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-rw-r--r--include/linux/seqlock.h51
1 files changed, 47 insertions, 4 deletions
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 0491d963d47e..8b97204f35a7 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -37,9 +37,25 @@
#include <linux/preempt.h>
#include <linux/lockdep.h>
#include <linux/compiler.h>
+#include <linux/kcsan-checks.h>
#include <asm/processor.h>
/*
+ * The seqlock interface does not prescribe a precise sequence of read
+ * begin/retry/end. For readers, typically there is a call to
+ * read_seqcount_begin() and read_seqcount_retry(), however, there are more
+ * esoteric cases which do not follow this pattern.
+ *
+ * As a consequence, we take the following best-effort approach for raw usage
+ * via seqcount_t under KCSAN: upon beginning a seq-reader critical section,
+ * pessimistically mark the next KCSAN_SEQLOCK_REGION_MAX memory accesses as
+ * atomics; if there is a matching read_seqcount_retry() call, no following
+ * memory operations are considered atomic. Usage of seqlocks via seqlock_t
+ * interface is not affected.
+ */
+#define KCSAN_SEQLOCK_REGION_MAX 1000
+
+/*
* Version using sequence counter only.
* This can be used when code has its own mutex protecting the
* updating starting before the write_seqcountbeqin() and ending
@@ -115,6 +131,7 @@ repeat:
cpu_relax();
goto repeat;
}
+ kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);
return ret;
}
@@ -131,6 +148,7 @@ static inline unsigned raw_read_seqcount(const seqcount_t *s)
{
unsigned ret = READ_ONCE(s->sequence);
smp_rmb();
+ kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);
return ret;
}
@@ -183,6 +201,7 @@ static inline unsigned raw_seqcount_begin(const seqcount_t *s)
{
unsigned ret = READ_ONCE(s->sequence);
smp_rmb();
+ kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);
return ret & ~1;
}
@@ -202,7 +221,8 @@ static inline unsigned raw_seqcount_begin(const seqcount_t *s)
*/
static inline int __read_seqcount_retry(const seqcount_t *s, unsigned start)
{
- return unlikely(s->sequence != start);
+ kcsan_atomic_next(0);
+ return unlikely(READ_ONCE(s->sequence) != start);
}
/**
@@ -225,6 +245,7 @@ static inline int read_seqcount_retry(const seqcount_t *s, unsigned start)
static inline void raw_write_seqcount_begin(seqcount_t *s)
{
+ kcsan_nestable_atomic_begin();
s->sequence++;
smp_wmb();
}
@@ -233,6 +254,7 @@ static inline void raw_write_seqcount_end(seqcount_t *s)
{
smp_wmb();
s->sequence++;
+ kcsan_nestable_atomic_end();
}
/**
@@ -243,6 +265,13 @@ static inline void raw_write_seqcount_end(seqcount_t *s)
* usual consistency guarantee. It is one wmb cheaper, because we can
* collapse the two back-to-back wmb()s.
*
+ * Note that writes surrounding the barrier should be declared atomic (e.g.
+ * via WRITE_ONCE): a) to ensure the writes become visible to other threads
+ * atomically, avoiding compiler optimizations; b) to document which writes are
+ * meant to propagate to the reader critical section. This is necessary because
+ * neither writes before and after the barrier are enclosed in a seq-writer
+ * critical section that would ensure readers are aware of ongoing writes.
+ *
* seqcount_t seq;
* bool X = true, Y = false;
*
@@ -262,18 +291,20 @@ static inline void raw_write_seqcount_end(seqcount_t *s)
*
* void write(void)
* {
- * Y = true;
+ * WRITE_ONCE(Y, true);
*
* raw_write_seqcount_barrier(seq);
*
- * X = false;
+ * WRITE_ONCE(X, false);
* }
*/
static inline void raw_write_seqcount_barrier(seqcount_t *s)
{
+ kcsan_nestable_atomic_begin();
s->sequence++;
smp_wmb();
s->sequence++;
+ kcsan_nestable_atomic_end();
}
static inline int raw_read_seqcount_latch(seqcount_t *s)
@@ -398,7 +429,9 @@ static inline void write_seqcount_end(seqcount_t *s)
static inline void write_seqcount_invalidate(seqcount_t *s)
{
smp_wmb();
+ kcsan_nestable_atomic_begin();
s->sequence+=2;
+ kcsan_nestable_atomic_end();
}
typedef struct {
@@ -430,11 +463,21 @@ typedef struct {
*/
static inline unsigned read_seqbegin(const seqlock_t *sl)
{
- return read_seqcount_begin(&sl->seqcount);
+ unsigned ret = read_seqcount_begin(&sl->seqcount);
+
+ kcsan_atomic_next(0); /* non-raw usage, assume closing read_seqretry() */
+ kcsan_flat_atomic_begin();
+ return ret;
}
static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
{
+ /*
+ * Assume not nested: read_seqretry() may be called multiple times when
+ * completing read critical section.
+ */
+ kcsan_flat_atomic_end();
+
return read_seqcount_retry(&sl->seqcount, start);
}

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