lkml.org 
[lkml]   [2021]   [Dec]   [17]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRe: [PATCH v1 01/11] seqlock: provide lockdep-free raw_seqcount_t variant
Date


> On Dec 17, 2021, at 9:49 AM, David Hildenbrand <david@redhat.com> wrote:
>
> On 17.12.21 18:29, David Hildenbrand wrote:
>> On 17.12.21 18:02, Nadav Amit wrote:
>>>
>>>
>>>> On Dec 17, 2021, at 3:30 AM, David Hildenbrand <david@redhat.com> wrote:
>>>>
>>>> Sometimes it is required to have a seqcount implementation that uses
>>>> a structure with a fixed and minimal size -- just a bare unsigned int --
>>>> independent of the kernel configuration. This is especially valuable, when
>>>> the raw_ variants of the seqlock function will be used and the additional
>>>> lockdep part of the seqcount_t structure remains essentially unused.
>>>>
>>>> Let's provide a lockdep-free raw_seqcount_t variant that can be used via
>>>> the raw functions to have a basic seqlock.
>>>>
>>>> The target use case is embedding a raw_seqcount_t in the "struct page",
>>>> where we really want a minimal size and cannot tolerate a sudden grow of
>>>> the seqcount_t structure resulting in a significant "struct page"
>>>> increase or even a layout change.
>>>>
>>>> Provide raw_read_seqcount_retry(), to make it easy to match to
>>>> raw_read_seqcount_begin() in the code.
>>>>
>>>> Let's add a short documentation as well.
>>>>
>>>> Note: There might be other possible users for raw_seqcount_t where the
>>>> lockdep part might be completely unused and just wastes memory --
>>>> essentially any users that only use the raw_ function variants.
>>>>
>>>
>>> Is it possible to force some policy when raw_seqcount_t is used to
>>> prevent its abuse? For instance not to allow to acquire other (certain?)
>>> locks when it is held?
>>>
>>
>> Good question ... in this series we won't be taking additional locks on
>> the reader or the writer side. Something like lockdep_forbid() /
>> lockdep_allow() to disallow any kind of locking. I haven't heard of
>> anything like that, maybe someone reading along has a clue?
>>
>> The writer side might be easy to handle, but some seqcount operations
>> that don't do the full read()->retry() cycle are problematic
>> (->raw_read_seqcount).
>
> Sorry, I forgot to mention an important point: the raw_seqcount_t
> doesn't give you any additional "power" to abuse.
>
> You can just use the ordinary seqcount_t with the raw_ functions. One
> example is mm->write_protect_seq . So whatever we would want to "invent"
> should also apply to the raw_ functions in general -- which might be
> undesired or impossible (IIRC IRQ context).
>

Thanks for the clarification. I was unfamiliar with
raw_read_seqcount_begin() (and friends). Indeed it is very very rarely
used.
\
 
 \ /
  Last update: 2021-12-17 19:02    [W:0.102 / U:0.396 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site