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SubjectRe: epoll with ONESHOT possibly fails to deliver events
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Hi list, 

Using strace, I checked that my program is using epoll api as I described. Here is a fragment of the strace output that demonstrates my use:

recvfrom(161, "GET / HTTP/1.1\r\nHost: 10.12.0.1:"..., 90, 0, NULL, NULL) = 90
sendto(161, "HTTP/1.1 200 OK\r\nDate: Tue, 09 O"..., 323, 0, NULL, 0) = 323
write(6, "\1\0\0\0\0\0\0\0", 8) = 8
recvfrom(161, 0x7f05ef6c3070, 90, 0, 0, 0) = -1 EAGAIN (Resource temporarily unavailable)
epoll_ctl(7, EPOLL_CTL_MOD, 161, {EPOLLIN|EPOLLONESHOT|EPOLLET, {u32=161, u64=4294967457}}) = 0
epoll_wait(7, {{EPOLLIN, {u32=161, u64=4294967457}}, {EPOLLIN, {u32=160, u64=16673999036704882848}}, {EPOLLIN, {u32=162, u64=22028646743015586}}}, 64, 0) = 3

I.e. we do the following (1) receive until EAGAIN, (2) register socket with epoll_ctl. In addition epoll_wait is called repeatedly, often following (2), as in the fragment above.

Is this considered a correct usage of the epoll API? If not, what is wrong with this usage?

Thanks,
Andi

On Dec 11, 2012, at 5:23 PM, Andreas Voellmy <andreas.voellmy@yale.edu> wrote:

> Hi list,
>
> I am using epoll for the Linux (version 3.4.0) implementation of the event notification subsystem of GHC's (Glasgow Haskell Compiler) RTS (runtime system). I am running into a bug that has only popped up using many cores (> 16) and under particular kind of load. I've been debugging for a couple of days now, and I can't find the error in the way that I am using epoll. I'm starting to wonder whether I am either misunderstanding the semantics of epoll and TCP sockets (likely) or there may be a bug in epoll itself (less likely).
>
> Here is a simplified version of my epoll usage: My program is a multithreaded web server. I have one thread per TCP socket and each socket is marked non-blocking. Each thread serving a client socket repeats the following:
>
> 1. receive a single http request's worth of bytes.
> 2. send an http response.
>
> For both steps, the thread will do a non-blocking operation (either recv or send) and if and only if the call returns EWOULDBLOCK or EAGAIN, then it calls epoll_ctl to register the socket and then it blocks on a condition variable. When the condition variable is signaled, it will continue where it left off (either about to recv or about to send). The epoll_ctl is performed with operation EPOLL_CTL_ADD if this is the first time the socket is being registered and otherwise is done with EPOLL_CTL_MOD. The events field is EPOLLIN | EPOLLET | EPOLLONESHOT.
>
> Another thread, distinct from all of the threads serving particular sockets, is perfoming epoll_wait calls. When sockets are returned as being ready from an epoll_wait call, the thread signals to the condition variable for the socket. Since I am using EPOLLONESHOT, I assume that there is no need to also perform epoll_ctl with EPOLL_CTL_DEL here.
>
> This guarantees that I only wait for epoll to signal a file's readiness if (a) we hit EAGAIN or EWOULDBLOCK in a recv or send, and (b) we call epoll_ctl to re-arm (or arm if on the first time) the socket on epoll.
>
> The problem I am encountering is that sometimes a thread will block waiting for the readiness signal and will never get notified, even though there is data to be read. This behavior seems to go away when I remove EPOLLONESHOT flag when registering the event.
>
> Is my use of epoll (as I described here) OK? Is the following sequence possible?
>
> 1. epoll reports activity on socket previously registered with ONESHOT; now socket is deactivated in epoll.
> 2. call to recv on socket returns EAGAIN or EWOULDBLOCK
> 3. data arrives on socket
> 4. epoll_ctl call rearms socket with epoll (with ONESHOT flag).
> 5. epoll_wait never returns the socket as being ready.
>
> Do I need to first call epoll_ctl and then call recv until I get to EAGAIN, or is it correct to call epoll_ctl for the file only after I've hit EAGAIN on a recv?
>
> I have looked over the epoll source here: http://git.kernel.org/?p=linux/kernel/git/stable/linux-stable.git;a=blob;f=fs/eventpoll.c;h=c0b3c70ee87a2b8e0e46c01a87d63ac692aecc71;hb=refs/heads/linux-3.4.y and I don't see how EPOLLONESHOT could result in the event sequence above, but I'm not that familiar with the code, so it would be great if others can confirm as well.
>
> I am not subscribed to the kernel list, so please include my email on replies.
>
> Cheers,
> Andi



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