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[212.8.243.115]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-45491f8d4c3sm40592308f8f.34.2026.05.13.05.39.30 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 13 May 2026 05:39:31 -0700 (PDT) Message-ID: <22a3be4458799dfe31980142b60c1f6ed63b2a36.camel@redhat.com> Subject: Re: [RFC PATCH v2 01/10] rv/da: fix monitor start ordering and memory ordering for monitoring flag From: Gabriele Monaco To: wen.yang@linux.dev Cc: linux-trace-kernel@vger.kernel.org, linux-kernel@vger.kernel.org, Steven Rostedt Date: Wed, 13 May 2026 14:39:29 +0200 In-Reply-To: <8af5ba4bd93d2acb8a546e8e47ced974a87c1eb8.1778522945.git.wen.yang@linux.dev> References: <8af5ba4bd93d2acb8a546e8e47ced974a87c1eb8.1778522945.git.wen.yang@linux.dev> Autocrypt: addr=gmonaco@redhat.com; prefer-encrypt=mutual; keydata=mDMEZuK5YxYJKwYBBAHaRw8BAQdAmJ3dM9Sz6/Hodu33Qrf8QH2bNeNbOikqYtxWFLVm0 1a0JEdhYnJpZWxlIE1vbmFjbyA8Z21vbmFjb0BrZXJuZWwub3JnPoiZBBMWCgBBFiEEysoR+AuB3R Zwp6j270psSVh4TfIFAmjKX2MCGwMFCQWjmoAFCwkIBwICIgIGFQoJCAsCBBYCAwECHgcCF4AACgk Q70psSVh4TfIQuAD+JulczTN6l7oJjyroySU55Fbjdvo52xiYYlMjPG7dCTsBAMFI7dSL5zg98I+8 cXY1J7kyNsY6/dcipqBM4RMaxXsOtCRHYWJyaWVsZSBNb25hY28gPGdtb25hY29AcmVkaGF0LmNvb T6InAQTFgoARAIbAwUJBaOagAULCQgHAgIiAgYVCgkICwIEFgIDAQIeBwIXgBYhBMrKEfgLgd0WcK eo9u9KbElYeE3yBQJoymCyAhkBAAoJEO9KbElYeE3yjX4BAJ/ETNnlHn8OjZPT77xGmal9kbT1bC1 7DfrYVISWV2Y1AP9HdAMhWNAvtCtN2S1beYjNybuK6IzWYcFfeOV+OBWRDQ== Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.60.1 (3.60.1-1.fc44) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Tue, 2026-05-12 at 02:24 +0800, wen.yang@linux.dev wrote: > From: Wen Yang >=20 > da_monitor_start() set monitoring=3D1 before calling da_monitor_init_hook= (), > may racing with the sched_switch handler: >=20 > =C2=A0 da_monitor_start()=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 sched_switch handler > =C2=A0 -------------------------=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0 --------------------------------- > =C2=A0 da_mon->monitoring =3D 1; > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 if (da_monitoring= (da_mon))=C2=A0 /* true=C2=A0 */ > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0 ha_start_timer_ns(...); > =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2= =A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0= =C2=A0 /* hrtimer->base =3D=3D NULL, crash */ > =C2=A0 da_monitor_init_hook(da_mon); > =C2=A0 /* hrtimer_setup() sets base */ >=20 > Fix the ordering and pair with release/acquire semantics: >=20 > =C2=A0 da_monitor_init_hook(da_mon); > =C2=A0 smp_store_release(&da_mon->monitoring, 1);=C2=A0=C2=A0=C2=A0 /* da= _monitor_start()=C2=A0 */ > =C2=A0 return smp_load_acquire(&da_mon->monitoring); /* da_monitoring()= =C2=A0=C2=A0=C2=A0=C2=A0 */ >=20 > On ARM64 a plain STR + LDR does not form a release-acquire pair, so > the load can observe monitoring=3D1 while hrtimer->base is still NULL. > The plain accesses are also data races under KCSAN. >=20 > Use WRITE_ONCE for the monitoring=3D0 store in da_monitor_reset() to > cover the reset path. >=20 > Fixes: 792575348ff7 ("rv/include: Add deterministic automata monitor > definition via C macros") > Signed-off-by: Wen Yang Thanks for the fix! There are probably more than a few of those bugs since most monitors are implicitly serialised because their events are serialised.. Looks good to me. Reviewed-by: Gabriele Monaco See minor comments below: > --- > =C2=A0include/rv/da_monitor.h | 8 +++++--- > =C2=A01 file changed, 5 insertions(+), 3 deletions(-) >=20 > diff --git a/include/rv/da_monitor.h b/include/rv/da_monitor.h > index 39765ff6f098..00ded3d5ab3f 100644 > --- a/include/rv/da_monitor.h > +++ b/include/rv/da_monitor.h > @@ -82,7 +82,7 @@ static void react(enum states curr_state, enum events e= vent) > =C2=A0static inline void da_monitor_reset(struct da_monitor *da_mon) > =C2=A0{ > =C2=A0 da_monitor_reset_hook(da_mon); > - da_mon->monitoring =3D 0; > + WRITE_ONCE(da_mon->monitoring, 0); > =C2=A0 da_mon->curr_state =3D model_get_initial_state(); > =C2=A0} > =C2=A0 > @@ -95,8 +95,9 @@ static inline void da_monitor_reset(struct da_monitor > *da_mon) > =C2=A0static inline void da_monitor_start(struct da_monitor *da_mon) > =C2=A0{ > =C2=A0 da_mon->curr_state =3D model_get_initial_state(); > - da_mon->monitoring =3D 1; > =C2=A0 da_monitor_init_hook(da_mon); > + /* Pairs with smp_load_acquire in da_monitoring(). */ I wonder if these comment are really adding value, pairing smp_load_acquire= / smp_store_release is the by-the-book usage and everything is here. But feel free to leave it if you think it's clearer. Thanks, Gabriele > + smp_store_release(&da_mon->monitoring, 1); > =C2=A0} > =C2=A0 > =C2=A0/* > @@ -104,7 +105,8 @@ static inline void da_monitor_start(struct da_monitor > *da_mon) > =C2=A0 */ > =C2=A0static inline bool da_monitoring(struct da_monitor *da_mon) > =C2=A0{ > - return da_mon->monitoring; > + /* Pairs with smp_store_release in da_monitor_start(). */ > + return smp_load_acquire(&da_mon->monitoring); > =C2=A0} > =C2=A0 > =C2=A0/*