Messages in this thread | | | Date | Mon, 11 Sep 2006 00:33:42 +0400 | From | Oleg Nesterov <> | Subject | Re: [PATCH] vt: Rework the console spawning variables. |
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On 09/10, Eric W. Biederman wrote: > > Oleg Nesterov <oleg@tv-sign.ru> writes: > > > On 09/09, Eric W. Biederman wrote: > >> > >> This patch does several things. > >> - The variables used are moved into a structure and declared in vt_kern.h > >> - A spinlock is added so we don't have SMP races updating the values. > >> - Instead of raw pid_t value a struct_pid is used to guard against > >> pid wrap around issues, if the daemon to spawn a new console dies. > > > > I am not arguing against this patch, but it's a pity we can't use 'struct pid' > > lockless. What dou you think about this: > > Actually with xchg I can use a reference counted struct pid lockless. > > ... > > Perhaps: > void update_pid(struct pid **ref, struct pid *new) > { > struct pid *old; > get_pid(new); > old = xchg(ref, new); > put_pid(old); > }
This can't work. This put_pid() can actually free the memory, while 'old' is still in use (lockless).
> rcu is definitely not the solution in these cases as the struct pid > is stored for a long time so we need the reference count.
Surely we need the reference count, I don't understand you. Look at put_pid_rcu().
That said,
> In the general case you have more then one variable you want to keep > in sync and you need the lock for that.
Yes.
> But since I can write it as a moderately clear one liner in the > case that matters I don't much care.
Ok.
Oleg.
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