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Date:      Sun, 07 Jul 2013 14:42:43 -0700
From:      Alfred Perlstein <alfred@freebsd.org>
To:        freebsd-threads@freebsd.org
Subject:   Re: Accessing struct pthread from kernel
Message-ID:  <51D9E0D3.3040802@freebsd.org>
In-Reply-To: <20130707213458.GH91021@kib.kiev.ua>
References:  <CACYV=-H_T98QPWOLS=0iHxgt1Om-sR3p8ogqFCsZHFiZDLtosg@mail.gmail.com> <20130707213458.GH91021@kib.kiev.ua>

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On 7/7/13 2:34 PM, Konstantin Belousov wrote:
> On Sat, Jul 06, 2013 at 01:22:05AM +0200, Davide Italiano wrote:
>> Hi,
>> as a preliminary step in the implementation of adaptive spinning for
>> umtx, I'm switching the pthread/umtx code so that a thread that
>> acquires a pthread_mutex writes the address of struct pthread in the
>> owner field of the lock instead of the thread id (tid). This is
>> because having struct pthread pointer allows easily to access
>> informations of the thread, and makes easy to get the state of the
>> thread exported from the kernel (once this will be implemented).
>>
>> For what concerns the libthr side, the internal function
>> _get_curthread() goes into the TLS to obtain the struct field of
>> curthread, so I'm done.
>> OTOH, I'm quite unsure instead about how to get address of struct
>> pthread for curthread from the kernel side (for example, in
>> do_lock_umutex() sys/kern/kern_umtx.c).
> You should not, see below.
>
>> I guess I need to write some MD code because the TLS is different on
>> the various architecture supported in FreeBSD, and as a first step I
>> focused on amd64.
>> It looks like from the SDM that %%fs register points to the base of
>> the TLS, so I think that accessing using curthread->td_pcb->pcb_fsbase
>> (and then adding the proper offset to acces the right field) is a
>> viable solution to do this. Am I correct?
>> In particular what worries me is if the read of 'struct pthread' for
>> curthread from the TLS register is atomic with respect to preemptions.
>>
>> Alternatively, is there an (easier way) to accomplish this task?
> Coupling the libthr thread structure and kernel makes the ABI cast in
> stone and avoids most possibilities of changing the libthr internals.
> The same is true for kernel accessing the userspace TLS area of the thread.
>
> If you want kernel<->usermode communication of the thread run state,
> and possibly also a way to advisory prevent a preemption of the
> spinning usermode thread, you should create a dedicated parameter block
> communicated from usermode to kernel on thread creation. For the main
> thread, the block could be allocated by kernel by image activator,
> placed on the stack and its address passed to the usermode by auxv.
>
> Note that you cannot access the usermode from the context switch
> code. Wiring the corresponding page is very wasteful (think about a
> process with 10,000 threads) and still does not provide any guarantees
> since usermode can unmap or remap the range. You might see my 'fast
> sigprocmask' patches where somewhat similar idea was implemented.
>
> I also recommend you to look at the Solaris schedctl(2).

About wiring the page, why not allow the kernel/userland to pack 
multiple threads into a single page?  I'm not sure how many bytes of 
context are required, but that would be much more efficient.  We used 
such tricks (packing multiple context blocks into a single page) in some 
scheduler magic we had on another project to allow the scheduler to 
access a page in userland.

-Alfred





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