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Date:   Tue, 2 Apr 2019 08:23:34 -0700
From:   "Paul E. McKenney" <paulmck@linux.ibm.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     rcu@vger.kernel.org, linux-kernel <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        dipankar <dipankar@in.ibm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        rostedt <rostedt@goodmis.org>,
        David Howells <dhowells@redhat.com>,
        Eric Dumazet <edumazet@google.com>,
        fweisbec <fweisbec@gmail.com>, Oleg Nesterov <oleg@redhat.com>,
        "Joel Fernandes, Google" <joel@joelfernandes.org>,
        linux-nvdimm@lists.01.org, dri-devel@lists.freedesktop.org,
        amd-gfx@lists.freedesktop.org
Subject: Re: [PATCH RFC tip/core/rcu 0/4] Forbid static SRCU use in modules
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On Tue, Apr 02, 2019 at 11:14:40AM -0400, Mathieu Desnoyers wrote:
> ----- On Apr 2, 2019, at 10:28 AM, paulmck paulmck@linux.ibm.com wrote:
> 
> > Hello!
> > 
> > This series prohibits use of DEFINE_SRCU() and DEFINE_STATIC_SRCU()
> > by loadable modules.  The reason for this prohibition is the fact
> > that using these two macros within modules requires that the size of
> > the reserved region be increased, which is not something we want to
> > be doing all that often.  Instead, loadable modules should define an
> > srcu_struct and invoke init_srcu_struct() from their module_init function
> > and cleanup_srcu_struct() from their module_exit function.  Note that
> > modules using call_srcu() will also need to invoke srcu_barrier() from
> > their module_exit function.
> 
> This arbitrary API limitation seems weird.
> 
> Isn't there a way to allow modules to use DEFINE_SRCU and DEFINE_STATIC_SRCU
> while implementing them with dynamic allocation under the hood ?

Although call_srcu() already has initialization hooks, some would
also be required in srcu_read_lock(), and I am concerned about adding
memory allocation at that point, especially given the possibility
of memory-allocation failure.  And the possibility that the first
srcu_read_lock() happens in an interrupt handler or similar.

Or am I missing a trick here?

							Thanx, Paul

> Thanks,
> 
> Mathieu
> 
> 
> > 
> > This series consist of the following:
> > 
> > 1.	Dynamically allocate dax_srcu.
> > 
> > 2.	Dynamically allocate drm_unplug_srcu.
> > 
> > 3.	Dynamically allocate kfd_processes_srcu.
> > 
> > These build and have been subjected to 0day testing, but might also need
> > testing by someone having the requisite hardware.
> > 
> >							Thanx, Paul
> > 
> > ------------------------------------------------------------------------
> > 
> > drivers/dax/super.c                        |   10 +++++-
> > drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd.c |    5 +++
> > drivers/gpu/drm/amd/amdkfd/kfd_process.c   |    2 -
> > drivers/gpu/drm/drm_drv.c                  |    8 ++++
> > include/linux/srcutree.h                   |   19 +++++++++--
> > kernel/rcu/rcuperf.c                       |   40 +++++++++++++++++++-----
> > kernel/rcu/rcutorture.c                    |   48 +++++++++++++++++++++--------
> >  7 files changed, 105 insertions(+), 27 deletions(-)
> 
> -- 
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com
>