linux-kernel - Re: [PATCH v6 1/5] random: Blocking API for accessing nonblocking

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Message-ID: <2404915.vbuKFfjVR8@tachyon.chronox.de>
Date:	Tue, 19 May 2015 09:35:15 +0200
From:	Stephan Mueller <smueller@...onox.de>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	Theodore Ts'o <tytso@....edu>, pebolle@...cali.nl,
	andreas.steffen@...ongswan.org, sandyinchina@...il.com,
	linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org
Subject: Re: [PATCH v6 1/5] random: Blocking API for accessing nonblocking_pool

Am Dienstag, 19. Mai 2015, 15:22:27 schrieb Herbert Xu:

Hi Herbert,

> On Tue, May 19, 2015 at 07:58:25AM +0200, Stephan Mueller wrote:
> > Herbert, do you have any ideas?
> 
> On the /dev/random side,
> 
> 1) Add a struct module argument in addition to func/data.
> 2) Grab module ref count when func/data is registered.
> 3) Drop module ref count after func returns.
> 
> On the drbg side,
> 
> 1) Allocate data pointer before func/data registration, it should
> contain a flag indicating whether drbg is still alive.
> 2) In cra_exit, zap the flag in allocated data.
> 3) In func immediately return if flag indicates drbg is dead.
> 4) Free allocated data pointer when func is done.
> 
> Obviously you need to add some locking so that func doesn't race
> against cra_exit or any other drbg paths that it intersects.

Thank you for the hints. I will follow your guidance.

Just for my edification: why is this (rather complex sounding) approach 
preferred over a simple cancel API? Other async APIs (e.g. the AIO syscalls 
with io_cancel) have such cancel operations.

Such cancel function would be as simple as:

void get_blocking_random_bytes_cancel(void *private)
{
        struct random_work *rw, *tmp;

        mutex_lock(&random_wait_list_mutex);
        list_for_each_entry_safe(rw, tmp, &random_wait_list, list) {
                if (private == rw->rw_private) {
                        list_del(&rw->list);
                        kfree(rw);
                        break;
                }
        }
        mutex_unlock(&random_wait_list_mutex);
}

-- 
Ciao
Stephan
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