Re: [PATCH] shrink_dcache_parent() races against shrink_dcache_memory()

[Kirill Korotaev]

I like your idea, but some comments below... I doubt it works.
I will think it over a bit later...

Kirill
P.S. it's not easily reproducable. Before my fix it took us 3-6 hours on 
automated stress testing to hit this bug. Right now I can't setup it for 
testing, maybe in a week or so.

> > On Mon, Jan 23, Kirill Korotaev wrote:
> >
> > [snip] 
> >
> > Hmm, will think about that one again. shrink_dcache_parent() and
> > shrink_dcache_memory()/dput() are not racing against each other now since the
> > reference counting is done before giving up dcache_lock and the select_parent
> > could start.
> >
> > Regards,
> > 	Jan
>
>
> I have been playing around with a possible solution to the problem.
> I have not been able to reproduce this issue, hence I am unable to verify
> if the patch below fixes the problem. I have run the system with this
> patch and verified that no obvious badness is observed.
>
> Kirill, Jan if you can easily reproduce the problem, could you
> try this patch and review it as well for correctness of the solution?
>
> All callers that try to free memory set the PF_MEMALLOC flag, we check
> if the super block is going away due to an unmount, if so we ask the
> allocator to return.
>
> The patch adds additional cost of holding the sb_lock for each dentry
> being pruned. It holds sb_lock under dentry->d_lock and dcache_lock,
> I am not sure about the locking order of these locks.
>
> Signed-off-by: Balbir Singh <balbir@xxxxxxxxxx>
> ---
>
>  fs/dcache.c |   23 +++++++++++++++++++++++
>  1 files changed, 23 insertions(+)
>
> diff -puN fs/dcache.c~dcache_race_fix2 fs/dcache.c
> --- linux-2.6/fs/dcache.c~dcache_race_fix2	2006-01-24 11:05:46.000000000 +0530
> +++ linux-2.6-balbir/fs/dcache.c	2006-01-24 11:05:46.000000000 +0530
> @@ -425,6 +425,29 @@ static void prune_dcache(int count)
>   			spin_unlock(&dentry->d_lock);
>  			continue;
>  		}
> +
> +		/*
> +		 * Note to reviewers: our current lock order is dcache_lock,
> +		 * dentry->d_lock & sb_lock. Could this create a deadlock?
> +		 */
> +		spin_lock(&sb_lock);
<<<< 1. sb_lock doesn't protect atomic_read() anyhow...
<<<<    I mean, sb_lock is not required to read its value...
> +		if (!atomic_read(&dentry->d_sb->s_active)) {
> +			/*
> +			 * Race condition, umount and other pruning is happening
> +			 * in parallel.
> +			 */
> +			if (current->flags & PF_MEMALLOC) {
> +				/*
> +				 * let the allocator leave this dentry alone
> +				 */
> +				spin_unlock(&sb_lock);
> +				spin_unlock(&dentry->d_lock);
> +				spin_unlock(&dcache_lock);
> +				return;
<<<< you should not return, but rather 'continue'. otherwise you skip 
_all_ dentries, even from active super blocks.
> +			}
> +		}
> +		spin_unlock(&sb_lock);
> +
<<<< and here, when you drop sb_lock, and dentry->d_lock/dcache_lock in 
prune_dentry() it looks to me that we have exactly the same situation as 
it was without your patch:
<<<< another CPU can start umount in parallel.
<<<< maybe sb_lock barrier helps this somehow, but I can't see how yet...

<<<< another idea: down_read(&sb->s_umount) probably could help...
<<<< because it will block the whole umount operation...
<<<< but we can't take it under dcache_lock...
>  		prune_one_dentry(dentry);
>  	}
>  	spin_unlock(&dcache_lock);
>
> Thanks,
> Balbir
> _


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