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Date:   Thu, 29 Sep 2022 11:07:14 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     rcu@vger.kernel.org
Cc:     linux-kernel@vger.kernel.org, kernel-team@fb.com,
        rostedt@goodmis.org, tglx@linutronix.de, john.ogness@linutronix.de,
        pmladek@suse.com
Subject: [PATCH v2 rcu 0/8] NMI-safe SRCU reader API
Message-ID: <20220929180714.GA2874192@paulmck-ThinkPad-P17-Gen-1>
Reply-To: paulmck@kernel.org
References: <20220921144620.GA1200846@paulmck-ThinkPad-P17-Gen-1>
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Hello!

This RFC series provides the second version of an NMI-safe SRCU reader API
in the guise of srcu_read_lock_nmisafe() and srcu_read_unlock_nmisafe().
A given srcu_struct structure must use either the traditional
srcu_read_lock() and srcu_read_unlock() API or the new _nmisafe() API:
Mixing and matching is not permitted.  So much so that kernels built
with CONFIG_PROVE_RCU=y will complain if you try it.

The reason for this restriction is that I have yet to find a use case
that is not a accident waiting to happen.  And if free intermixing
were permitted, it is pretty much a given that someone somewhere will
get confused and use srcu_read_lock_nmisafe() within NMI handlers and
srcu_read_lock() elsewhere, which will not (repeat, NOT) provide NMI
safety.

I do not expect to push this into the v6.1 merge window.  However, if
the printk() series that needs it goes in, then I will push it as a fix
for the resulting regression.

The series is as follows:

1.	Convert ->srcu_lock_count and ->srcu_unlock_count to atomic.

2.	Create an srcu_read_lock_nmisafe() and srcu_read_unlock_nmisafe().

3.	Check for consistent per-CPU per-srcu_struct NMI safety.

4.	Check for consistent global per-srcu_struct NMI safety.

5.	Add ARCH_HAS_NMI_SAFE_THIS_CPU_OPS Kconfig option.

6.	Add ARCH_HAS_NMI_SAFE_THIS_CPU_OPS Kconfig option.

7.	Add ARCH_HAS_NMI_SAFE_THIS_CPU_OPS Kconfig option.

8.	Add ARCH_HAS_NMI_SAFE_THIS_CPU_OPS Kconfig option.

Changes since v1 RFC:

1.	Added enabling patches for arm64, loongarch, s390, and x86.
	These have what appear to me to be NMI-safe this_cpu_inc()
	implementations.

2.	Fix a build error on !SMP kernels built without SRCU.

3.	Fix a build error on !SMP kernels.

						Thanx, Paul

------------------------------------------------------------------------

 b/arch/arm64/Kconfig       |    1 
 b/arch/loongarch/Kconfig   |    1 
 b/arch/s390/Kconfig        |    1 
 b/arch/x86/Kconfig         |    1 
 b/include/linux/srcu.h     |   39 +++++++++++++++++++++
 b/include/linux/srcutiny.h |   11 ++++++
 b/include/linux/srcutree.h |    4 +-
 b/kernel/rcu/Kconfig       |    3 +
 b/kernel/rcu/rcutorture.c  |   11 ++++--
 b/kernel/rcu/srcutree.c    |   24 ++++++-------
 include/linux/srcu.h       |    4 +-
 include/linux/srcutiny.h   |    4 +-
 include/linux/srcutree.h   |   12 +++++-
 kernel/rcu/srcutree.c      |   82 +++++++++++++++++++++++++++++++++++++++------
 14 files changed, 166 insertions(+), 32 deletions(-)