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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Fri, 16 Mar 2007 14:58:57 +0300
From:	Pavel Emelianov <xemul@...ru>
To:	Eric Dumazet <dada1@...mosbay.com>
CC:	Oleg Nesterov <oleg@...sign.ru>,
	"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

Eric Dumazet wrote:
> On Friday 16 March 2007 11:57, Pavel Emelianov wrote:
>> 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.
> 
> The finding of the zero bit is done without lock. (Search/lookup)
> 
> Then , the reservation of the found bit (test_and_set_bit) is done, and 
> decrement of nr_free. It may fail because the search was done lockless.

:\ I do understand how this algorithm works. What I don't
understand is why it is done so, if we take a global lock anyway.

> Finding a zero bit in a 4096 bytes array may consume about 6000 cycles on 
> modern hardware. Much more on SMP/NUMA machines, or on machines where 
> PAGE_SIZE is 64K instead of 4K :)
> 
> You don't want to hold pidmad_lock for so long period.

OK, thanks. That's explanations looks good.

> -
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@...r.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at  http://www.tux.org/lkml/
> 

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/