linux-kernel - (splice socket -> pipe) + EPOLLET -> epoll

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Message-ID: <CAEmTpZF3xV9TEpQ4QjuJBkNk_+Gi50CW-x5mxNq6LxpLuAtqwg@mail.gmail.com>
Date:	Thu, 13 Oct 2011 20:54:27 +0600
From:	Марк Коренберг 
	<socketpair@...il.com>
To:	linux-kernel@...r.kernel.org
Subject: (splice socket -> pipe) + EPOLLET -> epoll_wait does not not wake up !

The problem:

man 7 epoll said:
For  stream-oriented  files  (e.g.,  pipe, FIFO, stream socket), the
condition that the read/write I/O space is exhausted can also be
detected by checking the amount of data read from / written to the
target file descriptor.  For example, if you call read(2) by asking to
read a certain amount of data and read(2) returns a lower number of
bytes, you  can  be  sure of having exhausted the read I/O space for
the file descriptor.

I decide to use splice socket -> pipe instead of recv. So I have
registered socket's fd in epoll with EPOLLIN|EPOLLET.

When data appear in socket faster than I splice() it from socket, the
following sometimes appear:

1. in my code I sure, that pipe is empty.
2. my code do splice(socket, pipe, 65536)
3. splice return, say, 53248
4. my code accordingly to man, decide not to fire splice() again, as
it thinks that it will return EWOULDBLOCK=EAGAIN.
5. so, my code go to epoll_wait to wait for EPOLLIN on socket
6. epoll hangs.

This is not appear if I do just recv(). But it may be because speed is
lower, and some race condition in effect.

The hacked version of strace output is attached.

View attachment "strace.txt" of type "text/plain" (4488 bytes)