ipc/sem: optimize perform_atomic_semop()

This is the main workhorse that deals with semop user calls such that
the waitforzero or semval update operations, on the set, can complete on
not as the sma currently stands.  Currently, the set is iterated twice
(setting semval, then backwards for the sempid value).  Slowpaths, and
particularly SEM_UNDO calls, must undo any altered sem when it is
detected that the caller must block or has errored-out.

With larger sets, there can occur situations where this involves a lot
of cycles and can obviously be a suboptimal use of cached resources in
shared memory.  Ie, discarding CPU caches that are also calling semop
and have the sembuf cached (and can complete), while the current lock
holder doing the semop will block, error, or does a waitforzero
operation.

This patch proposes still iterating the set twice, but the first scan is
read-only, and we perform the actual updates afterward, once we know
that the call will succeed.  In order to not suffer from the overhead of
dealing with sops that act on the same sem_num, such (rare) cases use
perform_atomic_semop_slow(), which is exactly what we have now.
Duplicates are detected before grabbing sem_lock, and uses simple a
32/64-bit hash array variable to based on the sem_num we are working on.

In addition add some comments to when we expect to the caller to block.

[akpm@linux-foundation.org: coding-style fixes]
[colin.king@canonical.com: ensure we left shift a ULL rather than a 32 bit integer]
  Link: http://lkml.kernel.org/r/20161028181129.7311-1-colin.king@canonical.com
Link: http://lkml.kernel.org/r/20160921194603.GB21438@linux-80c1.suse
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Cc: Manfred Spraul <manfred@colorfullife.com>
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 102 insertions, 10 deletions

diff --git a/ipc/sem.c b/ipc/sem.c
index 1ff103299c28..c14883f5a4b5 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -115,7 +115,8 @@ struct sem_queue {
         struct sembuf           *sops;   /* array of pending operations */
         struct sembuf           *blocking; /* the operation that blocked */
         int                     nsops;   /* number of operations */
-        int                     alter;   /* does *sops alter the array? */
+        bool                    alter;   /* does *sops alter the array? */
+        bool                    dupsop;  /* sops on more than one sem_num */
 };
 
 /* Each task has a list of undo requests. They are executed automatically
@@ -587,15 +588,23 @@ SYSCALL_DEFINE3(semget, key_t, key, int, nsems, int, semflg)
 }
 
 /**
- * perform_atomic_semop - Perform (if possible) a semaphore operation
+ * perform_atomic_semop[_slow] - Attempt to perform semaphore
+ *                               operations on a given array.
  * @sma: semaphore array
  * @q: struct sem_queue that describes the operation
  *
+ * Caller blocking are as follows, based the value
+ * indicated by the semaphore operation (sem_op):
+ *
+ *  (1) >0 never blocks.
+ *  (2)  0 (wait-for-zero operation): semval is non-zero.
+ *  (3) <0 attempting to decrement semval to a value smaller than zero.
+ *
  * Returns 0 if the operation was possible.
  * Returns 1 if the operation is impossible, the caller must sleep.
- * Negative values are error codes.
+ * Returns <0 for error codes.
  */
-static int perform_atomic_semop(struct sem_array *sma, struct sem_queue *q)
+static int perform_atomic_semop_slow(struct sem_array *sma, struct sem_queue *q)
 {
         int result, sem_op, nsops, pid;
         struct sembuf *sop;
@@ -666,6 +675,72 @@ undo:
         return result;
 }
 
+static int perform_atomic_semop(struct sem_array *sma, struct sem_queue *q)
+{
+        int result, sem_op, nsops;
+        struct sembuf *sop;
+        struct sem *curr;
+        struct sembuf *sops;
+        struct sem_undo *un;
+
+        sops = q->sops;
+        nsops = q->nsops;
+        un = q->undo;
+
+        if (unlikely(q->dupsop))
+                return perform_atomic_semop_slow(sma, q);
+
+        /*
+         * We scan the semaphore set twice, first to ensure that the entire
+         * operation can succeed, therefore avoiding any pointless writes
+         * to shared memory and having to undo such changes in order to block
+         * until the operations can go through.
+         */
+        for (sop = sops; sop < sops + nsops; sop++) {
+                curr = sma->sem_base + sop->sem_num;
+                sem_op = sop->sem_op;
+                result = curr->semval;
+
+                if (!sem_op && result)
+                        goto would_block; /* wait-for-zero */
+
+                result += sem_op;
+                if (result < 0)
+                        goto would_block;
+
+                if (result > SEMVMX)
+                        return -ERANGE;
+
+                if (sop->sem_flg & SEM_UNDO) {
+                        int undo = un->semadj[sop->sem_num] - sem_op;
+
+                        /* Exceeding the undo range is an error. */
+                        if (undo < (-SEMAEM - 1) || undo > SEMAEM)
+                                return -ERANGE;
+                }
+        }
+
+        for (sop = sops; sop < sops + nsops; sop++) {
+                curr = sma->sem_base + sop->sem_num;
+                sem_op = sop->sem_op;
+                result = curr->semval;
+
+                if (sop->sem_flg & SEM_UNDO) {
+                        int undo = un->semadj[sop->sem_num] - sem_op;
+
+                        un->semadj[sop->sem_num] = undo;
+                }
+                curr->semval += sem_op;
+                curr->sempid = q->pid;
+        }
+
+        return 0;
+
+would_block:
+        q->blocking = sop;
+        return sop->sem_flg & IPC_NOWAIT ? -EAGAIN : 1;
+}
+
 static inline void wake_up_sem_queue_prepare(struct sem_queue *q, int error,
                                              struct wake_q_head *wake_q)
 {
@@ -1720,9 +1795,10 @@ SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __user *, tsops,
         struct sembuf fast_sops[SEMOPM_FAST];
         struct sembuf *sops = fast_sops, *sop;
         struct sem_undo *un;
-        int undos = 0, alter = 0, max, locknum;
+        int max, locknum;
+        bool undos = false, alter = false, dupsop = false;
         struct sem_queue queue;
-        unsigned long jiffies_left = 0;
+        unsigned long dup = 0, jiffies_left = 0;
         struct ipc_namespace *ns;
 
         ns = current->nsproxy->ipc_ns;
@@ -1736,10 +1812,12 @@ SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __user *, tsops,
                 if (sops == NULL)
                         return -ENOMEM;
         }
+
         if (copy_from_user(sops, tsops, nsops * sizeof(*tsops))) {
                 error =  -EFAULT;
                 goto out_free;
         }
+
         if (timeout) {
                 struct timespec _timeout;
                 if (copy_from_user(&_timeout, timeout, sizeof(*timeout))) {
@@ -1753,17 +1831,30 @@ SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __user *, tsops,
                 }
                 jiffies_left = timespec_to_jiffies(&_timeout);
         }
+
         max = 0;
         for (sop = sops; sop < sops + nsops; sop++) {
+                unsigned long mask = 1ULL << ((sop->sem_num) % BITS_PER_LONG);
+
                 if (sop->sem_num >= max)
                         max = sop->sem_num;
                 if (sop->sem_flg & SEM_UNDO)
-                        undos = 1;
-                if (sop->sem_op != 0)
-                        alter = 1;
+                        undos = true;
+                if (dup & mask) {
+                        /*
+                         * There was a previous alter access that appears
+                         * to have accessed the same semaphore, thus use
+                         * the dupsop logic. "appears", because the detection
+                         * can only check % BITS_PER_LONG.
+                         */
+                        dupsop = true;
+                }
+                if (sop->sem_op != 0) {
+                        alter = true;
+                        dup |= mask;
+                }
         }
 
-
         if (undos) {
                 /* On success, find_alloc_undo takes the rcu_read_lock */
                 un = find_alloc_undo(ns, semid);
@@ -1828,6 +1919,7 @@ SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __user *, tsops,
         queue.undo = un;
         queue.pid = task_tgid_vnr(current);
         queue.alter = alter;
+        queue.dupsop = dupsop;
 
         error = perform_atomic_semop(sma, &queue);
         if (error == 0) { /* non-blocking succesfull path */