diff options
author | Davidlohr Bueso <dave@stgolabs.net> | 2015-05-04 17:02:46 +0300 |
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committer | Ingo Molnar <mingo@kernel.org> | 2015-05-08 13:23:07 +0300 |
commit | fa6004ad4528153b699a4d5ce5ea6b33acce74cc (patch) | |
tree | eda96bac494cc6e9071e14b2d5741fa8f29958ea /ipc/mqueue.c | |
parent | 1d0dcb3ad9d336e6d6ee020a750a7f8d907e28de (diff) | |
download | linux-fa6004ad4528153b699a4d5ce5ea6b33acce74cc.tar.xz |
ipc/mqueue: Implement lockless pipelined wakeups
This patch moves the wakeup_process() invocation so it is not done under
the info->lock by making use of a lockless wake_q. With this change, the
waiter is woken up once it is STATE_READY and it does not need to loop
on SMP if it is still in STATE_PENDING. In the timeout case we still need
to grab the info->lock to verify the state.
This change should also avoid the introduction of preempt_disable() in -rt
which avoids a busy-loop which pools for the STATE_PENDING -> STATE_READY
change if the waiter has a higher priority compared to the waker.
Additionally, this patch micro-optimizes wq_sleep by using the cheaper
cousin of set_current_state(TASK_INTERRUPTABLE) as we will block no
matter what, thus get rid of the implied barrier.
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: George Spelvin <linux@horizon.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Chris Mason <clm@fb.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Manfred Spraul <manfred@colorfullife.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: dave@stgolabs.net
Link: http://lkml.kernel.org/r/1430748166.1940.17.camel@stgolabs.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Diffstat (limited to 'ipc/mqueue.c')
-rw-r--r-- | ipc/mqueue.c | 54 |
1 files changed, 33 insertions, 21 deletions
diff --git a/ipc/mqueue.c b/ipc/mqueue.c index 3aaea7ffd077..a24ba9fe5bb8 100644 --- a/ipc/mqueue.c +++ b/ipc/mqueue.c @@ -47,8 +47,7 @@ #define RECV 1 #define STATE_NONE 0 -#define STATE_PENDING 1 -#define STATE_READY 2 +#define STATE_READY 1 struct posix_msg_tree_node { struct rb_node rb_node; @@ -571,15 +570,12 @@ static int wq_sleep(struct mqueue_inode_info *info, int sr, wq_add(info, sr, ewp); for (;;) { - set_current_state(TASK_INTERRUPTIBLE); + __set_current_state(TASK_INTERRUPTIBLE); spin_unlock(&info->lock); time = schedule_hrtimeout_range_clock(timeout, 0, HRTIMER_MODE_ABS, CLOCK_REALTIME); - while (ewp->state == STATE_PENDING) - cpu_relax(); - if (ewp->state == STATE_READY) { retval = 0; goto out; @@ -907,11 +903,15 @@ out_name: * list of waiting receivers. A sender checks that list before adding the new * message into the message array. If there is a waiting receiver, then it * bypasses the message array and directly hands the message over to the - * receiver. - * The receiver accepts the message and returns without grabbing the queue - * spinlock. Therefore an intermediate STATE_PENDING state and memory barriers - * are necessary. The same algorithm is used for sysv semaphores, see - * ipc/sem.c for more details. + * receiver. The receiver accepts the message and returns without grabbing the + * queue spinlock: + * + * - Set pointer to message. + * - Queue the receiver task for later wakeup (without the info->lock). + * - Update its state to STATE_READY. Now the receiver can continue. + * - Wake up the process after the lock is dropped. Should the process wake up + * before this wakeup (due to a timeout or a signal) it will either see + * STATE_READY and continue or acquire the lock to check the state again. * * The same algorithm is used for senders. */ @@ -919,21 +919,29 @@ out_name: /* pipelined_send() - send a message directly to the task waiting in * sys_mq_timedreceive() (without inserting message into a queue). */ -static inline void pipelined_send(struct mqueue_inode_info *info, +static inline void pipelined_send(struct wake_q_head *wake_q, + struct mqueue_inode_info *info, struct msg_msg *message, struct ext_wait_queue *receiver) { receiver->msg = message; list_del(&receiver->list); - receiver->state = STATE_PENDING; - wake_up_process(receiver->task); - smp_wmb(); + wake_q_add(wake_q, receiver->task); + /* + * Rely on the implicit cmpxchg barrier from wake_q_add such + * that we can ensure that updating receiver->state is the last + * write operation: As once set, the receiver can continue, + * and if we don't have the reference count from the wake_q, + * yet, at that point we can later have a use-after-free + * condition and bogus wakeup. + */ receiver->state = STATE_READY; } /* pipelined_receive() - if there is task waiting in sys_mq_timedsend() * gets its message and put to the queue (we have one free place for sure). */ -static inline void pipelined_receive(struct mqueue_inode_info *info) +static inline void pipelined_receive(struct wake_q_head *wake_q, + struct mqueue_inode_info *info) { struct ext_wait_queue *sender = wq_get_first_waiter(info, SEND); @@ -944,10 +952,9 @@ static inline void pipelined_receive(struct mqueue_inode_info *info) } if (msg_insert(sender->msg, info)) return; + list_del(&sender->list); - sender->state = STATE_PENDING; - wake_up_process(sender->task); - smp_wmb(); + wake_q_add(wake_q, sender->task); sender->state = STATE_READY; } @@ -965,6 +972,7 @@ SYSCALL_DEFINE5(mq_timedsend, mqd_t, mqdes, const char __user *, u_msg_ptr, struct timespec ts; struct posix_msg_tree_node *new_leaf = NULL; int ret = 0; + WAKE_Q(wake_q); if (u_abs_timeout) { int res = prepare_timeout(u_abs_timeout, &expires, &ts); @@ -1049,7 +1057,7 @@ SYSCALL_DEFINE5(mq_timedsend, mqd_t, mqdes, const char __user *, u_msg_ptr, } else { receiver = wq_get_first_waiter(info, RECV); if (receiver) { - pipelined_send(info, msg_ptr, receiver); + pipelined_send(&wake_q, info, msg_ptr, receiver); } else { /* adds message to the queue */ ret = msg_insert(msg_ptr, info); @@ -1062,6 +1070,7 @@ SYSCALL_DEFINE5(mq_timedsend, mqd_t, mqdes, const char __user *, u_msg_ptr, } out_unlock: spin_unlock(&info->lock); + wake_up_q(&wake_q); out_free: if (ret) free_msg(msg_ptr); @@ -1149,14 +1158,17 @@ SYSCALL_DEFINE5(mq_timedreceive, mqd_t, mqdes, char __user *, u_msg_ptr, msg_ptr = wait.msg; } } else { + WAKE_Q(wake_q); + msg_ptr = msg_get(info); inode->i_atime = inode->i_mtime = inode->i_ctime = CURRENT_TIME; /* There is now free space in queue. */ - pipelined_receive(info); + pipelined_receive(&wake_q, info); spin_unlock(&info->lock); + wake_up_q(&wake_q); ret = 0; } if (ret == 0) { |