[PATCH v2 12/18] perf rwsem: Add down_read_trylock() and down_write_trylock()

[Ian Rogers]

On-demand memory shrinking (perf_session__shrink(), dsos__shrink(),
symbols__shrink()) can be triggered from OOM recovery inside allocation
wrappers while the current thread already holds a rw_semaphore (such as
symbols->lock, dsos->lock, or machines->lock). Non-blocking trylock
primitives are needed so shrinking paths can safely skip any locked
container without deadlocking.

Add non-blocking down_read_trylock() and down_write_trylock() wrappers
around pthread_rwlock_tryrdlock() / pthread_rwlock_trywrlock() and
pthread_mutex_trylock() along with SHARED_TRYLOCK_FUNCTION and
EXCLUSIVE_TRYLOCK_FUNCTION thread-safety annotations.

Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
---
 tools/perf/util/mutex.h |  3 +++
 tools/perf/util/rwsem.c | 20 ++++++++++++++++++++
 tools/perf/util/rwsem.h |  2 ++
 3 files changed, 25 insertions(+)

diff --git a/tools/perf/util/mutex.h b/tools/perf/util/mutex.h
index 5de1065ea2b9..de7dd6d8326c 100644
--- a/tools/perf/util/mutex.h
+++ b/tools/perf/util/mutex.h
@@ -57,6 +57,8 @@
 /* Documents functions that try to acquire a lock, and return success or failure. */
 #define EXCLUSIVE_TRYLOCK_FUNCTION(...) \
 	__attribute__((exclusive_trylock_function(__VA_ARGS__)))
+#define SHARED_TRYLOCK_FUNCTION(...) \
+	__attribute__((shared_trylock_function(__VA_ARGS__)))
 
 /* Documents a function that expects a mutex to be held prior to entry. */
 #define EXCLUSIVE_LOCKS_REQUIRED(...) __attribute__((exclusive_locks_required(__VA_ARGS__)))
@@ -78,6 +80,7 @@
 #define SHARED_LOCK_FUNCTION(...)
 #define UNLOCK_FUNCTION(...)
 #define EXCLUSIVE_TRYLOCK_FUNCTION(...)
+#define SHARED_TRYLOCK_FUNCTION(...)
 #define EXCLUSIVE_LOCKS_REQUIRED(...)
 #define SHARED_LOCKS_REQUIRED(...)
 #define NO_THREAD_SAFETY_ANALYSIS
diff --git a/tools/perf/util/rwsem.c b/tools/perf/util/rwsem.c
index 9d26832398db..37fa08bae5e9 100644
--- a/tools/perf/util/rwsem.c
+++ b/tools/perf/util/rwsem.c
@@ -37,6 +37,16 @@ int down_read(struct rw_semaphore *sem)
 #endif
 }
 
+bool down_read_trylock(struct rw_semaphore *sem)
+	NO_THREAD_SAFETY_ANALYSIS
+{
+#if RWS_ERRORCHECK
+	return mutex_trylock(&sem->mtx);
+#else
+	return perf_singlethreaded ? true : pthread_rwlock_tryrdlock(&sem->lock) == 0;
+#endif
+}
+
 int up_read(struct rw_semaphore *sem)
 	NO_THREAD_SAFETY_ANALYSIS
 {
@@ -59,6 +69,16 @@ int down_write(struct rw_semaphore *sem)
 #endif
 }
 
+bool down_write_trylock(struct rw_semaphore *sem)
+	NO_THREAD_SAFETY_ANALYSIS
+{
+#if RWS_ERRORCHECK
+	return mutex_trylock(&sem->mtx);
+#else
+	return perf_singlethreaded ? true : pthread_rwlock_trywrlock(&sem->lock) == 0;
+#endif
+}
+
 int up_write(struct rw_semaphore *sem)
 	NO_THREAD_SAFETY_ANALYSIS
 {
diff --git a/tools/perf/util/rwsem.h b/tools/perf/util/rwsem.h
index b102d8143181..782016c3e01f 100644
--- a/tools/perf/util/rwsem.h
+++ b/tools/perf/util/rwsem.h
@@ -22,9 +22,11 @@ int init_rwsem(struct rw_semaphore *sem);
 int exit_rwsem(struct rw_semaphore *sem);
 
 int down_read(struct rw_semaphore *sem) SHARED_LOCK_FUNCTION(sem);
+bool down_read_trylock(struct rw_semaphore *sem) SHARED_TRYLOCK_FUNCTION(true, sem);
 int up_read(struct rw_semaphore *sem) UNLOCK_FUNCTION(sem);
 
 int down_write(struct rw_semaphore *sem) EXCLUSIVE_LOCK_FUNCTION(sem);
+bool down_write_trylock(struct rw_semaphore *sem) EXCLUSIVE_TRYLOCK_FUNCTION(true, sem);
 int up_write(struct rw_semaphore *sem) UNLOCK_FUNCTION(sem);
 
 #endif /* _PERF_RWSEM_H */
-- 
2.56.0.313.g2479322e68-goog