svn commit: r267760 - head/sys/kern

[Konstantin Belousov]

On Fri, Jul 11, 2014 at 01:19:25PM +0200, Mateusz Guzik wrote:
> On Fri, Jul 11, 2014 at 12:55:51PM +0300, Konstantin Belousov wrote:
> > On Fri, Jul 11, 2014 at 04:43:51AM +0200, Mateusz Guzik wrote:
> > > In both cases the same mechanism blocks both exec and exit, this can be
> > > split if needed (p_lock would still cover exit, p_something would cover
> > > exec).
> > > 
> > > Here is a version with sx lock:
> > > 
> > > http://people.freebsd.org/~mjg/patches/exec-exit-hold-wait.patch
> > > 
> > > I'm not really happy with this. Reading foreign fdt is very rare and
> > > this adds lock + unlock for every exec and exit.
> > > 
> > > On the other hand mere counter version is rather simple:
> > > 
> > > http://people.freebsd.org/~mjg/patches/exec-exit-hold-nolock.patch
> > > 
> > > I don't have strong opinion here, but prefer the latter.
> > 
> > I suggest the name 'imagelock' for the beast.
> > 
> 
> Sounds good.
> 
> > The nolock version requires two atomics on both entry and leave from the
> > protected region, while sx-locked variant requires only one atomic for
> > entry and leave.
> > 
> > I am not sure why you decided to acquire p->p_keeplock in after the
> > proc lock in pget(), which indeed causes the complications of dropping
> > the proc_lock and rechecking to avoid LOR.  Did you tried to add a flag
> > to pfind*() functions to indicate that p_keeplock should be acquired,
> > instead ?
> 
> Lock is taken later to avoid waiting for finished exec/exit of processes
> we cannot return, so that e.g. procstat -fa does not trip over that
> much.
> 
> Right now only PROC_LOCK guarantees stability of p->p_ucred across pget
> operation. Without that the code would have to crget() and various
> functions modified to accept cred instead of proc, or 'imagelock'
> mechanism would have to be extended to also protect against cred
> changes.
No, you could get both locks, imagelock first, proc_lock next.

> 
> That said, the code could be indeed changed to sx requiring one atomic
> on entry and leave, but that would still leave us with such atomics in
> exit and exec and the last 2 are way more common than the first one,
> thus I prefer counter case which only adds lock + unlock on leave.
> 
> -- 
> Mateusz Guzik <mjguzik gmail.com>
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