess.
Instead, his change would result in the "recovery" scenario at worst when
the impatient sysadmin goes on a rampage.
Ben
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