linux-kernel - Re: [PATCH] srcu: Fix warning to permit SRCU-fast readers in NMI handlers

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Message-ID: <aXwZyxqsxgOyZHTU@tardis.local>
Date: Thu, 29 Jan 2026 18:39:07 -0800
From: Boqun Feng <boqun@...nel.org>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
	rcu@...r.kernel.org, bpf@...r.kernel.org,
	Andrii Nakryiko <andrii@...nel.org>,
	Alexei Starovoitov <ast@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] srcu: Fix warning to permit SRCU-fast readers in NMI
 handlers

On Thu, Jan 29, 2026 at 05:30:44PM -0800, Paul E. McKenney wrote:
> srcu: Fix warning to permit SRCU-fast readers in NMI handlers
> 

This line seems to be duplicated ;)

> SRCU-fast is designed to be used in NMI handlers, even going so far
> as to use atomic operations for architectures supporting NMIs but not
> providing NMI-safe per-CPU atomic operations.  However, the WARN_ON_ONCE()
> in __srcu_check_read_flavor() complains if SRCU-fast is used in an NMI
> handler.  This commit therefore modifies that WARN_ON_ONCE() to avoid
> such complaints.
> 
> Reported-by: Steven Rostedt <rostedt@...dmis.org>
> Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> Tested-by: Steven Rostedt <rostedt@...dmis.org>
> Cc: Andrii Nakryiko <andrii@...nel.org>
> Cc: Alexei Starovoitov <ast@...nel.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: bpf@...r.kernel.org
> 

I queued it with the duplicate line removed, please see below.

Regards,
Boqun

---------------------->8
Subject: [PATCH] srcu: Fix warning to permit SRCU-fast readers in NMI handlers

SRCU-fast is designed to be used in NMI handlers, even going so far
as to use atomic operations for architectures supporting NMIs but not
providing NMI-safe per-CPU atomic operations.  However, the WARN_ON_ONCE()
in __srcu_check_read_flavor() complains if SRCU-fast is used in an NMI
handler.  This commit therefore modifies that WARN_ON_ONCE() to avoid
such complaints.

[boqun: Remove the duplicate title line]

Reported-by: Steven Rostedt <rostedt@...dmis.org>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
Tested-by: Steven Rostedt <rostedt@...dmis.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: Boqun Feng <boqun@...nel.org>
---
 kernel/rcu/srcutree.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index c469c708fdd6..66ba6a2f83d3 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -789,7 +789,8 @@ void __srcu_check_read_flavor(struct srcu_struct *ssp, int read_flavor)
 	struct srcu_data *sdp;
 
 	/* NMI-unsafe use in NMI is a bad sign, as is multi-bit read_flavor values. */
-	WARN_ON_ONCE((read_flavor != SRCU_READ_FLAVOR_NMI) && in_nmi());
+	WARN_ON_ONCE(read_flavor != SRCU_READ_FLAVOR_NMI &&
+		     read_flavor != SRCU_READ_FLAVOR_FAST && in_nmi());
 	WARN_ON_ONCE(read_flavor & (read_flavor - 1));
 
 	sdp = raw_cpu_ptr(ssp->sda);
--