linux-kernel - Re: [patch V2 30/37] rseq: Split up rseq_exit_to_user

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Message-ID: <fe00b744-eef0-4c5c-902b-a3e2bbd7a8bd@efficios.com>
Date: Tue, 26 Aug 2025 11:45:42 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Thomas Gleixner <tglx@...utronix.de>, LKML <linux-kernel@...r.kernel.org>
Cc: Jens Axboe <axboe@...nel.dk>, Peter Zijlstra <peterz@...radead.org>,
 "Paul E. McKenney" <paulmck@...nel.org>, Boqun Feng <boqun.feng@...il.com>,
 Paolo Bonzini <pbonzini@...hat.com>, Sean Christopherson
 <seanjc@...gle.com>, Wei Liu <wei.liu@...nel.org>,
 Dexuan Cui <decui@...rosoft.com>, x86@...nel.org,
 Arnd Bergmann <arnd@...db.de>, Heiko Carstens <hca@...ux.ibm.com>,
 Christian Borntraeger <borntraeger@...ux.ibm.com>,
 Sven Schnelle <svens@...ux.ibm.com>, Huacai Chen <chenhuacai@...nel.org>,
 Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>
Subject: Re: [patch V2 30/37] rseq: Split up rseq_exit_to_user_mode()

On 2025-08-23 12:40, Thomas Gleixner wrote:
> Seperate the interrupt and syscall exit handling. Syscall exit does not

Separate

> require to clear the user_irq bit as it can't be set. On interrupt exit it
> can be set when the interrupt did not result in a scheduling event and
> therefore the return path did not invoke the TIF work handling, which would
> have cleared it.
> 
> The debug check for the event state is also not really required even when
> debug mode is enabled via the static key. Debug mode is largely aiding user
> space by enabling a larger amount of validation checks, which cause a
> segfault when a malformed critical section is detected. In production mode
> the critical section handling takes the content mostly as is and lets user
> space keep the pieces when it screwed up.
> 
> On kernel changes in that area the state check is useful, but that can be
> done when lockdep is enabled, which is anyway a required test scenario for
> fundamental changes.

Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>

> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> ---
>   include/linux/irq-entry-common.h |    4 ++--
>   include/linux/rseq_entry.h       |   21 +++++++++++++++++----
>   2 files changed, 19 insertions(+), 6 deletions(-)
> 
> --- a/include/linux/irq-entry-common.h
> +++ b/include/linux/irq-entry-common.h
> @@ -247,7 +247,7 @@ static __always_inline void __exit_to_us
>   static __always_inline void syscall_exit_to_user_mode_prepare(struct pt_regs *regs)
>   {
>   	__exit_to_user_mode_prepare(regs);
> -	rseq_exit_to_user_mode();
> +	rseq_syscall_exit_to_user_mode();
>   	__exit_to_user_mode_validate();
>   }
>   
> @@ -261,7 +261,7 @@ static __always_inline void syscall_exit
>   static __always_inline void irqentry_exit_to_user_mode_prepare(struct pt_regs *regs)
>   {
>   	__exit_to_user_mode_prepare(regs);
> -	rseq_exit_to_user_mode();
> +	rseq_irqentry_exit_to_user_mode();
>   	__exit_to_user_mode_validate();
>   }
>   
> --- a/include/linux/rseq_entry.h
> +++ b/include/linux/rseq_entry.h
> @@ -519,19 +519,31 @@ rseq_exit_to_user_mode_work(struct pt_re
>   
>   #endif /* !CONFIG_GENERIC_ENTRY */
>   
> -static __always_inline void rseq_exit_to_user_mode(void)
> +static __always_inline void rseq_syscall_exit_to_user_mode(void)
>   {
>   	struct rseq_event *ev = &current->rseq_event;
>   
>   	rseq_stat_inc(rseq_stats.exit);
>   
> -	if (static_branch_unlikely(&rseq_debug_enabled))
> +	/* Needed to remove the store for the !lockdep case */
> +	if (IS_ENABLED(CONFIG_LOCKDEP)) {
>   		WARN_ON_ONCE(ev->sched_switch);
> +		ev->events = 0;
> +	}
> +}
> +
> +static __always_inline void rseq_irqentry_exit_to_user_mode(void)
> +{
> +	struct rseq_event *ev = &current->rseq_event;
> +
> +	rseq_stat_inc(rseq_stats.exit);
> +
> +	lockdep_assert_once(!ev->sched_switch);
>   
>   	/*
>   	 * Ensure that event (especially user_irq) is cleared when the
>   	 * interrupt did not result in a schedule and therefore the
> -	 * rseq processing did not clear it.
> +	 * rseq processing could not clear it.
>   	 */
>   	ev->events = 0;
>   }
> @@ -551,7 +563,8 @@ static inline unsigned long rseq_exit_to
>   	return ti_work;
>   }
>   static inline void rseq_note_user_irq_entry(void) { }
> -static inline void rseq_exit_to_user_mode(void) { }
> +static inline void rseq_syscall_exit_to_user_mode(void) { }
> +static inline void rseq_irqentry_exit_to_user_mode(void) { }
>   static inline void rseq_debug_syscall_return(struct pt_regs *regs) { }
>   #endif /* !CONFIG_RSEQ */
>   
> 


-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com