ipc: introduce obtaining a lockless ipc object

Through ipc_lock() and therefore ipc_lock_check() we currently return the
locked ipc object.  This is not necessary for all situations and can,
therefore, cause unnecessary ipc lock contention.

Introduce analogous ipc_obtain_object() and ipc_obtain_object_check()
functions that only lookup and return the ipc object.

Both these functions must be called within the RCU read critical section.

[akpm@linux-foundation.org: propagate the ipc_obtain_object() errno from ipc_lock()]
Signed-off-by: Davidlohr Bueso <davidlohr.bueso@hp.com>
Signed-off-by: Rik van Riel <riel@redhat.com>
Reviewed-by: Chegu Vinod <chegu_vinod@hp.com>
Acked-by: Michel Lespinasse <walken@google.com>
Cc: Emmanuel Benisty <benisty.e@gmail.com>
Cc: Jason Low <jason.low2@hp.com>
Cc: Peter Hurley <peter@hurleysoftware.com>
Cc: Stanislav Kinsbursky <skinsbursky@parallels.com>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/ipc/util.c b/ipc/util.c
index 03eadd8fb0fda0ab5bb8a089a03311615b2ec28d..813804ebdebaaf24e0faa6f2174336b004a73ca3 100644 (file)
--- a/ipc/util.c
+++ b/ipc/util.c
@@ -668,6 +668,28 @@ void ipc64_perm_to_ipc_perm (struct ipc64_perm *in, struct ipc_perm *out)
        out->seq        = in->seq;
 }
 
+/**
+ * ipc_obtain_object
+ * @ids: ipc identifier set
+ * @id: ipc id to look for
+ *
+ * Look for an id in the ipc ids idr and return associated ipc object.
+ *
+ * Call inside the RCU critical section.
+ * The ipc object is *not* locked on exit.
+ */
+struct kern_ipc_perm *ipc_obtain_object(struct ipc_ids *ids, int id)
+{
+       struct kern_ipc_perm *out;
+       int lid = ipcid_to_idx(id);
+
+       out = idr_find(&ids->ipcs_idr, lid);
+       if (!out)
+               return ERR_PTR(-EINVAL);
+
+       return out;
+}
+
 /**
  * ipc_lock - Lock an ipc structure without rw_mutex held
  * @ids: IPC identifier set
@@ -675,32 +697,53 @@ void ipc64_perm_to_ipc_perm (struct ipc64_perm *in, struct ipc_perm *out)
  *
  * Look for an id in the ipc ids idr and lock the associated ipc object.
  *
- * The ipc object is locked on exit.
+ * The ipc object is locked on successful exit.
  */
-
 struct kern_ipc_perm *ipc_lock(struct ipc_ids *ids, int id)
 {
        struct kern_ipc_perm *out;
-       int lid = ipcid_to_idx(id);
 
        rcu_read_lock();
-       out = idr_find(&ids->ipcs_idr, lid);
-       if (out == NULL) {
-               rcu_read_unlock();
-               return ERR_PTR(-EINVAL);
-       }
+       out = ipc_obtain_object(ids, id);
+       if (IS_ERR(out))
+               goto err1;
 
        spin_lock(&out->lock);
-       
+
        /* ipc_rmid() may have already freed the ID while ipc_lock
         * was spinning: here verify that the structure is still valid
         */
-       if (out->deleted) {
-               spin_unlock(&out->lock);
-               rcu_read_unlock();
-               return ERR_PTR(-EINVAL);
-       }
+       if (!out->deleted)
+               return out;
 
+       spin_unlock(&out->lock);
+       out = ERR_PTR(-EINVAL);
+err1:
+       rcu_read_unlock();
+       return out;
+}
+
+/**
+ * ipc_obtain_object_check
+ * @ids: ipc identifier set
+ * @id: ipc id to look for
+ *
+ * Similar to ipc_obtain_object() but also checks
+ * the ipc object reference counter.
+ *
+ * Call inside the RCU critical section.
+ * The ipc object is *not* locked on exit.
+ */
+struct kern_ipc_perm *ipc_obtain_object_check(struct ipc_ids *ids, int id)
+{
+       struct kern_ipc_perm *out = ipc_obtain_object(ids, id);
+
+       if (IS_ERR(out))
+               goto out;
+
+       if (ipc_checkid(out, id))
+               return ERR_PTR(-EIDRM);
+out:
        return out;
 }
 

diff --git a/ipc/util.h b/ipc/util.h
index ac1480a4efd1499c5bb72bd85c99902a3ef693a2..bfc8d4ea6e46484dc1be9927670ccd02caa852c6 100644 (file)
--- a/ipc/util.h
+++ b/ipc/util.h
@@ -123,6 +123,7 @@ void ipc_rcu_getref(void *ptr);
 void ipc_rcu_putref(void *ptr);
 
 struct kern_ipc_perm *ipc_lock(struct ipc_ids *, int);
+struct kern_ipc_perm *ipc_obtain_object(struct ipc_ids *ids, int id);
 
 void kernel_to_ipc64_perm(struct kern_ipc_perm *in, struct ipc64_perm *out);
 void ipc64_perm_to_ipc_perm(struct ipc64_perm *in, struct ipc_perm *out);
@@ -168,6 +169,7 @@ static inline void ipc_unlock(struct kern_ipc_perm *perm)
 }
 
 struct kern_ipc_perm *ipc_lock_check(struct ipc_ids *ids, int id);
+struct kern_ipc_perm *ipc_obtain_object_check(struct ipc_ids *ids, int id);
 int ipcget(struct ipc_namespace *ns, struct ipc_ids *ids,
                        struct ipc_ops *ops, struct ipc_params *params);
 void free_ipcs(struct ipc_namespace *ns, struct ipc_ids *ids,