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authorLinus Torvalds <torvalds@linux-foundation.org>2019-12-11 11:46:19 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2019-12-11 11:46:19 -0800
commitd1c6a2aa02af09238ad09493eb3c8685ccc3fe12 (patch)
tree138bfecd61e39cbe7492eaba1db0092d1e8cc8e7
parent6794862a16ef41f753abd75c03a152836e4c8028 (diff)
pipe: simplify signal handling in pipe_read() and add comments
There's no need to separately check for signals while inside the locked region, since we're going to do "wait_event_interruptible()" right afterwards anyway, and the error handling is much simpler there. The check for whether we had already read anything was also redundant, since we no longer do the odd merging of reads when there are pending writers. But perhaps more importantly, this adds commentary about why we still need to wake up possible writers even though we didn't read any data, and why we can skip all the finishing touches now if we get a signal (or had a signal pending) while waiting for more data. [ This is a split-out cleanup from my "make pipe IO use exclusive wait queues" thing, which I can't apply because it triggers a nasty bug in the GNU make jobserver - Linus ] Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-rw-r--r--fs/pipe.c36
1 files changed, 29 insertions, 7 deletions
diff --git a/fs/pipe.c b/fs/pipe.c
index 87109e761fa5..04d004ee2e8c 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -364,17 +364,39 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
ret = -EAGAIN;
break;
}
- if (signal_pending(current)) {
- if (!ret)
- ret = -ERESTARTSYS;
- break;
- }
__pipe_unlock(pipe);
- if (was_full) {
+
+ /*
+ * We only get here if we didn't actually read anything.
+ *
+ * However, we could have seen (and removed) a zero-sized
+ * pipe buffer, and might have made space in the buffers
+ * that way.
+ *
+ * You can't make zero-sized pipe buffers by doing an empty
+ * write (not even in packet mode), but they can happen if
+ * the writer gets an EFAULT when trying to fill a buffer
+ * that already got allocated and inserted in the buffer
+ * array.
+ *
+ * So we still need to wake up any pending writers in the
+ * _very_ unlikely case that the pipe was full, but we got
+ * no data.
+ */
+ if (unlikely(was_full)) {
wake_up_interruptible_sync_poll(&pipe->wait, EPOLLOUT | EPOLLWRNORM);
kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
}
- wait_event_interruptible(pipe->wait, pipe_readable(pipe));
+
+ /*
+ * But because we didn't read anything, at this point we can
+ * just return directly with -ERESTARTSYS if we're interrupted,
+ * since we've done any required wakeups and there's no need
+ * to mark anything accessed. And we've dropped the lock.
+ */
+ if (wait_event_interruptible(pipe->wait, pipe_readable(pipe)) < 0)
+ return -ERESTARTSYS;
+
__pipe_lock(pipe);
was_full = pipe_full(pipe->head, pipe->tail, pipe->max_usage);
}