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Date:   Tue, 3 Mar 2020 11:25:40 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Jann Horn <jannh@google.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Darren Hart <dvhart@infradead.org>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] threads: Update PID limit comment according to futex
 UAPI change
Message-ID: <20200303102540.GC2579@hirez.programming.kicks-ass.net>
References: <20200302112939.8068-1-jannh@google.com>
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On Mon, Mar 02, 2020 at 12:29:39PM +0100, Jann Horn wrote:
> The futex UAPI changed back in commit 76b81e2b0e22 ("[PATCH] lightweight
> robust futexes updates 2"), which landed in v2.6.17: FUTEX_TID_MASK is now
> 0x3fffffff instead of 0x1fffffff. Update the corresponding comment in
> include/linux/threads.h.
> 
> Signed-off-by: Jann Horn <jannh@google.com>
> ---
>  include/linux/threads.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/threads.h b/include/linux/threads.h
> index 3086dba525e20..18d5a74bcc3dd 100644
> --- a/include/linux/threads.h
> +++ b/include/linux/threads.h
> @@ -29,7 +29,7 @@
>  
>  /*
>   * A maximum of 4 million PIDs should be enough for a while.
> - * [NOTE: PID/TIDs are limited to 2^29 ~= 500+ million, see futex.h.]
> + * [NOTE: PID/TIDs are limited to 2^30 ~= 1 billion, see FUTEX_TID_MASK.]
>   */
>  #define PID_MAX_LIMIT (CONFIG_BASE_SMALL ? PAGE_SIZE * 8 : \
>  	(sizeof(long) > 4 ? 4 * 1024 * 1024 : PID_MAX_DEFAULT))

I just noticed another mention of this in Documentation/robust-futex-ABI.txt
There it states that bit-29 is reserved for future use.

Thomas, do we want to release that bit and update all this?