From: Nicolai Hähnle Nicolai.Haehnle@amd.com

While adding our task as a waiter, detect if another task should back off because of us.

With this patch, we establish the invariant that the wait list contains at most one (sleeping) waiter with ww_ctx->acquired > 0, and this waiter will be the first waiter with a context.

Since only waiters with ww_ctx->acquired > 0 have to back off, this allows us to be much more economical with wakeups.

v2: rebase on v2 of earlier patches

Cc: Peter Zijlstra peterz@infradead.org Cc: Ingo Molnar mingo@redhat.com Cc: Maarten Lankhorst dev@mblankhorst.nl Cc: Daniel Vetter daniel@ffwll.ch Cc: Chris Wilson chris@chris-wilson.co.uk Cc: dri-devel@lists.freedesktop.org Signed-off-by: Nicolai Hähnle Nicolai.Haehnle@amd.com --- kernel/locking/mutex.c | 42 ++++++++++++++++++++++++++++++++---------- 1 file changed, 32 insertions(+), 10 deletions(-)

diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c index d63eebb..01e9438 100644 --- a/kernel/locking/mutex.c +++ b/kernel/locking/mutex.c @@ -609,23 +609,34 @@ void __sched ww_mutex_unlock(struct ww_mutex *lock) EXPORT_SYMBOL(ww_mutex_unlock);

static inline int __sched -__ww_mutex_lock_check_stamp(struct mutex *lock, struct ww_acquire_ctx *ctx) +__ww_mutex_lock_check_stamp(struct mutex *lock, struct mutex_waiter *waiter, + struct ww_acquire_ctx *ctx) { struct ww_mutex *ww = container_of(lock, struct ww_mutex, base); struct ww_acquire_ctx *hold_ctx = READ_ONCE(ww->ctx); + struct mutex_waiter *cur;

- if (!hold_ctx) - return 0; + if (hold_ctx && __ww_mutex_stamp_after(ctx, hold_ctx)) + goto deadlock;

- if (__ww_mutex_stamp_after(ctx, hold_ctx)) { -#ifdef CONFIG_DEBUG_MUTEXES - DEBUG_LOCKS_WARN_ON(ctx->contending_lock); - ctx->contending_lock = ww; -#endif - return -EDEADLK; + /* + * If there is a waiter in front of us that has a context, then its + * stamp is earlier than ours and we must back off. + */ + cur = waiter; + list_for_each_entry_continue_reverse(cur, &lock->wait_list, list) { + if (cur->ww_ctx) + goto deadlock; }

return 0; + +deadlock: +#ifdef CONFIG_DEBUG_MUTEXES + DEBUG_LOCKS_WARN_ON(ctx->contending_lock); + ctx->contending_lock = ww; +#endif + return -EDEADLK; }

static inline int __sched @@ -666,6 +677,16 @@ __ww_mutex_add_waiter(struct mutex_waiter *waiter, }

list_add_tail(&waiter->list, &cur->list); + + /* + * Wake up the waiter so that it gets a chance to back + * off. + */ + if (cur->ww_ctx->acquired > 0) { + debug_mutex_wake_waiter(lock, cur); + wake_up_process(cur->task); + } + return 0; }

@@ -767,7 +788,8 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass, }

if (use_ww_ctx && ww_ctx && ww_ctx->acquired > 0) { - ret = __ww_mutex_lock_check_stamp(lock, ww_ctx); + ret = __ww_mutex_lock_check_stamp(lock, &waiter, + ww_ctx); if (ret) goto err; }