linux-kernel - Re: [RFC] kernel/pid.c pid allocation wierdness

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Date:	Fri, 16 Mar 2007 13:57:39 +0300
From:	Pavel Emelianov <xemul@...ru>
To:	Oleg Nesterov <oleg@...sign.ru>
CC:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Sukadev Bhattiprolu <sukadev@...ibm.com>,
	Serge Hallyn <serue@...ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux Containers <containers@...ts.osdl.org>
Subject: Re: [RFC] kernel/pid.c pid allocation wierdness

Oleg Nesterov wrote:
> On 03/14, Eric W. Biederman wrote:
>> Pavel Emelianov <xemul@...ru> writes:
>>
>>> Hi.
>>>
>>> I'm looking at how alloc_pid() works and can't understand
>>> one (simple/stupid) thing.
>>>
>>> It first kmem_cache_alloc()-s a strct pid, then calls
>>> alloc_pidmap() and at the end it taks a global pidmap_lock()
>>> to add new pid to hash.
> 
> We need some global lock. pidmap_lock is already here, and it is
> only used to protect pidmap->page allocation. Iow, it is almost
> unused. So it was very natural to re-use it while implementing
> pidrefs.
> 
>>> The question is - why does alloc_pidmap() use at least
>>> two atomic ops and potentially loop to find a zero bit
>>> in pidmap? Why not call alloc_pidmap() under pidmap_lock
>>> and find zero pid in pidmap w/o any loops and atomics?
> 
> Currently we search for zero bit lockless, why do you want
> to do it under spin_lock ?

Search isn't lockless. Look:

while (1) {
   if (!test_and_set_bit(...)) {
       atomic_dec(&nr_free);
       return pid;
   }
   ...
}

we use two atomic operations to find and set a bit in a map.

> Oleg.
> 
> 

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