Date: Sun, 7 Jul 2013 10:03:49 -0700 From: Davide Italiano <davide@freebsd.org> To: Julian Elischer <julian@freebsd.org> Cc: freebsd-amd64@freebsd.org, freebsd-threads@freebsd.org Subject: Re: Accessing struct pthread from kernel Message-ID: <CACYV=-HbLaN6iaiDyoKNOUoBKP%2BbX7mEyvKsBE-wpTC5vTHQew@mail.gmail.com> In-Reply-To: <51D99BB7.1020101@freebsd.org> References: <CACYV=-H_T98QPWOLS=0iHxgt1Om-sR3p8ogqFCsZHFiZDLtosg@mail.gmail.com> <51D99BB7.1020101@freebsd.org>
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On Sun, Jul 7, 2013 at 9:47 AM, Julian Elischer <julian@freebsd.org> wrote: > 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) > Yes, I previously tried implementing something like what you propose, exploiting the fact that thr_create() pass the pointer to struct pthread to the kernel as argument. The problem I see here is that the main thread is not created by thr_create() but by fork(). So, another way to pass the struct to the kernel is needed. One might think about adding another system call for the purpose, but I guess this is overkill so if you have other suggestions I'll be happpy to hear. > for what it's worth the M:N threading system used to do something like this, > with a user callback straucture supplied to the kernel.. > > >> >> Thanks, >> > Thanks for your answer, -- Davide "There are no solved problems; there are only problems that are more or less solved" -- Henri Poincare
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