rseq: Simplify the event notification

Since commit 0190e4198e47 ("rseq: Deprecate RSEQ_CS_FLAG_NO_RESTART_ON_*
flags") the bits in task::rseq_event_mask are meaningless and just extra
work in terms of setting them individually.

Aside of that the only relevant point where an event has to be raised is
context switch. Neither the CPU nor MM CID can change without going through
a context switch.

Collapse them all into a single boolean which simplifies the code a lot and
remove the pointless invocations which have been sprinkled all over the
place for no value.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://patch.msgid.link/20251027084306.336978188@linutronix.de

3 files changed, 25 insertions, 72 deletions

diff --git a/include/linux/rseq.h b/include/linux/rseq.h
index d72ddf7ce903..241067bf20db 100644
--- a/include/linux/rseq.h
+++ b/include/linux/rseq.h
@@ -3,38 +3,8 @@
 #define _LINUX_RSEQ_H
 
 #ifdef CONFIG_RSEQ
-
-#include <linux/preempt.h>
 #include <linux/sched.h>
 
-#ifdef CONFIG_MEMBARRIER
-# define RSEQ_EVENT_GUARD	irq
-#else
-# define RSEQ_EVENT_GUARD	preempt
-#endif
-
-/*
- * Map the event mask on the user-space ABI enum rseq_cs_flags
- * for direct mask checks.
- */
-enum rseq_event_mask_bits {
-	RSEQ_EVENT_PREEMPT_BIT	= RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT_BIT,
-	RSEQ_EVENT_SIGNAL_BIT	= RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL_BIT,
-	RSEQ_EVENT_MIGRATE_BIT	= RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE_BIT,
-};
-
-enum rseq_event_mask {
-	RSEQ_EVENT_PREEMPT	= (1U << RSEQ_EVENT_PREEMPT_BIT),
-	RSEQ_EVENT_SIGNAL	= (1U << RSEQ_EVENT_SIGNAL_BIT),
-	RSEQ_EVENT_MIGRATE	= (1U << RSEQ_EVENT_MIGRATE_BIT),
-};
-
-static inline void rseq_set_notify_resume(struct task_struct *t)
-{
-	if (t->rseq)
-		set_tsk_thread_flag(t, TIF_NOTIFY_RESUME);
-}
-
 void __rseq_handle_notify_resume(struct ksignal *sig, struct pt_regs *regs);
 
 static inline void rseq_handle_notify_resume(struct pt_regs *regs)
@@ -43,35 +13,27 @@ static inline void rseq_handle_notify_resume(struct pt_regs *regs)
 		__rseq_handle_notify_resume(NULL, regs);
 }
 
-static inline void rseq_signal_deliver(struct ksignal *ksig,
-				       struct pt_regs *regs)
+static inline void rseq_signal_deliver(struct ksignal *ksig, struct pt_regs *regs)
 {
 	if (current->rseq) {
-		scoped_guard(RSEQ_EVENT_GUARD)
-			__set_bit(RSEQ_EVENT_SIGNAL_BIT, &current->rseq_event_mask);
+		current->rseq_event_pending = true;
 		__rseq_handle_notify_resume(ksig, regs);
 	}
 }
 
-/* rseq_preempt() requires preemption to be disabled. */
-static inline void rseq_preempt(struct task_struct *t)
+static inline void rseq_sched_switch_event(struct task_struct *t)
 {
-	__set_bit(RSEQ_EVENT_PREEMPT_BIT, &t->rseq_event_mask);
-	rseq_set_notify_resume(t);
-}
-
-/* rseq_migrate() requires preemption to be disabled. */
-static inline void rseq_migrate(struct task_struct *t)
-{
-	__set_bit(RSEQ_EVENT_MIGRATE_BIT, &t->rseq_event_mask);
-	rseq_set_notify_resume(t);
+	if (t->rseq) {
+		t->rseq_event_pending = true;
+		set_tsk_thread_flag(t, TIF_NOTIFY_RESUME);
+	}
 }
 
 static __always_inline void rseq_exit_to_user_mode(void)
 {
 	if (IS_ENABLED(CONFIG_DEBUG_RSEQ)) {
-		if (WARN_ON_ONCE(current->rseq && current->rseq_event_mask))
-			current->rseq_event_mask = 0;
+		if (WARN_ON_ONCE(current->rseq && current->rseq_event_pending))
+			current->rseq_event_pending = false;
 	}
 }
 
@@ -85,12 +47,12 @@ static inline void rseq_fork(struct task_struct *t, u64 clone_flags)
 		t->rseq = NULL;
 		t->rseq_len = 0;
 		t->rseq_sig = 0;
-		t->rseq_event_mask = 0;
+		t->rseq_event_pending = false;
 	} else {
 		t->rseq = current->rseq;
 		t->rseq_len = current->rseq_len;
 		t->rseq_sig = current->rseq_sig;
-		t->rseq_event_mask = current->rseq_event_mask;
+		t->rseq_event_pending = current->rseq_event_pending;
 	}
 }
 
@@ -99,15 +61,13 @@ static inline void rseq_execve(struct task_struct *t)
 	t->rseq = NULL;
 	t->rseq_len = 0;
 	t->rseq_sig = 0;
-	t->rseq_event_mask = 0;
+	t->rseq_event_pending = false;
 }
 
 #else /* CONFIG_RSEQ */
-static inline void rseq_set_notify_resume(struct task_struct *t) { }
 static inline void rseq_handle_notify_resume(struct pt_regs *regs) { }
 static inline void rseq_signal_deliver(struct ksignal *ksig, struct pt_regs *regs) { }
-static inline void rseq_preempt(struct task_struct *t) { }
-static inline void rseq_migrate(struct task_struct *t) { }
+static inline void rseq_sched_switch_event(struct task_struct *t) { }
 static inline void rseq_fork(struct task_struct *t, u64 clone_flags) { }
 static inline void rseq_execve(struct task_struct *t) { }
 static inline void rseq_exit_to_user_mode(void) { }
diff --git a/include/linux/sched.h b/include/linux/sched.h
index b469878de25c..6627c527c2c7 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1407,14 +1407,14 @@ struct task_struct {
 #endif /* CONFIG_NUMA_BALANCING */
 
 #ifdef CONFIG_RSEQ
-	struct rseq __user *rseq;
-	u32 rseq_len;
-	u32 rseq_sig;
+	struct rseq __user		*rseq;
+	u32				rseq_len;
+	u32				rseq_sig;
 	/*
-	 * RmW on rseq_event_mask must be performed atomically
+	 * RmW on rseq_event_pending must be performed atomically
 	 * with respect to preemption.
 	 */
-	unsigned long rseq_event_mask;
+	bool				rseq_event_pending;
 # ifdef CONFIG_DEBUG_RSEQ
 	/*
 	 * This is a place holder to save a copy of the rseq fields for
diff --git a/include/uapi/linux/rseq.h b/include/uapi/linux/rseq.h
index c233aae5eac9..1b76d508400c 100644
--- a/include/uapi/linux/rseq.h
+++ b/include/uapi/linux/rseq.h
@@ -114,20 +114,13 @@ struct rseq {
 	/*
 	 * Restartable sequences flags field.
 	 *
-	 * This field should only be updated by the thread which
-	 * registered this data structure. Read by the kernel.
-	 * Mainly used for single-stepping through rseq critical sections
-	 * with debuggers.
-	 *
-	 * - RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT
-	 *     Inhibit instruction sequence block restart on preemption
-	 *     for this thread.
-	 * - RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL
-	 *     Inhibit instruction sequence block restart on signal
-	 *     delivery for this thread.
-	 * - RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE
-	 *     Inhibit instruction sequence block restart on migration for
-	 *     this thread.
+	 * This field was initially intended to allow event masking for
+	 * single-stepping through rseq critical sections with debuggers.
+	 * The kernel does not support this anymore and the relevant bits
+	 * are checked for being always false:
+	 *	- RSEQ_CS_FLAG_NO_RESTART_ON_PREEMPT
+	 *	- RSEQ_CS_FLAG_NO_RESTART_ON_SIGNAL
+	 *	- RSEQ_CS_FLAG_NO_RESTART_ON_MIGRATE
 	 */
 	__u32 flags;