Re: [RFC][PATCH 0/4] locking: Add/convert context analysis bits

[Marco Elver <elver@google.com>]

On Wed, Jan 21, 2026 at 12:07PM +0100, Peter Zijlstra wrote:
> Hai
> 
> This is on top of tip/locking/core with these patches on:
> 
>   https://lkml.kernel.org/r/20260119094029.1344361-1-elver@google.com
> 
> and converts mutex, rtmutex, ww_mutex and futex to use the new context analysis
> bits.
> 
> There is one snafu:
> 
> ww_mutex_set_context_fastpath()'s data_race() usage doesn't stop the compiler
> from complaining when building a defconfig+PREEMPT_RT+LOCKDEP build:
> 
> ../kernel/locking/ww_mutex.h:439:24: error: calling function '__ww_mutex_has_waiters' requires holding raw_spinlock 'lock->base.rtmutex.wait_lock' exclusively [-Werror,-Wthread-safety-analysis]
>   439 |         if (likely(!data_race(__ww_mutex_has_waiters(&lock->base))))
>       |                               ^
> 1 error generated.

This works:

diff --git a/kernel/locking/ww_mutex.h b/kernel/locking/ww_mutex.h
index 73eed6b7f24e..561e2475954d 100644
--- a/kernel/locking/ww_mutex.h
+++ b/kernel/locking/ww_mutex.h
@@ -436,7 +436,8 @@ ww_mutex_set_context_fastpath(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
 	 * __ww_mutex_add_waiter() and makes sure we either observe ww->ctx
 	 * and/or !empty list.
 	 */
-	if (likely(!data_race(__ww_mutex_has_waiters(&lock->base))))
+	bool has_waiters = data_race(__ww_mutex_has_waiters(&lock->base));
+	if (likely(!has_waiters))
 		return;
 
 	/*


It appears that the _Pragma are ignored when the expression is inside
__builtin_expect(...). That's a bit inconvenient.

Another option is this given its exclusively used without holding this
lock:

diff --git a/kernel/locking/ww_mutex.h b/kernel/locking/ww_mutex.h
index 73eed6b7f24e..45a9c394fe91 100644
--- a/kernel/locking/ww_mutex.h
+++ b/kernel/locking/ww_mutex.h
@@ -71,7 +71,7 @@ __ww_mutex_owner(struct mutex *lock)
 }
 
 static inline bool
-__ww_mutex_has_waiters(struct mutex *lock)
+__ww_mutex_has_waiters_lockless(struct mutex *lock)
 {
 	return atomic_long_read(&lock->owner) & MUTEX_FLAG_WAITERS;
 }
@@ -151,10 +151,9 @@ __ww_mutex_owner(struct rt_mutex *lock)
 }
 
 static inline bool
-__ww_mutex_has_waiters(struct rt_mutex *lock)
-	__must_hold(&lock->rtmutex.wait_lock)
+__ww_mutex_has_waiters_lockless(struct rt_mutex *lock)
 {
-	return rt_mutex_has_waiters(&lock->rtmutex);
+	return data_race(rt_mutex_has_waiters(&lock->rtmutex));
 }
 
 static inline void lock_wait_lock(struct rt_mutex *lock, unsigned long *flags)
@@ -436,7 +435,7 @@ ww_mutex_set_context_fastpath(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
 	 * __ww_mutex_add_waiter() and makes sure we either observe ww->ctx
 	 * and/or !empty list.
 	 */
-	if (likely(!data_race(__ww_mutex_has_waiters(&lock->base))))
+	if (likely(!__ww_mutex_has_waiters_lockless(&lock->base)))
 		return;
 
 	/*