percpu alloc: introduce rseq_percpu_pool_ptr_offset

Introduce rseq_percpu_pool_ptr_offset to pre-decode the offset from
all __rseq_percpu pointers for a given pool. This is useful to prepare
offsets that would need to be calculated within rseq critical sections.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I42d2669ed5ed81d74aae551b721c9cb04b25fe45

2 files changed, 18 insertions, 0 deletions

diff --git a/include/rseq/percpu-alloc.h b/include/rseq/percpu-alloc.h
index 0761acc..0bd7a91 100644
--- a/include/rseq/percpu-alloc.h
+++ b/include/rseq/percpu-alloc.h
@@ -169,6 +169,16 @@ void *__rseq_percpu_ptr(void __rseq_percpu *ptr, int cpu);
 #define rseq_percpu_ptr(ptr, cpu)	((__typeof__(*(ptr)) *) __rseq_percpu_ptr(ptr, cpu))
 
 /*
+ * rseq_percpu_pool_cpu_offset: Return the offset from encoded to decoded percpu pointer.
+ *
+ * Calculate the offset from any __rseq_percpu pointer allocated from
+ * the pool to its associated per-cpu data for @cpu.
+ *
+ * This API is MT-safe.
+ */
+ptrdiff_t rseq_percpu_pool_ptr_offset(struct rseq_percpu_pool *pool, int cpu);
+
+/*
  * rseq_percpu_pool_set_create: Create a pool set.
  *
  * Create a set of pools. Its purpose is to offer a memory allocator API
diff --git a/src/rseq-percpu-alloc.c b/src/rseq-percpu-alloc.c
index 7d6498f..781a22d 100644
--- a/src/rseq-percpu-alloc.c
+++ b/src/rseq-percpu-alloc.c
@@ -116,6 +116,14 @@ void *__rseq_pool_percpu_ptr(struct rseq_percpu_pool *pool, int cpu, uintptr_t i
 	return pool->base + (pool->percpu_len * cpu) + item_offset;
 }
 
+ptrdiff_t rseq_percpu_pool_ptr_offset(struct rseq_percpu_pool *pool, int cpu)
+{
+	uintptr_t rseq_percpu_base = (uintptr_t) pool->index << POOL_INDEX_SHIFT;
+	uintptr_t refptr = (uintptr_t) __rseq_pool_percpu_ptr(pool, cpu, 0);
+
+	return (ptrdiff_t) (refptr - rseq_percpu_base);
+}
+
 void *__rseq_percpu_ptr(void __rseq_percpu *_ptr, int cpu)
 {
 	uintptr_t ptr = (uintptr_t) _ptr;