linux-kernel - Re: Use case for TASKS

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:   Tue, 16 May 2017 08:22:33 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     rostedt@...dmis.org, linux-kernel@...r.kernel.org
Subject: Re: Use case for TASKS_RCU


* Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:

> Hello!
> 
> The question of the use case for TASKS_RCU came up, and here is my
> understanding.  Steve will not be shy about correcting any misconceptions
> I might have.  ;-)
> 
> The use case is to support freeing of trampolines used in tracing/probing
> in CONFIG_PREEMPT=y kernels.  It is necessary to wait until any task
> executing in the trampoline in question has left it, taking into account
> that the trampoline's code might be interrupted and preempted.  However,
> the code in the trampolines is guaranteed never to context switch.
> 
> Note that in CONFIG_PREEMPT=n kernels, synchronize_sched() suffices.
> It is therefore tempting to think in terms of disabling preemption across
> the trampolines, but there is apparently not enough room to accommodate
> the needed preempt_disable() and preempt_enable() in the code invoking
> the trampoline, and putting the preempt_disable() and preempt_enable()
> in the trampoline itself fails because of the possibility of preemption
> just before the preempt_disable() and just after the preempt_enable().
> Similar reasoning rules out use of rcu_read_lock() and rcu_read_unlock().

So how was this solved before TASKS_RCU? Also, nothing uses call_rcu_tasks() at 
the moment, so it's hard for me to review its users. What am I missing?

Thanks,

	Ingo