linux-kernel - Re: [PATCH 2/3, V2] kernel: Move groups_sort to the caller of set

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Date:   Tue, 5 Dec 2017 15:23:26 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Thiago Rafael Becker <thiago.becker@...il.com>
Cc:     NeilBrown <neilb@...e.com>, bfields@...ldses.org,
        linux-nfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3, V2] kernel: Move groups_sort to the caller of
 set_groups.

On Tue, Dec 05, 2017 at 09:03:02PM -0200, Thiago Rafael Becker wrote:
> On Tue, 5 Dec 2017, Matthew Wilcox wrote:
> > It must be relatively common to sort an already-sorted array.  I wonder
> > if something like this patch would be worthwhile?
> 
> The bug happens when two threads enter sort_groups for the same group info
> in parallel, and one thread starts overwriting values that another thread
> may already have "heapified" or sorted.
> 
> Thread A                  Thread B
> Enter groups_sort
>                           Enter groups_sort
> .
> .
> .
> Return from groups_sort
>                           .
>                           .
>                           .
>                           Return from groups_sort
> 
> Wouldn't this patch just make both threads see the structure as unsorted and
> sort them?

I'm sorry, I wasn't clear.  I wasn't commenting on the original bug (and
I believe your analysis to be correct unless there's some locking which
prevents two calls to group_sort from happening at the same time).

I was wondering about whether our sort() implementation is the best that
it could possibly be, and whether having the trait of not modifying an
already-sorted array is worthwhile (eg it would not cause cachelines to
enter the dirty state if they were already clean).