* Request for documentation of PID handling functions
@ 2008-07-28 22:59 David Howells
2008-07-30 2:04 ` Pavel Emelyanov
0 siblings, 1 reply; 3+ messages in thread
From: David Howells @ 2008-07-28 22:59 UTC (permalink / raw)
To: Pavel Emelyanov; +Cc: dhowells, jmorris, linux-kernel
Hi Pavel,
Can you please provide documentation for the PID handling functions you
supplied in commit:
7af5729474b5b8ad385adadab78d6e723e7655a3
In particular I'd like to know what locks are required when. I'm trying to
determine if in this piece of code:
read_lock(&tasklist_lock);
ret = 0;
if (pid != task_pid_vnr(current))
ret = -EPERM;
read_unlock(&tasklist_lock);
the lock of tasklist_lock is necessary.
David
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: Request for documentation of PID handling functions
2008-07-28 22:59 Request for documentation of PID handling functions David Howells
@ 2008-07-30 2:04 ` Pavel Emelyanov
2008-07-30 9:56 ` Oleg Nesterov
0 siblings, 1 reply; 3+ messages in thread
From: Pavel Emelyanov @ 2008-07-30 2:04 UTC (permalink / raw)
To: David Howells; +Cc: Oleg Nesterov, jmorris, linux-kernel
David Howells wrote:
> Hi Pavel,
>
> Can you please provide documentation for the PID handling functions you
> supplied in commit:
>
> 7af5729474b5b8ad385adadab78d6e723e7655a3
>
> In particular I'd like to know what locks are required when. I'm trying to
> determine if in this piece of code:
>
> read_lock(&tasklist_lock);
> ret = 0;
> if (pid != task_pid_vnr(current))
> ret = -EPERM;
> read_unlock(&tasklist_lock);
>
> the lock of tasklist_lock is necessary.
The RCU protection is enough to get pids from tasks. However, the pids
are detached under the tasklist lock write-locked, so provided you're
*already* sitting under it the RCU is excessive.
However, Oleg pointed me out some time ago, that in some cases RCU-only
protection was not enough in some cases, so I put him in Cc.
> David
>
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: Request for documentation of PID handling functions
2008-07-30 2:04 ` Pavel Emelyanov
@ 2008-07-30 9:56 ` Oleg Nesterov
0 siblings, 0 replies; 3+ messages in thread
From: Oleg Nesterov @ 2008-07-30 9:56 UTC (permalink / raw)
To: Pavel Emelyanov; +Cc: David Howells, jmorris, linux-kernel
On 07/30, Pavel Emelyanov wrote:
>
> David Howells wrote:
> > Hi Pavel,
> >
> > Can you please provide documentation for the PID handling functions you
> > supplied in commit:
> >
> > 7af5729474b5b8ad385adadab78d6e723e7655a3
> >
> > In particular I'd like to know what locks are required when. I'm trying to
> > determine if in this piece of code:
> >
> > read_lock(&tasklist_lock);
> > ret = 0;
> > if (pid != task_pid_vnr(current))
> > ret = -EPERM;
> > read_unlock(&tasklist_lock);
> >
> > the lock of tasklist_lock is necessary.
>
> The RCU protection is enough to get pids from tasks. However, the pids
> are detached under the tasklist lock write-locked, so provided you're
> *already* sitting under it the RCU is excessive.
Yes, tasklist OR rcu is enough.
But, since the task == current, the code above doesn't need any locks at all.
Unless this code runs after exit_notify() of course, in that case we need
rcu/tasklist and task_pid_vnr(current) can return 0.
Oleg.
^ permalink raw reply [flat|nested] 3+ messages in thread
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2008-07-28 22:59 Request for documentation of PID handling functions David Howells
2008-07-30 2:04 ` Pavel Emelyanov
2008-07-30 9:56 ` Oleg Nesterov
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