Messages in this thread | | | From | (Eric W. Biederman) | Date | Sat, 22 Dec 2012 12:31:21 -0800 | Subject | Re: [PATCH review 2/3] pidns: Stop pid allocation when init dies |
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Oleg Nesterov <oleg@redhat.com> writes:
> On 12/21, Eric W. Biederman wrote: >> >> --- a/include/linux/pid_namespace.h >> +++ b/include/linux/pid_namespace.h >> @@ -21,7 +21,7 @@ struct pid_namespace { >> struct kref kref; >> struct pidmap pidmap[PIDMAP_ENTRIES]; >> int last_pid; >> - int nr_hashed; >> + unsigned int nr_hashed; >> struct task_struct *child_reaper; >> struct kmem_cache *pid_cachep; >> unsigned int level; >> @@ -42,6 +42,8 @@ struct pid_namespace { >> >> extern struct pid_namespace init_pid_ns; >> >> +#define PIDNS_HASH_ADDING (1U << 31) > > Yes, agreed. We can't rely on PF_EXITING/whatever, we need the explicit > flag.
The simpler and more comprehensible we can make this code the better. We have had too many surprises in this code because of complex failure modes.
> 1/2 looks fine too. Only one nit about init_pid_ns below...
Then I will add your acked-by to the first patch.
>> @@ -319,7 +318,7 @@ struct pid *alloc_pid(struct pid_namespace *ns) >> >> upid = pid->numbers + ns->level; >> spin_lock_irq(&pidmap_lock); >> - if (ns->nr_hashed < 0) >> + if (ns->nr_hashed < PIDNS_HASH_ADDING) > > I won't insist, but perhaps if "(!(nr_hashed & PIDNS_HASH_ADDING))" > looks more understandable.
I will stare at it both ways and post an updated patch.
I'm not certain which form I like better. Certainly the decrements are doing a double duty.
>> +void disable_pid_allocation(struct pid_namespace *ns) >> +{ >> + spin_lock_irq(&pidmap_lock); >> + if (ns->nr_hashed >= PIDNS_HASH_ADDING) > > Do we really need this check? It seems that PIDNS_HASH_ADDING > bit must be always set when disable_pid_allocation() is called. > >> + ns->nr_hashed -= PIDNS_HASH_ADDING; > > Anyway, nr_hashed &= ~PIDNS_HASH_ADDING looks simpler and doesn't > need a check.
That I agree with.
> But again, I won't insist this is minor and subjective. > >> struct pid *find_pid_ns(int nr, struct pid_namespace *ns) >> { >> struct hlist_node *elem; >> @@ -584,7 +591,7 @@ void __init pidmap_init(void) >> /* Reserve PID 0. We never call free_pidmap(0) */ >> set_bit(0, init_pid_ns.pidmap[0].page); >> atomic_dec(&init_pid_ns.pidmap[0].nr_free); >> - init_pid_ns.nr_hashed = 1; >> + init_pid_ns.nr_hashed = 1 + PIDNS_HASH_ADDING; > > The obly chunk which doesn't look exactly correct to me, although this > doesn't really matter. Hmm, actually the code was already wrong before > this patch. > > I think init_pid_ns.nr_hashed should be PIDNS_HASH_ADDING, we should not > add 1 to account the unused zero pid, and kernel_thread(kernel_init) was > not called yet.
Good point because the zero pid does not get hashed. Who knows perhaps with a little more evolution create_pid_ns can be used to create the initial pid namespace.
I am also going to add "BUILD_BUG_ON(PID_MAX_LIMIT >= PIDNS_HASH_ADDING);" to document that the pid values and PIDNS_HASH_ADDING can't overlap.
Eric
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