Date: Sun, 30 Jul 2006 13:47:47 -0700 From: "Kip Macy" <kip.macy@gmail.com> To: "Divacky Roman" <xdivac02@stud.fit.vutbr.cz> Cc: hackers@freebsd.org Subject: Re: VM question related to faults Message-ID: <b1fa29170607301347p7356d3d6jb922a4c51059d494@mail.gmail.com> In-Reply-To: <20060730200354.GA82547@stud.fit.vutbr.cz> References: <20060730105731.GA64955@stud.fit.vutbr.cz> <20060730200354.GA82547@stud.fit.vutbr.cz>
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>From kern_umtx.c:
static int
_do_lock(struct thread *td, struct umtx *umtx, long id, int timo)
{
struct umtx_q *uq;
intptr_t owner;
intptr_t old;
int error = 0;
uq = td->td_umtxq;
/*
* Care must be exercised when dealing with umtx structure. It
* can fault on any access.
*/
for (;;) {
/*
* Try the uncontested case. This should be done in userland.
*/
owner = casuptr((intptr_t *)&umtx->u_owner,
UMTX_UNOWNED, id);
/* The acquire succeeded. */
if (owner == UMTX_UNOWNED)
return (0);
/* The address was invalid. */
if (owner == -1)
return (EFAULT);
/* If no one owns it but it is contested try to acquire it. */
if (owner == UMTX_CONTESTED) {
owner = casuptr((intptr_t *)&umtx->u_owner,
UMTX_CONTESTED, id | UMTX_CONTESTED);
if (owner == UMTX_CONTESTED)
return (0);
/* The address was invalid. */
if (owner == -1)
return (EFAULT);
On 7/30/06, Divacky Roman <xdivac02@stud.fit.vutbr.cz> wrote:
> On Sun, Jul 30, 2006 at 12:57:32PM +0200, Divacky Roman wrote:
> > hi,
> >
> > while working on SoC linuxolator project I am in a need of this:
> >
> > I need to do some operation on memory like mem1 = mem1 + mem2 etc.
> > where the mem1/mem2 access can trigger fault. (memory not mapped or something)
>
> to make it clear.. I am trying to access user-space memory from kernel.
> This needs to be atomic (its an implementation of linux futexes)
>
> I need to check from kernel if some memory is accessible and then perform an
> operation on this memory. All atomically.
>
> hence I need two things - function which checks wheter the memory is accessible
> and something which makes it atomic (some mutex/something which prevents other
> process to enter VM to unmap/etc. the memory in question)
>
> hope its a bit more clear now
>
> roman
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