From: Florian Weimer <fweimer@redhat.com>
To: Carlos O'Donell <carlos@redhat.com>
Cc: libc-alpha@sourceware.org
Subject: Re: [PATCH] nptl: Add pthread_thread_number_np function
Date: Thu, 21 Dec 2017 11:03:00 -0000 [thread overview]
Message-ID: <609c8815-7d79-c504-0a1f-3eb6f82ead9d@redhat.com> (raw)
In-Reply-To: <547636b9-0c3a-c876-009f-c4f95ba1fa5b@redhat.com>
On 12/21/2017 10:26 AM, Carlos O'Donell wrote:
>> +The returned number is only unique with regards to the current process.
>> +It may be shared by subprocesses and other processes in the system.
>> +
>> +The initial (main) thread has number 1. Thread numbers are not
>> +necessarily assigned in a consecutive fashion. They bear no
>> +relationship to POSIX thread IDs (@code{pthread_t} values), process IDs
>> +or thread IDs assigned by the kernel.
>
> I would like us to add something like this:
> ~~~
> While the return type of this function is only 64-bits wide, the intent
> is not to impose an artificial limit on the number of threads that can be
> created by the runtime. In the future this interface may be extended
> to 128-bits to support creating as many threads as a user may need
> for the lifetime of the process.
> ~~~
>
> That way the intent of the interface and future changes are clear.
So how would a programmer use this interface in a future-proof way? I
think such a statement would raise more questions than it answers.
>> diff --git a/nptl/allocatestack.c b/nptl/allocatestack.c
>> index 1cc7893195..454df7740b 100644
>> --- a/nptl/allocatestack.c
>> +++ b/nptl/allocatestack.c
>> @@ -413,16 +413,28 @@ allocate_stack (const struct pthread_attr *attr, struct pthread **pdp,
>> assert (powerof2 (pagesize_m1 + 1));
>> assert (TCB_ALIGNMENT >= STACK_ALIGN);
>>
>> - /* Get the stack size from the attribute if it is set. Otherwise we
>> - use the default we determined at start time. */
>> - if (attr->stacksize != 0)
>> - size = attr->stacksize;
>> - else
>> - {
>> - lll_lock (__default_pthread_attr_lock, LLL_PRIVATE);
>> + uint64_t thread_number;
>> + lll_lock (__default_pthread_attr_lock, LLL_PRIVATE);
>> + {
>> + /* Number 1 is reserved for the initial thread. Reuse
>> + __default_pthread_attr_lock to avoid concurrent updates of this
>> + counter. */
>
> OK.
>
>> + static uint64_t global_thread_number = 1;
>> + thread_number = ++global_thread_number;
>
> Alright, here comes serious worry #1.
>
> If we say "Thread numbers are not necessarily assigned in a consecutive fashion.",
> and we assign them in a consecutive fashion, users will ignore this statement
> and use what empirically appears to be true.
The above does not actually assign thread numbers in a consecutive
fashion, from an application perspective because the implementation can
create its own threads for its own internal use. (librt and libanl do
this.)
> People start relying on this counter incrementing from 1 upwards.
>
> People start using this monotonic property for indexing.
>
> Soon we can't change it because it's implied API behaviour.
>
> I think we should disabuse them from doing something low cost to roll the value:
>
> * Do nothing for thread 1, leaving it 1.
> * Check global_thread_number for overflow instead.
> * Pick a random number of bits to roll between 0-63 (picked at process startup)
> * Roll the value by some that number of bits.
> * Use the rolled value as the thread_number
Not sure if I understand this. Do you want us to start at a random
value? Or assign IDs randomly? The latter will have a collision much
sooner.
I can switch the thread numbers to a fixed, but random-looking
permutation of the integers in [0, 2**64), but this looks excessive.
In my opinion, we need to assume at one point that programmers read the
documentation.
Thanks,
Florian
next prev parent reply other threads:[~2017-12-21 11:03 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-12-14 18:56 Florian Weimer
2017-12-14 20:24 ` Florian Weimer
2017-12-14 23:34 ` Nix
2017-12-15 6:41 ` Florian Weimer
2017-12-15 0:29 ` Andrew Pinski
2017-12-15 7:47 ` Florian Weimer
2017-12-15 7:54 ` Andrew Pinski
2017-12-15 4:08 ` Carlos O'Donell
2017-12-15 7:48 ` Florian Weimer
2017-12-20 8:06 ` Carlos O'Donell
2017-12-20 14:34 ` Florian Weimer
2017-12-20 17:58 ` Carlos O'Donell
2017-12-21 9:26 ` Carlos O'Donell
2017-12-21 11:03 ` Florian Weimer [this message]
2017-12-21 19:19 ` Carlos O'Donell
2017-12-22 16:25 ` Florian Weimer
2017-12-22 17:09 ` Carlos O'Donell
2017-12-22 17:43 ` Joseph Myers
2017-12-22 19:39 ` Florian Weimer
2017-12-22 20:02 ` Joseph Myers
2017-12-22 22:11 ` Florian Weimer
2018-03-02 18:04 ` Rich Felker
2018-03-02 18:08 ` Rich Felker
2018-03-09 17:23 ` Florian Weimer
2018-03-09 23:29 ` Carlos O'Donell
2018-03-02 14:42 Florian Weimer
2018-03-02 17:16 ` Joseph Myers
2018-05-15 13:42 ` Florian Weimer
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=609c8815-7d79-c504-0a1f-3eb6f82ead9d@redhat.com \
--to=fweimer@redhat.com \
--cc=carlos@redhat.com \
--cc=libc-alpha@sourceware.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).