From 9a4b99fce659c03699f1cb5003ebe7c57c084d49 Mon Sep 17 00:00:00 2001 From: Davidlohr Bueso Date: Fri, 26 Feb 2021 09:50:26 -0800 Subject: kernel/futex: Kill rt_mutex_next_owner() Update wake_futex_pi() and kill the call altogether. This is possible because: (i) The case of fixup_owner() in which the pi_mutex was stolen from the signaled enqueued top-waiter which fails to trylock and doesn't see a current owner of the rtmutex but needs to acknowledge an non-enqueued higher priority waiter, which is the other alternative. This used to be handled by rt_mutex_next_owner(), which guaranteed fixup_pi_state_owner('newowner') never to be nil. Nowadays the logic is handled by an EAGAIN loop, without the need of rt_mutex_next_owner(). Specifically: c1e2f0eaf015 (futex: Avoid violating the 10th rule of futex) 9f5d1c336a10 (futex: Handle transient "ownerless" rtmutex state correctly) (ii) rt_mutex_next_owner() and rt_mutex_top_waiter() are semantically equivalent, as of: c28d62cf52d7 (locking/rtmutex: Handle non enqueued waiters gracefully in remove_waiter()) So instead of keeping the call around, just use the good ole rt_mutex_top_waiter(). No change in semantics. Signed-off-by: Davidlohr Bueso Signed-off-by: Thomas Gleixner Acked-by: Peter Zijlstra (Intel) Link: https://lore.kernel.org/r/20210226175029.50335-1-dave@stgolabs.net --- kernel/futex.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-) (limited to 'kernel/futex.c') diff --git a/kernel/futex.c b/kernel/futex.c index e68db7745039..db8002dbca7a 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -1494,13 +1494,14 @@ static void mark_wake_futex(struct wake_q_head *wake_q, struct futex_q *q) static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_pi_state *pi_state) { u32 curval, newval; + struct rt_mutex_waiter *top_waiter; struct task_struct *new_owner; bool postunlock = false; DEFINE_WAKE_Q(wake_q); int ret = 0; - new_owner = rt_mutex_next_owner(&pi_state->pi_mutex); - if (WARN_ON_ONCE(!new_owner)) { + top_waiter = rt_mutex_top_waiter(&pi_state->pi_mutex); + if (WARN_ON_ONCE(!top_waiter)) { /* * As per the comment in futex_unlock_pi() this should not happen. * @@ -1513,6 +1514,8 @@ static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_pi_state *pi_ goto out_unlock; } + new_owner = top_waiter->task; + /* * We pass it to the next owner. The WAITERS bit is always kept * enabled while there is PI state around. We cleanup the owner -- cgit From a1565aa4699847febfdfd6af3bf06ca17a9e16af Mon Sep 17 00:00:00 2001 From: Davidlohr Bueso Date: Fri, 26 Feb 2021 09:50:27 -0800 Subject: kernel/futex: Make futex_wait_requeue_pi() only call fixup_owner() A small cleanup that allows for fixup_pi_state_owner() only to be called from fixup_owner(), and make requeue_pi uniformly call fixup_owner() regardless of the state in which the fixup is actually needed. Of course this makes the caller's first pi_state->owner != current check redundant, but that should't really matter. Signed-off-by: Davidlohr Bueso Signed-off-by: Thomas Gleixner Acked-by: Peter Zijlstra (Intel) Link: https://lore.kernel.org/r/20210226175029.50335-2-dave@stgolabs.net --- kernel/futex.c | 11 +++++------ 1 file changed, 5 insertions(+), 6 deletions(-) (limited to 'kernel/futex.c') diff --git a/kernel/futex.c b/kernel/futex.c index db8002dbca7a..ee09995d707b 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -3241,15 +3241,14 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags, * reference count. */ - /* Check if the requeue code acquired the second futex for us. */ + /* + * Check if the requeue code acquired the second futex for us and do + * any pertinent fixup. + */ if (!q.rt_waiter) { - /* - * Got the lock. We might not be the anticipated owner if we - * did a lock-steal - fix up the PI-state in that case. - */ if (q.pi_state && (q.pi_state->owner != current)) { spin_lock(q.lock_ptr); - ret = fixup_pi_state_owner(uaddr2, &q, current); + ret = fixup_owner(uaddr2, &q, true); /* * Drop the reference to the pi state which * the requeue_pi() code acquired for us. -- cgit From a3f2428d2b9c9ca70f52818774a2f6e0e30a0f0b Mon Sep 17 00:00:00 2001 From: Davidlohr Bueso Date: Fri, 26 Feb 2021 09:50:28 -0800 Subject: kernel/futex: Move hb unlock out of unqueue_me_pi() This improves the code readability, and the locking more obvious as it becomes symmetric for the caller. Signed-off-by: Davidlohr Bueso Signed-off-by: Thomas Gleixner Acked-by: Peter Zijlstra (Intel) Link: https://lore.kernel.org/r/20210226175029.50335-3-dave@stgolabs.net --- kernel/futex.c | 10 +++------- 1 file changed, 3 insertions(+), 7 deletions(-) (limited to 'kernel/futex.c') diff --git a/kernel/futex.c b/kernel/futex.c index ee09995d707b..dcd6132485e1 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -2318,19 +2318,15 @@ retry: /* * PI futexes can not be requeued and must remove themself from the - * hash bucket. The hash bucket lock (i.e. lock_ptr) is held on entry - * and dropped here. + * hash bucket. The hash bucket lock (i.e. lock_ptr) is held. */ static void unqueue_me_pi(struct futex_q *q) - __releases(q->lock_ptr) { __unqueue_futex(q); BUG_ON(!q->pi_state); put_pi_state(q->pi_state); q->pi_state = NULL; - - spin_unlock(q->lock_ptr); } static int __fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q, @@ -2913,8 +2909,8 @@ no_block: if (res) ret = (res < 0) ? res : 0; - /* Unqueue and drop the lock */ unqueue_me_pi(&q); + spin_unlock(q.lock_ptr); goto out; out_unlock_put_key: @@ -3290,8 +3286,8 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags, if (res) ret = (res < 0) ? res : 0; - /* Unqueue and drop the lock. */ unqueue_me_pi(&q); + spin_unlock(q.lock_ptr); } if (ret == -EINTR) { -- cgit From c2e4bfe0eef313eeb1c4c9d921be7a9d91d5d71a Mon Sep 17 00:00:00 2001 From: Davidlohr Bueso Date: Fri, 26 Feb 2021 09:50:29 -0800 Subject: kernel/futex: Explicitly document pi_lock for pi_state owner fixup This seems to belong in the serialization and lifetime rules section. pi_state_update_owner() will take the pi_mutex's owner's pi_lock to do whatever fixup, successful or not. Signed-off-by: Davidlohr Bueso Signed-off-by: Thomas Gleixner Acked-by: Peter Zijlstra (Intel) Link: https://lore.kernel.org/r/20210226175029.50335-4-dave@stgolabs.net --- kernel/futex.c | 1 + 1 file changed, 1 insertion(+) (limited to 'kernel/futex.c') diff --git a/kernel/futex.c b/kernel/futex.c index dcd6132485e1..475055715371 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -981,6 +981,7 @@ static inline void exit_pi_state_list(struct task_struct *curr) { } * p->pi_lock: * * p->pi_state_list -> pi_state->list, relation + * pi_mutex->owner -> pi_state->owner, relation * * pi_state->refcount: * -- cgit