[PATCH] [PATCH] Improve sysvipc_find_ipc()

[Manfred Spraul]

Initially noticed by Rafael Aquini, see
https://lore.kernel.org/lkml/20210809203554.1562989-1-aquini@xxxxxxxxxx/

The algorithm used in sysvipc_find_ipc() is highly inefficient.
It actually needs to find the next used index in an idr, and it uses
a for loop to locate that entry.

But: The IDR API contains idr_get_next(), thus switch the code to use
idr_get_next().

In addition: Update a few comments.

Signed-off-by: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
---
 ipc/util.c | 43 ++++++++++++++++++++-----------------------
 1 file changed, 20 insertions(+), 23 deletions(-)

diff --git a/ipc/util.c b/ipc/util.c
index 0027e47626b7..083fd6dba1a1 100644
--- a/ipc/util.c
+++ b/ipc/util.c
@@ -783,35 +783,32 @@ struct pid_namespace *ipc_seq_pid_ns(struct seq_file *s)
 }
 
 /*
- * This routine locks the ipc structure found at least at position pos.
+ * This routine locks the ipc structure found at least at index pos.
  */
 static struct kern_ipc_perm *sysvipc_find_ipc(struct ipc_ids *ids, loff_t pos,
 					      loff_t *new_pos)
 {
+	int tmpidx;
 	struct kern_ipc_perm *ipc;
-	int total, id;
-
-	total = 0;
-	for (id = 0; id < pos && total < ids->in_use; id++) {
-		ipc = idr_find(&ids->ipcs_idr, id);
-		if (ipc != NULL)
-			total++;
-	}
 
-	ipc = NULL;
-	if (total >= ids->in_use)
-		goto out;
+	tmpidx = pos;
 
-	for (; pos < ipc_mni; pos++) {
-		ipc = idr_find(&ids->ipcs_idr, pos);
-		if (ipc != NULL) {
-			rcu_read_lock();
-			ipc_lock_object(ipc);
-			break;
-		}
+	ipc = idr_get_next(&ids->ipcs_idr, &tmpidx);
+	if (ipc != NULL) {
+		rcu_read_lock();
+		ipc_lock_object(ipc);
+		/*
+		 * We found the object with the index tmpidx.
+		 * For next search, start with tmpidx+1
+		 */
+		*new_pos = tmpidx + 1;
+	} else {
+		/*
+		 * EOF. seq_file can't notice that, thus
+		 * move the offset by one.
+		 */
+		*new_pos = pos + 1;
 	}
-out:
-	*new_pos = pos + 1;
 	return ipc;
 }
 
@@ -829,7 +826,7 @@ static void *sysvipc_proc_next(struct seq_file *s, void *it, loff_t *pos)
 }
 
 /*
- * File positions: pos 0 -> header, pos n -> ipc id = n - 1.
+ * File positions: pos 0 -> header, pos n -> ipc idx = n - 1.
  * SeqFile iterator: iterator value locked ipc pointer or SEQ_TOKEN_START.
  */
 static void *sysvipc_proc_start(struct seq_file *s, loff_t *pos)
@@ -854,7 +851,7 @@ static void *sysvipc_proc_start(struct seq_file *s, loff_t *pos)
 	if (*pos == 0)
 		return SEQ_START_TOKEN;
 
-	/* Find the (pos-1)th ipc */
+	/* Find the ipc object with the index >= (pos-1) */
 	return sysvipc_find_ipc(ids, *pos - 1, pos);
 }
 
-- 
2.31.1



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