linux-ext4 - Re: [PATCH] ext4: Fix the soft lockup with multi block allocator.

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Message-ID: <20080109191108.GA13294@skywalker>
Date:	Thu, 10 Jan 2008 00:41:08 +0530
From:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:	Jan Kara <jack@...e.cz>, Mingming Cao <cmm@...ibm.com>
Cc:	tytso@....edu, adilger@....com, bzzz@....com,
	linux-ext4@...r.kernel.org
Subject: Re: [PATCH] ext4: Fix the soft lockup with multi block allocator.

On Wed, Jan 09, 2008 at 07:44:30PM +0100, Jan Kara wrote:
> On Wed 09-01-08 23:54:28, Aneesh Kumar K.V wrote:
> > On Wed, Jan 09, 2008 at 01:10:41PM +0100, Jan Kara wrote:
> > > > With the multi block allocator when we don't have prealloc space we discard
> > > > @@ -3790,7 +3782,9 @@ repeat:
> > > >  
> > > >  	/* if we still need more blocks and some PAs were used, try again */
> > > >  	if (free < needed && busy) {
> > > > +		busy = 0;
> > > >  		ext4_unlock_group(sb, group);
> > > > +		schedule_timeout(HZ);
> > > >  		goto repeat;
> > > >  	}
> > >   Hmm, wouldn't just schedule() be enough here? That would give a good
> > > chance to other processes to proceed and we would avoid this artificial
> > > wait of 1s which is quite ugly IMO.
> > > 
> > 
> > But then who will wake up the task ?. I have the below comment added to
> > the patch in the patch queue.
>   As far as I know, you don't have to wake-up the task explicitely.
> Scheduler will simply schedule the task sometime in future (it is a similar
> situation as if the task got preempted in the kernel).

I missed the current->state = TASK_UNINTERRUPTIBLE; in that code piece.
So yes without setting the task state yes it would put it back to the run
queue.Infact schedule_timeout() without changing the task state also
behaves similarly. Now that that we know that it if fine just to have a
schedule() there since schedule_timeout() was just behaving as
schedule(). I guess we should make the change you suggested. In that
case we can remove the comment which says we need to add the wait queue.

Mingming,

Do you want me to send a patch or can you make the modification ?

-aneesh

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