From a28c8b9db8a1014aa572cd19a3bdb9ddebd3e555 Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Sat, 7 Dec 2019 13:21:01 -0800 Subject: [PATCH] pipe: remove 'waiting_writers' merging logic This code is ancient, and goes back to when we only had a single page for the pipe buffers. The exact history is hidden in the mists of time (ie "before git", and in fact predates the BK repository too). At that long-ago point in time, it actually helped to try to merge big back-and-forth pipe reads and writes, and not limit pipe reads to the single pipe buffer in length just because that was all we had at a time. However, since then we've expanded the pipe buffers to multiple pages, and this logic really doesn't seem to make sense. And a lot of it is somewhat questionable (ie "hmm, the user asked for a non-blocking read, but we see that there's a writer pending, so let's wait anyway to get the extra data that the writer will have"). But more importantly, it makes the "go to sleep" logic much less obvious, and considering the wakeup issues we've had, I want to make for less of those kinds of things. Cc: David Howells Signed-off-by: Linus Torvalds --- fs/pipe.c | 19 +++++-------------- fs/splice.c | 21 ++++----------------- include/linux/pipe_fs_i.h | 2 -- 3 files changed, 9 insertions(+), 33 deletions(-) diff --git a/fs/pipe.c b/fs/pipe.c index bcc2192..58f236c 100644 --- a/fs/pipe.c +++ b/fs/pipe.c @@ -348,18 +348,11 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to) if (!pipe->writers) break; - if (!pipe->waiting_writers) { - /* syscall merging: Usually we must not sleep - * if O_NONBLOCK is set, or if we got some data. - * But if a writer sleeps in kernel space, then - * we can wait for that data without violating POSIX. - */ - if (ret) - break; - if (filp->f_flags & O_NONBLOCK) { - ret = -EAGAIN; - break; - } + if (ret) + break; + if (filp->f_flags & O_NONBLOCK) { + ret = -EAGAIN; + break; } if (signal_pending(current)) { if (!ret) @@ -540,9 +533,7 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from) wake_up_interruptible_sync_poll(&pipe->wait, EPOLLIN | EPOLLRDNORM); kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN); } - pipe->waiting_writers++; pipe_wait(pipe); - pipe->waiting_writers--; was_empty = pipe_empty(head, pipe->tail); } diff --git a/fs/splice.c b/fs/splice.c index fa1f377..3009652 100644 --- a/fs/splice.c +++ b/fs/splice.c @@ -559,7 +559,7 @@ static int splice_from_pipe_next(struct pipe_inode_info *pipe, struct splice_des if (!pipe->writers) return 0; - if (!pipe->waiting_writers && sd->num_spliced) + if (sd->num_spliced) return 0; if (sd->flags & SPLICE_F_NONBLOCK) @@ -1098,9 +1098,7 @@ static int wait_for_space(struct pipe_inode_info *pipe, unsigned flags) return -EAGAIN; if (signal_pending(current)) return -ERESTARTSYS; - pipe->waiting_writers++; pipe_wait(pipe); - pipe->waiting_writers--; } } @@ -1482,11 +1480,9 @@ static int ipipe_prep(struct pipe_inode_info *pipe, unsigned int flags) } if (!pipe->writers) break; - if (!pipe->waiting_writers) { - if (flags & SPLICE_F_NONBLOCK) { - ret = -EAGAIN; - break; - } + if (flags & SPLICE_F_NONBLOCK) { + ret = -EAGAIN; + break; } pipe_wait(pipe); } @@ -1527,9 +1523,7 @@ static int opipe_prep(struct pipe_inode_info *pipe, unsigned int flags) ret = -ERESTARTSYS; break; } - pipe->waiting_writers++; pipe_wait(pipe); - pipe->waiting_writers--; } pipe_unlock(pipe); @@ -1751,13 +1745,6 @@ static int link_pipe(struct pipe_inode_info *ipipe, i_tail++; } while (len); - /* - * return EAGAIN if we have the potential of some data in the - * future, otherwise just return 0 - */ - if (!ret && ipipe->waiting_writers && (flags & SPLICE_F_NONBLOCK)) - ret = -EAGAIN; - pipe_unlock(ipipe); pipe_unlock(opipe); diff --git a/include/linux/pipe_fs_i.h b/include/linux/pipe_fs_i.h index 44f2245..dbcfa68 100644 --- a/include/linux/pipe_fs_i.h +++ b/include/linux/pipe_fs_i.h @@ -38,7 +38,6 @@ struct pipe_buffer { * @readers: number of current readers of this pipe * @writers: number of current writers of this pipe * @files: number of struct file referring this pipe (protected by ->i_lock) - * @waiting_writers: number of writers blocked waiting for room * @r_counter: reader counter * @w_counter: writer counter * @fasync_readers: reader side fasync @@ -56,7 +55,6 @@ struct pipe_inode_info { unsigned int readers; unsigned int writers; unsigned int files; - unsigned int waiting_writers; unsigned int r_counter; unsigned int w_counter; struct page *tmp_page; -- 2.7.4