linux-kernel - Re: [PATCH v2 14/16] srcu: Create an SRCU-fast-updown API

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Message-ID: <9a91159e-4411-443c-b0cc-e0cbc5426b11@paulmck-laptop>
Date: Wed, 26 Nov 2025 09:09:15 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: rcu@...r.kernel.org, linux-kernel@...r.kernel.org, kernel-team@...a.com,
	rostedt@...dmis.org, Andrii Nakryiko <andrii@...nel.org>,
	Alexei Starovoitov <ast@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>, bpf@...r.kernel.org
Subject: Re: [PATCH v2 14/16] srcu: Create an SRCU-fast-updown API

On Wed, Nov 26, 2025 at 03:06:46PM +0100, Frederic Weisbecker wrote:
> Le Tue, Nov 25, 2025 at 07:54:33AM -0800, Paul E. McKenney a écrit :
> > On Tue, Nov 25, 2025 at 03:18:25PM +0100, Frederic Weisbecker wrote:
> > > Le Wed, Nov 05, 2025 at 12:32:14PM -0800, Paul E. McKenney a écrit :
> > > > This commit creates an SRCU-fast-updown API, including
> > > > DEFINE_SRCU_FAST_UPDOWN(), DEFINE_STATIC_SRCU_FAST_UPDOWN(),
> > > > __init_srcu_struct_fast_updown(), init_srcu_struct_fast_updown(),
> > > > srcu_read_lock_fast_updown(), srcu_read_unlock_fast_updown(),
> > > > __srcu_read_lock_fast_updown(), and __srcu_read_unlock_fast_updown().
> > > > 
> > > > These are initially identical to their SRCU-fast counterparts, but both
> > > > SRCU-fast and SRCU-fast-updown will be optimized in different directions
> > > > by later commits.  SRCU-fast will lack any sort of srcu_down_read() and
> > > > srcu_up_read() APIs, which will enable extremely efficient NMI safety.
> > > > For its part, SRCU-fast-updown will not be NMI safe, which will enable
> > > > reasonably efficient implementations of srcu_down_read_fast() and
> > > > srcu_up_read_fast().
> > > 
> > > Doing a last round of reviews before sitting down on a pull request,
> > > I think the changelog in this one should mention what are the expected
> > > uses of SRCU-fast-updown, since the RCU-TASK-TRACE conversion bits aren't
> > > there for this merge window yet.
> > 
> > The RCU Tasks Trace conversion is helped by RCU-fast.  RCU-fast-updown
> > is needed for Andrii's uretprobes code in order to get rid of the
> > read-side memory barriers while still allowing entering the reader at
> > task level while exiting it in a timer handler.
> > 
> > Does any of that help?
> > 
> > Oh, and commit-by-commit testing passed this past evening, so still
> > looking good there!
> 
> Ok, here is the new proposed changelog accordingly:

Looks good to me, thank you!

Nit:  s/usecases/use cases/

							Thanx, Paul

> ----
> srcu: Create an SRCU-fast-updown API
> 
> This commit creates an SRCU-fast-updown API, including
> DEFINE_SRCU_FAST_UPDOWN(), DEFINE_STATIC_SRCU_FAST_UPDOWN(),
> __init_srcu_struct_fast_updown(), init_srcu_struct_fast_updown(),
> srcu_read_lock_fast_updown(), srcu_read_unlock_fast_updown(),
> __srcu_read_lock_fast_updown(), and __srcu_read_unlock_fast_updown().
> 
> These are initially identical to their SRCU-fast counterparts, but both
> SRCU-fast and SRCU-fast-updown will be optimized in different directions
> by later commits. SRCU-fast will lack any sort of srcu_down_read() and
> srcu_up_read() APIs, which will enable extremely efficient NMI safety.
> For its part, SRCU-fast-updown will not be NMI safe, which will enable
> reasonably efficient implementations of srcu_down_read_fast() and
> srcu_up_read_fast().
> 
> This API fork happens to meet two different future usecases.
> 
> * SRCU-fast will become the reimplementation basis for RCU-TASK-TRACE
>   for consolidation. Since RCU-TASK-TRACE must be NMI safe, SRCU-fast
>   must be as well.
> 
> * SRCU-fast-updown will be needed for uretprobes code in order to get
>   rid of the read-side memory barriers while still allowing entering the
>   reader at task level while exiting it in a timer handler.
> 
> This commit also adds rcutorture tests for the new APIs.  This
> (annoyingly) needs to be in the same commit for bisectability.  With this
> commit, the 0x8 value tests SRCU-fast-updown.  However, most SRCU-fast
> testing will be via the RCU Tasks Trace wrappers.
> 
> [ paulmck: Apply s/0x8/0x4/ missing change per Boqun Feng feedback. ]
> [ paulmck: Apply Akira Yokosawa feedback. ]
> 
> Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> Cc: Andrii Nakryiko <andrii@...nel.org>
> Cc: Alexei Starovoitov <ast@...nel.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: <bpf@...r.kernel.org>
> Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> 
> -- 
> Frederic Weisbecker
> SUSE Labs