Re: [PATCH 1/4] locking/ww_mutex: Simplify use_ww_ctx & ww_ctx handling

[Davidlohr Bueso]

On Tue, 16 Mar 2021, Waiman Long wrote:

The use_ww_ctx flag is passed to mutex_optimistic_spin(), but the
function doesn't use it. The frequent use of the (use_ww_ctx && ww_ctx)
combination is repetitive.

I always found that very fugly.

In fact, ww_ctx should not be used at all if !use_ww_ctx.  Simplify
ww_mutex code by dropping use_ww_ctx from mutex_optimistic_spin() an
clear ww_ctx if !use_ww_ctx. In this way, we can replace (use_ww_ctx &&
ww_ctx) by just (ww_ctx).

Signed-off-by: Waiman Long <long...@redhat.com>

Acked-by: Davidlohr Bueso <dbu...@suse.de>

---
kernel/locking/mutex.c | 25 ++++++++++++++-----------
1 file changed, 14 insertions(+), 11 deletions(-)

diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index adb935090768..622ebdfcd083 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -626,7 +626,7 @@ static inline int mutex_can_spin_on_owner(struct mutex 
*lock)
 */
static __always_inline bool
mutex_optimistic_spin(struct mutex *lock, struct ww_acquire_ctx *ww_ctx,
-                     const bool use_ww_ctx, struct mutex_waiter *waiter)
+                     struct mutex_waiter *waiter)
{
        if (!waiter) {
                /*
@@ -702,7 +702,7 @@ mutex_optimistic_spin(struct mutex *lock, struct 
ww_acquire_ctx *ww_ctx,
#else
static __always_inline bool
mutex_optimistic_spin(struct mutex *lock, struct ww_acquire_ctx *ww_ctx,
-                     const bool use_ww_ctx, struct mutex_waiter *waiter)
+                     struct mutex_waiter *waiter)
{
        return false;
}
@@ -922,6 +922,9 @@ __mutex_lock_common(struct mutex *lock, long state, 
unsigned int subclass,
        struct ww_mutex *ww;
        int ret;

+       if (!use_ww_ctx)
+               ww_ctx = NULL;
+
        might_sleep();

#ifdef CONFIG_DEBUG_MUTEXES
@@ -929,7 +932,7 @@ __mutex_lock_common(struct mutex *lock, long state, 
unsigned int subclass,
#endif

        ww = container_of(lock, struct ww_mutex, base);
-       if (use_ww_ctx && ww_ctx) {
+       if (ww_ctx) {
                if (unlikely(ww_ctx == READ_ONCE(ww->ctx)))
                        return -EALREADY;

@@ -946,10 +949,10 @@ __mutex_lock_common(struct mutex *lock, long state, 
unsigned int subclass,
        mutex_acquire_nest(&lock->dep_map, subclass, 0, nest_lock, ip);

        if (__mutex_trylock(lock) ||
-           mutex_optimistic_spin(lock, ww_ctx, use_ww_ctx, NULL)) {
+           mutex_optimistic_spin(lock, ww_ctx, NULL)) {
                /* got the lock, yay! */
                lock_acquired(&lock->dep_map, ip);
-               if (use_ww_ctx && ww_ctx)
+               if (ww_ctx)
                        ww_mutex_set_context_fastpath(ww, ww_ctx);
                preempt_enable();
                return 0;
@@ -960,7 +963,7 @@ __mutex_lock_common(struct mutex *lock, long state, 
unsigned int subclass,
         * After waiting to acquire the wait_lock, try again.
         */
        if (__mutex_trylock(lock)) {
-               if (use_ww_ctx && ww_ctx)
+               if (ww_ctx)
                        __ww_mutex_check_waiters(lock, ww_ctx);

                goto skip_wait;
@@ -1013,7 +1016,7 @@ __mutex_lock_common(struct mutex *lock, long state, 
unsigned int subclass,
                        goto err;
                }

-               if (use_ww_ctx && ww_ctx) {
+               if (ww_ctx) {
                        ret = __ww_mutex_check_kill(lock, &waiter, ww_ctx);
                        if (ret)
                                goto err;
@@ -1026,7 +1029,7 @@ __mutex_lock_common(struct mutex *lock, long state, 
unsigned int subclass,
                 * ww_mutex needs to always recheck its position since its 
waiter
                 * list is not FIFO ordered.
                 */
-               if ((use_ww_ctx && ww_ctx) || !first) {
+               if (ww_ctx || !first) {
                        first = __mutex_waiter_is_first(lock, &waiter);
                        if (first)
                                __mutex_set_flag(lock, MUTEX_FLAG_HANDOFF);
@@ -1039,7 +1042,7 @@ __mutex_lock_common(struct mutex *lock, long state, 
unsigned int subclass,
                 * or we must see its unlock and acquire.
                 */
                if (__mutex_trylock(lock) ||
-                   (first && mutex_optimistic_spin(lock, ww_ctx, use_ww_ctx, 
&waiter)))
+                   (first && mutex_optimistic_spin(lock, ww_ctx, &waiter)))
                        break;

                spin_lock(&lock->wait_lock);
@@ -1048,7 +1051,7 @@ __mutex_lock_common(struct mutex *lock, long state, 
unsigned int subclass,
acquired:
        __set_current_state(TASK_RUNNING);

-       if (use_ww_ctx && ww_ctx) {
+       if (ww_ctx) {
                /*
                 * Wound-Wait; we stole the lock (!first_waiter), check the
                 * waiters as anyone might want to wound us.
@@ -1068,7 +1071,7 @@ __mutex_lock_common(struct mutex *lock, long state, 
unsigned int subclass,
        /* got the lock - cleanup and rejoice! */
        lock_acquired(&lock->dep_map, ip);

-       if (use_ww_ctx && ww_ctx)
+       if (ww_ctx)
                ww_mutex_lock_acquired(ww, ww_ctx);

        spin_unlock(&lock->wait_lock);
--
2.18.1