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Subject: Re: [GIT PULL] Please pull proc and exec work for 5.7-rc1
From:   Waiman Long <longman@redhat.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     "Eric W. Biederman" <ebiederm@xmission.com>,
        Ingo Molnar <mingo@kernel.org>, Will Deacon <will@kernel.org>,
        Bernd Edlinger <bernd.edlinger@hotmail.de>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Alexey Gladkov <gladkov.alexey@gmail.com>
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Date:   Fri, 3 Apr 2020 19:23:23 -0400
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On 4/3/20 7:16 PM, Waiman Long wrote:
> On 4/3/20 4:59 PM, Linus Torvalds wrote:
>> On Fri, Apr 3, 2020 at 1:41 PM Waiman Long <longman@redhat.com> wrote:
>>> Another alternative is to add new functions like down_read_unfair() that
>>> perform unfair read locking for its callers. That will require less code
>>> change, but the calling functions have to make the right choice.
>> I'd prefer the static choice model - and I'd hide this in some
>> "task_cred_read_lock()" function anyway rather than have the users do
>> "mutex_lock_killable(&task->signal->cred_guard_mutex)" like they do
>> now.
>>
>> How nasty would it be to add the "upgrade" op? I took a quick look,
>> but that just made me go "Waiman would know" ;)
>>
>>              Linus
>>
> With static choice, you mean defined at init time. Right? In that case,
> you don't really need a special encapsulation function.
>
> With upgrade, if there is only one reader, it is pretty straight
> forward. With more than one readers, it gets more complicated as we have
> to wait for other readers to unlock. We can spin for a certain period of
> time. After that, that reader can use the handoff mechanism by queuing
> itself in front the wait queue before releasing the read lock and go to
> sleep. That will make sure that it will get the lock once all the other
> readers exits. For an unfair rwsem, the writer cannot assert the handoff
> bit and so it shouldn't interfere with this upgrade process.
>
> If there are multiple upgrade readers, only one can win the race. The
> others have to release the read lock and queue themselves as writers.
> Will that be acceptable?

Alternatively, we could assert that only one reader can do the upgrade
and do a WARN_ON_ONCE() if multiple concurrent upgrade attempts is detected.

Regards,
Longman