Accessing struct pthread from kernel

Julian Elischer julian at
Sun Jul 7 16:48:08 UTC 2013

On 7/6/13 7:22 AM, 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).
> 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?
there is a struct pthread for each tid (and visa versa)
you could notify the kernel of it when you make a new thread.
  (new arg/call)

for what it's worth the M:N threading system used to do something like 
with a user callback straucture supplied to the kernel..

> Thanks,

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