From: Jason Baron <[email protected]>
Date: Mon, 20 Apr 2015 20:05:13 +0000 (GMT)
> Under tcp memory pressure, calling epoll_wait() in edge triggered
> mode after -EAGAIN, can result in an indefinite hang in epoll_wait(),
> even when there is suffcient memory available to continue making
> progress. The problem is that __sk_mem_schedule() can return 0,
> under memory pressure without having set the SOCK_NOSPACE flag. Thus,
> even though all the outstanding packets have been acked, we never
> get the EPOLLOUT that we are expecting from epoll_wait().
>
> This issue is currently limited to epoll when used in edge trigger
> mode, since 'tcp_poll()', does in fact currently set SOCK_NOSPACE.
> This is sufficient for poll()/select() and epoll() in level trigger
> mode. However, in edge trigger mode, epoll() is relying on the write
> path to set SOCK_NOSPACE. So I view this patch as bringing us into
> sync with poll()/select() and epoll() level trigger behavior.
Can you explain exactly how epoll in edge trigger mode is
depending upon SOCK_NOSPACE being set in this way? I tried
to read the epoll code and it just seems to call ->poll()
in the normal way when returning event state.
Also, there are exactly two call sites of sk_stream_wait_space()
for TCP, and they both look like this:
====================
wait_for_sndbuf:
set_bit(SOCK_NOSPACE, &sk->sk_socket->flags);
wait_for_memory:
tcp_push(sk, flags & ~MSG_MORE, mss_now,
TCP_NAGLE_PUSH, size_goal);
if ((err = sk_stream_wait_memory(sk, &timeo)) != 0)
goto do_error;
====================
Definitely, the person who wrote this code intended SOCK_NOSPACE to be
set only when we are waiting for sndbuf space rather than just memory.
At a minimum, I need a more detailed commit log message for this,
showing the exact code paths in epoll() that have this requirement and
thus create the looping condition. Because with a casual scan of the
epoll code I could not figure it out.
Thanks.
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