在 2021/6/9 下午7:07, Pavel Begunkov 写道:
There is a complaint against sys_io_uring_enter() blocking if it submits
stdin reads. The problem is in __io_file_supports_async(), which
sees that it's a cdev and allows it to be processed inline.
Punt char devices using generic rules of io_file_supports_async(),
including checking for presence of *_iter() versions of rw callbacks.
Apparently, it will affect most of cdevs with some exceptions like
null and zero devices.
Signed-off-by: Pavel Begunkov <asml.silence@xxxxxxxxx>
---
"...For now, just ensure that anything potentially problematic is done
inline". I believe this part is outdated, but what use cases we miss?
Anything that we care about?
IMHO the best option is to do like in this patch and add
(read,write)_iter(), to places we care about.
/dev/[u]random, consoles, any else?
This reminds me another thing, once I did nowait read on a brd(block
ramdisk), I saw a 10%~30% regression after __io_file_supports_async()
added. brd is bio based device (block layer doesn't support nowait IO
for this kind of device), so theoretically it makes sense to punt it to
iowq threads in advance in __io_file_supports_async(), but actually
what originally happen is: IOCB_NOWAIT is not delivered to block
layer(REQ_NOWAIT) and then the IO request is executed inline (It seems
brd device won't block). This finally makes 'check it in advance'
slower..
fs/io_uring.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/fs/io_uring.c b/fs/io_uring.c
index 42380ed563c4..44d1859f0dfb 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -2616,7 +2616,7 @@ static bool __io_file_supports_async(struct file *file, int rw)
return true;
return false;
}
- if (S_ISCHR(mode) || S_ISSOCK(mode))
+ if (S_ISSOCK(mode))
return true;
if (S_ISREG(mode)) {
if (IS_ENABLED(CONFIG_BLOCK) &&
