locking/seqlock: Support Clang's context analysis

Add support for Clang's context analysis for seqlock_t.

Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20251219154418.3592607-12-elver@google.com

Documentation/dev-tools/context-analysis.rst

@@ -79,7 +79,7 @@ Supported Kernel Primitives
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Currently the following synchronization primitives are supported:
-`raw_spinlock_t`, `spinlock_t`, `rwlock_t`, `mutex`.
+`raw_spinlock_t`, `spinlock_t`, `rwlock_t`, `mutex`, `seqlock_t`.
 
 For context locks with an initialization function (e.g., `spin_lock_init()`),
 calling this function before initializing any guarded members or globals

include/linux/seqlock.h

@@ -816,6 +816,7 @@ static __always_inline void write_seqcount_latch_end(seqcount_latch_t *s)
 	do {								\
 		spin_lock_init(&(sl)->lock);				\
 		seqcount_spinlock_init(&(sl)->seqcount, &(sl)->lock);	\
+		__assume_ctx_lock(sl);					\
 	} while (0)
 
 /**
@@ -832,6 +833,7 @@ static __always_inline void write_seqcount_latch_end(seqcount_latch_t *s)
  * Return: count, to be passed to read_seqretry()
  */
 static inline unsigned read_seqbegin(const seqlock_t *sl)
+	__acquires_shared(sl) __no_context_analysis
 {
 	return read_seqcount_begin(&sl->seqcount);
 }
@@ -848,6 +850,7 @@ static inline unsigned read_seqbegin(const seqlock_t *sl)
  * Return: true if a read section retry is required, else false
  */
 static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
+	__releases_shared(sl) __no_context_analysis
 {
 	return read_seqcount_retry(&sl->seqcount, start);
 }
@@ -872,6 +875,7 @@ static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
  * _irqsave or _bh variants of this function instead.
  */
 static inline void write_seqlock(seqlock_t *sl)
+	__acquires(sl) __no_context_analysis
 {
 	spin_lock(&sl->lock);
 	do_write_seqcount_begin(&sl->seqcount.seqcount);
@@ -885,6 +889,7 @@ static inline void write_seqlock(seqlock_t *sl)
  * critical section of given seqlock_t.
  */
 static inline void write_sequnlock(seqlock_t *sl)
+	__releases(sl) __no_context_analysis
 {
 	do_write_seqcount_end(&sl->seqcount.seqcount);
 	spin_unlock(&sl->lock);
@@ -898,6 +903,7 @@ static inline void write_sequnlock(seqlock_t *sl)
  * other write side sections, can be invoked from softirq contexts.
  */
 static inline void write_seqlock_bh(seqlock_t *sl)
+	__acquires(sl) __no_context_analysis
 {
 	spin_lock_bh(&sl->lock);
 	do_write_seqcount_begin(&sl->seqcount.seqcount);
@@ -912,6 +918,7 @@ static inline void write_seqlock_bh(seqlock_t *sl)
  * write_seqlock_bh().
  */
 static inline void write_sequnlock_bh(seqlock_t *sl)
+	__releases(sl) __no_context_analysis
 {
 	do_write_seqcount_end(&sl->seqcount.seqcount);
 	spin_unlock_bh(&sl->lock);
@@ -925,6 +932,7 @@ static inline void write_sequnlock_bh(seqlock_t *sl)
  * other write sections, can be invoked from hardirq contexts.
  */
 static inline void write_seqlock_irq(seqlock_t *sl)
+	__acquires(sl) __no_context_analysis
 {
 	spin_lock_irq(&sl->lock);
 	do_write_seqcount_begin(&sl->seqcount.seqcount);
@@ -938,12 +946,14 @@ static inline void write_seqlock_irq(seqlock_t *sl)
  * seqlock_t write side section opened with write_seqlock_irq().
  */
 static inline void write_sequnlock_irq(seqlock_t *sl)
+	__releases(sl) __no_context_analysis
 {
 	do_write_seqcount_end(&sl->seqcount.seqcount);
 	spin_unlock_irq(&sl->lock);
 }
 
 static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
+	__acquires(sl) __no_context_analysis
 {
 	unsigned long flags;
 
@@ -976,6 +986,7 @@ static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
  */
 static inline void
 write_sequnlock_irqrestore(seqlock_t *sl, unsigned long flags)
+	__releases(sl) __no_context_analysis
 {
 	do_write_seqcount_end(&sl->seqcount.seqcount);
 	spin_unlock_irqrestore(&sl->lock, flags);
@@ -998,6 +1009,7 @@ write_sequnlock_irqrestore(seqlock_t *sl, unsigned long flags)
  * The opened read section must be closed with read_sequnlock_excl().
  */
 static inline void read_seqlock_excl(seqlock_t *sl)
+	__acquires_shared(sl) __no_context_analysis
 {
 	spin_lock(&sl->lock);
 }
@@ -1007,6 +1019,7 @@ static inline void read_seqlock_excl(seqlock_t *sl)
  * @sl: Pointer to seqlock_t
  */
 static inline void read_sequnlock_excl(seqlock_t *sl)
+	__releases_shared(sl) __no_context_analysis
 {
 	spin_unlock(&sl->lock);
 }
@@ -1021,6 +1034,7 @@ static inline void read_sequnlock_excl(seqlock_t *sl)
  * from softirq contexts.
  */
 static inline void read_seqlock_excl_bh(seqlock_t *sl)
+	__acquires_shared(sl) __no_context_analysis
 {
 	spin_lock_bh(&sl->lock);
 }
@@ -1031,6 +1045,7 @@ static inline void read_seqlock_excl_bh(seqlock_t *sl)
  * @sl: Pointer to seqlock_t
  */
 static inline void read_sequnlock_excl_bh(seqlock_t *sl)
+	__releases_shared(sl) __no_context_analysis
 {
 	spin_unlock_bh(&sl->lock);
 }
@@ -1045,6 +1060,7 @@ static inline void read_sequnlock_excl_bh(seqlock_t *sl)
  * hardirq context.
  */
 static inline void read_seqlock_excl_irq(seqlock_t *sl)
+	__acquires_shared(sl) __no_context_analysis
 {
 	spin_lock_irq(&sl->lock);
 }
@@ -1055,11 +1071,13 @@ static inline void read_seqlock_excl_irq(seqlock_t *sl)
  * @sl: Pointer to seqlock_t
  */
 static inline void read_sequnlock_excl_irq(seqlock_t *sl)
+	__releases_shared(sl) __no_context_analysis
 {
 	spin_unlock_irq(&sl->lock);
 }
 
 static inline unsigned long __read_seqlock_excl_irqsave(seqlock_t *sl)
+	__acquires_shared(sl) __no_context_analysis
 {
 	unsigned long flags;
 
@@ -1089,6 +1107,7 @@ static inline unsigned long __read_seqlock_excl_irqsave(seqlock_t *sl)
  */
 static inline void
 read_sequnlock_excl_irqrestore(seqlock_t *sl, unsigned long flags)
+	__releases_shared(sl) __no_context_analysis
 {
 	spin_unlock_irqrestore(&sl->lock, flags);
 }
@@ -1125,6 +1144,7 @@ read_sequnlock_excl_irqrestore(seqlock_t *sl, unsigned long flags)
  * parameter of the next read_seqbegin_or_lock() iteration.
  */
 static inline void read_seqbegin_or_lock(seqlock_t *lock, int *seq)
+	__acquires_shared(lock) __no_context_analysis
 {
 	if (!(*seq & 1))	/* Even */
 		*seq = read_seqbegin(lock);
@@ -1140,6 +1160,7 @@ static inline void read_seqbegin_or_lock(seqlock_t *lock, int *seq)
  * Return: true if a read section retry is required, false otherwise
  */
 static inline int need_seqretry(seqlock_t *lock, int seq)
+	__releases_shared(lock) __no_context_analysis
 {
 	return !(seq & 1) && read_seqretry(lock, seq);
 }
@@ -1153,6 +1174,7 @@ static inline int need_seqretry(seqlock_t *lock, int seq)
  * with read_seqbegin_or_lock() and validated by need_seqretry().
  */
 static inline void done_seqretry(seqlock_t *lock, int seq)
+	__no_context_analysis
 {
 	if (seq & 1)
 		read_sequnlock_excl(lock);
@@ -1180,6 +1202,7 @@ static inline void done_seqretry(seqlock_t *lock, int seq)
  */
 static inline unsigned long
 read_seqbegin_or_lock_irqsave(seqlock_t *lock, int *seq)
+	__acquires_shared(lock) __no_context_analysis
 {
 	unsigned long flags = 0;
 
@@ -1205,6 +1228,7 @@ read_seqbegin_or_lock_irqsave(seqlock_t *lock, int *seq)
  */
 static inline void
 done_seqretry_irqrestore(seqlock_t *lock, int seq, unsigned long flags)
+	__no_context_analysis
 {
 	if (seq & 1)
 		read_sequnlock_excl_irqrestore(lock, flags);
@@ -1225,6 +1249,7 @@ struct ss_tmp {
 };
 
 static __always_inline void __scoped_seqlock_cleanup(struct ss_tmp *sst)
+	__no_context_analysis
 {
 	if (sst->lock)
 		spin_unlock(sst->lock);
@@ -1254,6 +1279,7 @@ extern void __scoped_seqlock_bug(void);
 
 static __always_inline void
 __scoped_seqlock_next(struct ss_tmp *sst, seqlock_t *lock, enum ss_state target)
+	__no_context_analysis
 {
 	switch (sst->state) {
 	case ss_done:
@@ -1296,9 +1322,19 @@ __scoped_seqlock_next(struct ss_tmp *sst, seqlock_t *lock, enum ss_state target)
 	}
 }
 
+/*
+ * Context analysis no-op helper to release seqlock at the end of the for-scope;
+ * the alias analysis of the compiler will recognize that the pointer @s is an
+ * alias to @_seqlock passed to read_seqbegin(_seqlock) below.
+ */
+static __always_inline void __scoped_seqlock_cleanup_ctx(struct ss_tmp **s)
+	__releases_shared(*((seqlock_t **)s)) __no_context_analysis {}
+
 #define __scoped_seqlock_read(_seqlock, _target, _s)			\
 	for (struct ss_tmp _s __cleanup(__scoped_seqlock_cleanup) =	\
-	     { .state = ss_lockless, .data = read_seqbegin(_seqlock) };	\
+	     { .state = ss_lockless, .data = read_seqbegin(_seqlock) }, \
+	     *__UNIQUE_ID(ctx) __cleanup(__scoped_seqlock_cleanup_ctx) =\
+		(struct ss_tmp *)_seqlock;				\
 	     _s.state != ss_done;					\
 	     __scoped_seqlock_next(&_s, _seqlock, _target))
 

include/linux/seqlock_types.h

@@ -81,13 +81,14 @@ SEQCOUNT_LOCKNAME(mutex,        struct mutex,    true,     mutex)
  *    - Comments on top of seqcount_t
  *    - Documentation/locking/seqlock.rst
  */
-typedef struct {
+context_lock_struct(seqlock) {
 	/*
 	 * Make sure that readers don't starve writers on PREEMPT_RT: use
 	 * seqcount_spinlock_t instead of seqcount_t. Check __SEQ_LOCK().
 	 */
 	seqcount_spinlock_t seqcount;
 	spinlock_t lock;
-} seqlock_t;
+};
+typedef struct seqlock seqlock_t;
 
 #endif /* __LINUX_SEQLOCK_TYPES_H */

lib/test_context-analysis.c

@@ -6,6 +6,7 @@
 
 #include <linux/build_bug.h>
 #include <linux/mutex.h>
+#include <linux/seqlock.h>
 #include <linux/spinlock.h>
 
 /*
@@ -208,3 +209,52 @@ static void __used test_mutex_cond_guard(struct test_mutex_data *d)
 		d->counter++;
 	}
 }
+
+struct test_seqlock_data {
+	seqlock_t sl;
+	int counter __guarded_by(&sl);
+};
+
+static void __used test_seqlock_init(struct test_seqlock_data *d)
+{
+	seqlock_init(&d->sl);
+	d->counter = 0;
+}
+
+static void __used test_seqlock_reader(struct test_seqlock_data *d)
+{
+	unsigned int seq;
+
+	do {
+		seq = read_seqbegin(&d->sl);
+		(void)d->counter;
+	} while (read_seqretry(&d->sl, seq));
+}
+
+static void __used test_seqlock_writer(struct test_seqlock_data *d)
+{
+	unsigned long flags;
+
+	write_seqlock(&d->sl);
+	d->counter++;
+	write_sequnlock(&d->sl);
+
+	write_seqlock_irq(&d->sl);
+	d->counter++;
+	write_sequnlock_irq(&d->sl);
+
+	write_seqlock_bh(&d->sl);
+	d->counter++;
+	write_sequnlock_bh(&d->sl);
+
+	write_seqlock_irqsave(&d->sl, flags);
+	d->counter++;
+	write_sequnlock_irqrestore(&d->sl, flags);
+}
+
+static void __used test_seqlock_scoped(struct test_seqlock_data *d)
+{
+	scoped_seqlock_read (&d->sl, ss_lockless) {
+		(void)d->counter;
+	}
+}