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authorMax Kellermann <max.kellermann@ionos.com>2023-09-21 09:57:55 +0200
committerChristian Brauner <brauner@kernel.org>2023-10-19 11:02:48 +0200
commit478dbf12176700f28d836dd03ae93a6888278230 (patch)
treee6ccefe57219ed1d32cbac4ad597b1c9b5af8d19
parentdfaabf916b1ca83cfac856745db2fc9d57d9b13a (diff)
fs/pipe: use spinlock in pipe_read() only if there is a watch_queue
If there is no watch_queue, holding the pipe mutex is enough to prevent concurrent writes, and we can avoid the spinlock. O_NOTIFICATION_QUEUE is an exotic and rarely used feature, and of all the pipes that exist at any given time, only very few actually have a watch_queue, therefore it appears worthwile to optimize the common case. This patch does not optimize pipe_resize_ring() where the spinlocks could be avoided as well; that does not seem like a worthwile optimization because this function is not called often. Related commits: - commit 8df441294dd3 ("pipe: Check for ring full inside of the spinlock in pipe_write()") - commit b667b8673443 ("pipe: Advance tail pointer inside of wait spinlock in pipe_read()") - commit 189b0ddc2451 ("pipe: Fix missing lock in pipe_resize_ring()") Signed-off-by: Max Kellermann <max.kellermann@ionos.com> Message-Id: <20230921075755.1378787-4-max.kellermann@ionos.com> Reviewed-by: David Howells <dhowells@redhat.com> Signed-off-by: Christian Brauner <brauner@kernel.org>
-rw-r--r--fs/pipe.c43
1 files changed, 32 insertions, 11 deletions
diff --git a/fs/pipe.c b/fs/pipe.c
index 939def02c18c..1362e2ec2211 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -227,6 +227,36 @@ static inline bool pipe_readable(const struct pipe_inode_info *pipe)
return !pipe_empty(head, tail) || !writers;
}
+static inline unsigned int pipe_update_tail(struct pipe_inode_info *pipe,
+ struct pipe_buffer *buf,
+ unsigned int tail)
+{
+ pipe_buf_release(pipe, buf);
+
+ /*
+ * If the pipe has a watch_queue, we need additional protection
+ * by the spinlock because notifications get posted with only
+ * this spinlock, no mutex
+ */
+ if (pipe_has_watch_queue(pipe)) {
+ spin_lock_irq(&pipe->rd_wait.lock);
+#ifdef CONFIG_WATCH_QUEUE
+ if (buf->flags & PIPE_BUF_FLAG_LOSS)
+ pipe->note_loss = true;
+#endif
+ pipe->tail = ++tail;
+ spin_unlock_irq(&pipe->rd_wait.lock);
+ return tail;
+ }
+
+ /*
+ * Without a watch_queue, we can simply increment the tail
+ * without the spinlock - the mutex is enough.
+ */
+ pipe->tail = ++tail;
+ return tail;
+}
+
static ssize_t
pipe_read(struct kiocb *iocb, struct iov_iter *to)
{
@@ -320,17 +350,8 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
buf->len = 0;
}
- if (!buf->len) {
- pipe_buf_release(pipe, buf);
- spin_lock_irq(&pipe->rd_wait.lock);
-#ifdef CONFIG_WATCH_QUEUE
- if (buf->flags & PIPE_BUF_FLAG_LOSS)
- pipe->note_loss = true;
-#endif
- tail++;
- pipe->tail = tail;
- spin_unlock_irq(&pipe->rd_wait.lock);
- }
+ if (!buf->len)
+ tail = pipe_update_tail(pipe, buf, tail);
total_len -= chars;
if (!total_len)
break; /* common path: read succeeded */