linux-kernel - Re: [PATCH] list_lru: Prefetch neighboring list entries before acquiring lock

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Date:   Wed, 29 Nov 2017 13:53:19 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Vladimir Davydov <vdavydov.dev@...il.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Dave Chinner <david@...morbit.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] list_lru: Prefetch neighboring list entries before
 acquiring lock

On Wed, 29 Nov 2017 09:17:34 -0500 Waiman Long <longman@...hat.com> wrote:

> The list_lru_del() function removes the given item from the LRU list.
> The operation looks simple, but it involves writing into the cachelines
> of the two neighboring list entries in order to get the deletion done.
> That can take a while if the cachelines aren't there yet, thus
> prolonging the lock hold time.
> 
> To reduce the lock hold time, the cachelines of the two neighboring
> list entries are now prefetched before acquiring the list_lru_node's
> lock.
> 
> Using a multi-threaded test program that created a large number
> of dentries and then killed them, the execution time was reduced
> from 38.5s to 36.6s after applying the patch on a 2-socket 36-core
> 72-thread x86-64 system.

Patch looks good.

Can someone (Dave?) please explain why list_lru_del() supports deletion
of an already list_empty(item)?  This seems a rather dangerous thing to
encourage.  Use cases I can think of are:

a) item is already reliably deleted, so why the heck was the caller
   calling list_lru_del() and 

b) item might be concurrently deleted by another thread, in which case
   the race loser is likely to hit a use-after-free.

Is there a good use case here?