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Re: [PATCH 1/4] xfs: wake up cil->xc_commit_wait while removing ctx from

To: Jeff Liu <jeff.liu@xxxxxxxxxx>
Subject: Re: [PATCH 1/4] xfs: wake up cil->xc_commit_wait while removing ctx from cil->xc_committing
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Thu, 2 Jan 2014 11:45:03 +1100
Cc: Mark Tinguely <tinguely@xxxxxxx>, "xfs@xxxxxxxxxxx" <xfs@xxxxxxxxxxx>
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <52C4286C.5040007@xxxxxxxxxx>
References: <52B98292.5040002@xxxxxxxxxx> <52C18F56.70709@xxxxxxx> <52C4286C.5040007@xxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
On Wed, Jan 01, 2014 at 10:38:36PM +0800, Jeff Liu wrote:
> On 12/30 2013 23:20 PM, Mark Tinguely wrote:
> > On 12/24/13 06:48, Jeff Liu wrote:
> >> From: Jie Liu<jeff.liu@xxxxxxxxxx>
> >>
> >> I can easily to hit a hang up while running fsstress and shutting down
> >> XFS on SSD via the tests below:
> <snip>
> >>
> >>     Task1                    Task2
> >>
> >>                      list_add(&ctx->committing,&cil->xc_committing);
> >>
> >> xlog_wait(&cil->xc_commit_wait..)
> >> schedule()...
> >>
> >>                      Aborting!! list_del(&ctx->committing);
> >>                      wake_up_all(&cil->xc_commit_wait);<-- MISSING!
> >>
> >> As a result, we should handle this situation in xlog_cil_committed().
> >>
> >> Signed-off-by: Jie Liu<jeff.liu@xxxxxxxxxx>
> >> ---
> >>   fs/xfs/xfs_log_cil.c | 2 ++
> >>   1 file changed, 2 insertions(+)
> >>
> >> diff --git a/fs/xfs/xfs_log_cil.c b/fs/xfs/xfs_log_cil.c
> >> index 5eb51fc..8c7e9c7 100644
> >> --- a/fs/xfs/xfs_log_cil.c
> >> +++ b/fs/xfs/xfs_log_cil.c
> >> @@ -406,6 +406,8 @@ xlog_cil_committed(
> >>
> >>       spin_lock(&ctx->cil->xc_push_lock);
> >>       list_del(&ctx->committing);
> >> +    if (abort)
> >> +        wake_up_all(&ctx->cil->xc_commit_wait);
> >>       spin_unlock(&ctx->cil->xc_push_lock);
> >>
> >>       xlog_cil_free_logvec(ctx->lv_chain);
> > 
> > Hi Jeff, I hope you had a good break,
> Thanks :)
> > 
> > So you are saying the wakeup in the CIL push error path missing?
> Yes.
> 
> > I agree with that. But I don't like adding a new wake up to
> > xlog_cil_committed(), which is after the log buffer is written.

Hi Mark, any particular reason why you don't like this? It would be
great if you could explain why you don't like something up front so
we don't have to guess at your reasons or wait for another round
trip in the conversation to find them out....

> IMO this callback would be called if any problem is happened before
> the log buffer is written as well, e.g, 
> xlog_cil_push()->xfs_log_notify() <-- failed
>                       | 
>                       |->xlog_cil_committed()

Right, it's the generic CIL commit handler and it can be called
directly or from log IO completion.

The question is this: it is safe to wake up waiters from log IO
completion if that is where an abort is first triggered from (i.e.
on log IO error). From what I can see, it is safe to do the wakeup
on abort because the iclog iwe attach the IO completion callback to
in xlog_cil_push() cannot be put under IO until we release the
reference gained in xfs_log_done().

But this raises an interesting question - the wakeup in
xlog_cil_push() is done before the log IO for the checkpoint is
complete, so the wakeup is occurring on checkpoint processing
completion, not iclog IO completion. i.e. the actual log force
sleeping still needs to wait for log IO completion to occur after
then CIL has been pushed. This occurs in the _xfs_log_force{_lsn}()
wrappers, where iclog state changes are waited for.

Why is this important? The iclog write/flush wakeups are all done
from IO completion context, except for the force shutdown case,
which calls xlog_state_do_callback(log, XFS_LI_ABORTED, NULL); to
trigger wakeups and aborts via the log IO completion callbacks on
all the outstanding iclogs.

IOWs, we've already got a design pattern that says:

        - run log force wakeups from IO completions
        - on shutdown, run IO completions directly to abort pending
          log operations

So, really, issuing wakeups from iclog IO completion on log aborts
or errors is exactly what we currently do to ensure that shutdowns
don't leave processes waiting on log force completion behind. So
from that perspective, adding the wakeup on abort to
xlog_cil_committed() seems like the right approach to take.

Actually, there's more issues here: xlog_cil_push() leaks a
reference to the iclog when it triggers the error path via
xfs_log_notify() failure. At this point we always need to release
the iclog. Hence if xfs_log_notify() were to always add the IO
completion to the iclog and xlog_cil_committed() issued wakeups on
abort errors, then we could completely ignore the log state in
xfs_log_notify() and have xfs_log_release_iclog() capture the IO
error and the subsequent shutdown would handle the aborts and
wakeups....

Hmmm, then xfs_log_notify could go away, and the callback list could
be made a lockless list and the ic_callback_lock could go away,
too....

> Besides, the CTX will be removed from the committing list here but
> there might still have waiters sleeping on it.

Right, if we get an abort from log IO completion, then we may not
have any other wakeup vector that can be triggered. Triggering them
from IO completion ensures that even forced shutdowns have a trigger
for wakeups...

SO, for the hang issue, I think the minimal patch is OK, but we
should look to clean up the logic and fix the leaked iclog reference
on xfs_log_notify() failure as well.

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx

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