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From: Christopher Lameter <cl@linux.com>
To: Waiman Long <longman@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Pekka Enberg <penberg@kernel.org>,
	David Rientjes <rientjes@google.com>,
	Joonsoo Kim <iamjoonsoo.kim@lge.com>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	selinux@vger.kernel.org, Paul Moore <paul@paul-moore.com>,
	Stephen Smalley <sds@tycho.nsa.gov>,
	Eric Paris <eparis@parisplace.org>,
	"Peter Zijlstra (Intel)" <peterz@infradead.org>,
	Oleg Nesterov <oleg@redhat.com>
Subject: Re: [PATCH 1/4] mm: Implement kmem objects freeing queue
Date: Fri, 22 Mar 2019 17:47:18 +0000	[thread overview]
Message-ID: <01000169a683a0ed-3fa1b014-8efa-4c8f-a7e1-958e9eccd693-000000@email.amazonses.com> (raw)
In-Reply-To: <20190321214512.11524-2-longman@redhat.com>

On Thu, 21 Mar 2019, Waiman Long wrote:

> When releasing kernel data structures, freeing up the memory
> occupied by those objects is usually the last step. To avoid races,
> the release operation is commonly done with a lock held. However, the
> freeing operations do not need to be under lock, but are in many cases.
>
> In some complex cases where the locks protect many different memory
> objects, that can be a problem especially if some memory debugging
> features like KASAN are enabled. In those cases, freeing memory objects
> under lock can greatly lengthen the lock hold time. This can even lead
> to soft/hard lockups in some extreme cases.
>
> To make it easer to defer freeing memory objects until after unlock,
> a kernel memory freeing queue mechanism is now added. It is modelled
> after the wake_q mechanism for waking up tasks without holding a lock.

It is already pretty easy. You just store the pointer to the slab object
in a local variable, finish all the unlocks and then free the objects.
This is done in numerous places of the kernel.

I fear that the automated mechanism will make the code more difficult to
read and result in a loss of clarity of the sequencing of events in
releasing locks and objects.

Also there is already kfree_rcu which does a similar thing to what you are
proposing here and is used in numerous places.


  reply	other threads:[~2019-03-22 17:47 UTC|newest]

Thread overview: 27+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-03-21 21:45 [PATCH 0/4] Signal: Fix hard lockup problem in flush_sigqueue() Waiman Long
2019-03-21 21:45 ` [PATCH 1/4] mm: Implement kmem objects freeing queue Waiman Long
2019-03-22 17:47   ` Christopher Lameter [this message]
2019-03-22 17:47     ` Christopher Lameter
2019-03-21 21:45 ` [PATCH 2/4] signal: Make flush_sigqueue() use free_q to release memory Waiman Long
2019-03-22  1:52   ` Matthew Wilcox
2019-03-22 11:16     ` Oleg Nesterov
2019-03-22 16:10       ` Waiman Long
2019-03-22 17:50         ` Christopher Lameter
2019-03-22 17:50           ` Christopher Lameter
2019-03-22 18:12           ` Waiman Long
2019-03-22 19:39             ` Christopher Lameter
2019-03-22 19:39               ` Christopher Lameter
2019-03-22 19:59               ` Matthew Wilcox
2019-03-25 14:15                 ` Christopher Lameter
2019-03-25 14:15                   ` Christopher Lameter
2019-03-25 15:26                   ` Matthew Wilcox
2019-03-25 16:16                     ` Christopher Lameter
2019-03-25 16:16                       ` Christopher Lameter
2019-03-26 13:36                   ` Oleg Nesterov
2019-03-26 13:29           ` Oleg Nesterov
2019-03-21 21:45 ` [PATCH 3/4] signal: Add free_uid_to_q() Waiman Long
2019-03-21 21:45 ` [PATCH 4/4] mm: Do periodic rescheduling when freeing objects in kmem_free_up_q() Waiman Long
2019-03-21 22:00   ` Peter Zijlstra
2019-03-22 14:35     ` Waiman Long
2019-03-22 10:15 ` [PATCH 0/4] Signal: Fix hard lockup problem in flush_sigqueue() Matthew Wilcox
2019-03-22 11:49   ` Oleg Nesterov

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