debugging bind9 debian (1:9.5.1.dfsg.P3-1+lenny1) hangs

Nick Jones nick.jones at network-box.com
Fri Jan 8 04:38:00 UTC 2010 

On Wed, 2010-01-06 at 19:47 -0800, Mike wrote:
> Can someone help me understand? Here is a snippet from the strace output:
> 
> 
> 19120 <... futex resumed> )             = 1
> 19120 epoll_ctl(8, EPOLL_CTL_DEL, 517, {EPOLLIN, {u32=517, u64=517}}) = 0
For EPOLL_CTL_DEL, the last argument, the epoll_event structure, is
ignored, thus needn't be populated with the fd.  For safety it should be
memset(..., 0, sizeof(struct epoll_event));  According to the man page,
for kernels 2.6.9 and onward, this structure can be NULL, but for an
application not tied to a particular platform there is no certainty that
this will be run on such a kernel, so the bind developers have rightly
used a valid structure, but again, probably should be memset 0.

> 19120 epoll_wait(8, {{EPOLLIN, {u32=5, u64=13240600445115695109}}}, 64, 
> -1) = 1
The fact that the u64 field is such a wild value suggests that the
epoll_event structure used when doing epoll_ctl(..., EPOLL_CTL_ADD, ...)
was not memset 0, it really should be.

> 19120 read(5, "\5\2\0\0\375\377\377\377"..., 8) = 8
> 19120 epoll_ctl(8, EPOLL_CTL_DEL, 517, {EPOLLIN, {u32=517, u64=517}}) = 
> -1 ENOENT (No such file or directory)
> 19120 epoll_ctl(8, EPOLL_CTL_DEL, 517, {EPOLLOUT, {u32=517, u64=517}}) = 
> -1 ENOENT (No such file or directory)
We are calling EPOLL_CTL_DEL again on fd 517, we have already done that.
In addition, we are calling it with differing event flags, which is not
necessary, as EPOLL_CTL_DEL deletes the fd entirely.

> 19120 read(5, "\5\2\0\0\373\377\377\377"..., 8) = 8
> 19120 epoll_ctl(8, EPOLL_CTL_DEL, 517, {EPOLLIN, {u32=517, u64=517}}) = 
> -1 ENOENT (No such file or directory)
> 19120 epoll_ctl(8, EPOLL_CTL_DEL, 517, {EPOLLOUT, {u32=517, u64=517}}) = 
> -1 ENOENT (No such file or directory)
And again... all these system calls add stress to the system

> 19120 close(517)                        = 0
Now 517 is closed, return value of 0 implies it was a valid fd

> 19120 read(5, 0xb64cb28c, 8)            = -1 EAGAIN (Resource 
> temporarily unavailable)
> 19120 epoll_wait(8, {{EPOLLIN, {u32=520, u64=2233382994440}}}, 64, -1) = 1
Again, looks like the epoll_event structure wasn't memset 0 before fd
520 was EPOLL_CTL_ADD'd.  It's a bit scary that we are getting a read on
fd 520, but the maxevents value is 64.  I've never tested this
behaviour, but I would hope the kernel would round robin notifications
on the set of fds for the process, not simply report events on the first
64 fds in numerical order.

> 19120 futex(0xb74d2048, FUTEX_WAKE_OP_PRIVATE, 1, 1, 0xb74d2044, 
> {FUTEX_OP_SET, 0, FUTEX_OP_CMP_GT, 1} <unfinished ...>
> 19118 <... futex resumed> )             = 0
> 19118 futex(0xb74d2010, FUTEX_WAIT_PRIVATE, 2, NULL <unfinished ...>
> 19120 <... futex resumed> )             = 1
> 19120 futex(0xb74d2010, FUTEX_WAKE_PRIVATE, 1 <unfinished ...>
> 19118 <... futex resumed> )             = 0
> 19118 futex(0xb74d2010, FUTEX_WAKE_PRIVATE, 1) = 0
> 19118 gettimeofday({1262638342, 657830}, NULL) = 0
> 19118 futex(0xb5b5ac70, FUTEX_WAIT_PRIVATE, 2, NULL <unfinished ...>
> 19120 <... futex resumed> )             = 1
> 19120 futex(0xb5b5ac70, FUTEX_WAKE_PRIVATE, 1 <unfinished ...>
> 19118 <... futex resumed> )             = 0
> 19118 recvmsg(520, {msg_name(16)={sa_family=AF_INET, sin_port=htons(53), 
> sin_addr=inet_addr("213.130.47.22")}, 
> msg_iov(1)=[{"\345\23\204\20\0\1\0\1\0\5\0\v\5auth5\2ns\3gin\3ntt\3net"..., 
> 4096}], msg_controllen=20, {cmsg_len=20, cmsg_level=SOL_SOCKET, 
> cmsg_type=0x1d /* SCM_??? */, ...}, msg_flags=0}, 0) = 402
> 19118 futex(0xb5b5ac70, FUTEX_WAKE_PRIVATE, 1) = 0
> 19118 recvmsg(520, 0xb74ccf54, 0)       = -1 EAGAIN (Resource 
> temporarily unavailable)
> 19118 write(7, "\10\2\0\0\375\377\377\377"..., 8) = 8
> 19118 gettimeofday({1262638342, 657830}, NULL) = 0
> 19118 gettimeofday({1262638342, 657830}, NULL) = 0
> 19118 gettimeofday({1262638342, 657830}, NULL) = 0
> 19118 gettimeofday({1262638342, 657830}, NULL) = 0
> 19118 gettimeofday({1262638342, 657830}, NULL) = 0
> 19118 gettimeofday({1262638342, 657830}, NULL) = 0
> 19118 gettimeofday({1262638342, 657830}, NULL) = 0
> 19118 gettimeofday({1262638342, 657830}, NULL) = 0
> 19118 gettimeofday({1262638342, 657830}, NULL) = 0
> 19118 gettimeofday({1262638342, 657830}, NULL) = 0
> 19118 gettimeofday({1262638342, 657830}, NULL) = 0
> 19118 gettimeofday({1262638342, 657830}, NULL) = 0
> 19118 gettimeofday({1262638342, 657830}, NULL) = 0
> 19118 gettimeofday({1262638342, 657830}, NULL) = 0
> 19118 gettimeofday({1262638342, 657830}, NULL) = 0
> 19118 futex(0xb8e61e90, FUTEX_WAIT_PRIVATE, 2, NULL <unfinished ...>
> 19120 <... futex resumed> )             = 1
> 19120 futex(0xb8e61e90, FUTEX_WAKE_PRIVATE, 1 <unfinished ...>
> 19118 <... futex resumed> )             = 0
> 19118 futex(0xb8e61e90, FUTEX_WAKE_PRIVATE, 1) = 0
> 19118 write(7, "\10\2\0\0\373\377\377\377"..., 8) = 8
> 19118 gettimeofday({1262638342, 657830}, NULL) = 0
> 19118 gettimeofday({1262638342, 657830}, NULL) = 0
> 19118 gettimeofday({1262638342, 657830}, NULL) = 0
> 19118 futex(0xb74d2048, FUTEX_WAIT_PRIVATE, 85, NULL <unfinished ...>
> 19120 <... futex resumed> )             = 1
> 19120 epoll_ctl(8, EPOLL_CTL_DEL, 520, {EPOLLIN, {u32=520, u64=520}}) = 0
> 19120 epoll_wait(8, {{EPOLLIN, {u32=5, u64=13240600445115695109}}}, 64, 
> -1) = 1
> 19120 read(5, "\10\2\0\0\375\377\377\377"..., 8) = 8
> 19120 epoll_ctl(8, EPOLL_CTL_DEL, 520, {EPOLLIN, {u32=520, u64=520}}) = 
> -1 ENOENT (No such file or directory)
> 19120 epoll_ctl(8, EPOLL_CTL_DEL, 520, {EPOLLOUT, {u32=520, u64=520}}) = 
> -1 ENOENT (No such file or directory)
> 19120 read(5, "\10\2\0\0\373\377\377\377"..., 8) = 8
> 19120 epoll_ctl(8, EPOLL_CTL_DEL, 520, {EPOLLIN, {u32=520, u64=520}}) = 
> -1 ENOENT (No such file or directory)
> 19120 epoll_ctl(8, EPOLL_CTL_DEL, 520, {EPOLLOUT, {u32=520, u64=520}}) = 
> -1 ENOENT (No such file or directory)
The same pattern again of EPOLL_CTL_DEL mutiple times.  I wonder if the
goal was to manage registration of read and write flags separately?
With epoll, I've found that one has to maintain a record of what the
user of the fd requests (read/write) and what has already been set in
previous calls to epoll_ctl, if there is a difference, then use
EPOLL_CTL_MOD or EPOLL_CTL_DEL

> 19120 close(520)                        = 0
> 19120 read(5, 0xb64cb28c, 8)            = -1 EAGAIN (Resource 
> temporarily unavailable)
> 19120 epoll_wait(8,  <unfinished ...>
> 19119 <... futex resumed> )             = -1 ETIMEDOUT (Connection timed 
> out)
> 19119 gettimeofday({1262638342, 857733}, NULL) = 0
> 19119 futex(0xb74d2048, FUTEX_WAKE_OP_PRIVATE, 1, 1, 0xb74d2044, 
> {FUTEX_OP_SET, 0, FUTEX_OP_CMP_GT, 1} <unfinished ...>
> 19118 <... futex resumed> )             = 0
> 19118 futex(0xb74d2010, FUTEX_WAIT_PRIVATE, 2, NULL <unfinished ...>
> 19119 <... futex resumed> )             = 1
> 19119 futex(0xb74d2010, FUTEX_WAKE_PRIVATE, 1 <unfinished ...>
> 19118 <... futex resumed> )             = 0
> 19118 futex(0xb74d2010, FUTEX_WAKE_PRIVATE, 1) = 0
> 19118 gettimeofday({1262638342, 857733}, NULL) = 0
> 19118 futex(0xb74d4010, FUTEX_WAIT_PRIVATE, 2, NULL <unfinished ...>
> 19119 <... futex resumed> )             = 1
> 19119 futex(0xb74d4010, FUTEX_WAKE_PRIVATE, 1 <unfinished ...>
> 19118 <... futex resumed> )             = 0
> 19118 futex(0xb74d4040, FUTEX_WAIT_PRIVATE, 2, NULL <unfinished ...>
> 19119 <... futex resumed> )             = 1
> 19119 clock_gettime(CLOCK_REALTIME, {1262638342, 857733237}) = 0
> 19119 futex(0xb74d4040, FUTEX_WAKE_PRIVATE, 1 <unfinished ...>
> 19118 <... futex resumed> )             = 0
> 19118 futex(0xb74d4044, FUTEX_WAKE_OP_PRIVATE, 1, 1, 0xb74d4040, 
> {FUTEX_OP_SET, 0, FUTEX_OP_CMP_GT, 1}) = 0
> 19118 futex(0xb74d4010, FUTEX_WAKE_PRIVATE, 1) = 0
> 19118 gettimeofday({1262638342, 857733}, NULL) = 0
> 19118 read(9, 0xb74cc958, 40)           = -1 EAGAIN (Resource 
> temporarily unavailable)
> 19118 read(9, 0xb74cc958, 40)           = -1 EAGAIN (Resource 
> temporarily unavailable)
> 19118 read(9, 0xb74cc958, 40)           = -1 EAGAIN (Resource 
> temporarily unavailable)
> 19118 read(9, 0xb74cc958, 40)           = -1 EAGAIN (Resource 
> temporarily unavailable)
> 19118 read(9, 0xb74cc958, 40)           = -1 EAGAIN (Resource 
> temporarily unavailable)
> 19118 read(9, 0xb74cc958, 40)           = -1 EAGAIN (Resource 
> temporarily unavailable)
> 19118 read(9, 0xb74cc958, 40)           = -1 EAGAIN (Resource 
> temporarily unavailable)
> 19118 read(9, 0xb74cc958, 40)           = -1 EAGAIN (Resource 
> temporarily unavailable)
> 19118 read(9, 0xb74cc958, 40)           = -1 EAGAIN (Resource 
> temporarily unavailable)
> 19118 read(9, 0xb74cc958, 40)           = -1 EAGAIN (Resource 
> temporarily unavailable)
> 19118 read(9, 0xb74cc958, 40)           = -1 EAGAIN (Resource 
> temporarily unavailable)
> 19118 read(9, 0xb74cc958, 40)           = -1 EAGAIN (Resource 
> temporarily unavailable)
> 19118 read(9, 0xb74cc958, 40)           = -1 EAGAIN (Resource 
> temporarily unavailable)
> 19118 socket(PF_INET6, SOCK_DGRAM, IPPROTO_UDP) = 6
> 19118 fcntl64(6, F_DUPFD, 512)          = 515
fd 515 begins its life here...

> 19118 close(6)                          = 0
> 19118 fcntl64(515, F_GETFL)             = 0x2 (flags O_RDWR)
> 19118 fcntl64(515, F_SETFL, O_RDWR|O_NONBLOCK) = 0
> 19118 setsockopt(515, SOL_SOCKET, SO_TIMESTAMP, [1], 4) = 0
> 19118 setsockopt(515, SOL_IPV6, 0x31 /* IPV6_??? */, [1], 4) = 0
> 19118 getsockopt(515, SOL_SOCKET, SO_RCVBUF, [111616], [4]) = 0
> 19118 setsockopt(515, SOL_IPV6, IPV6_V6ONLY, [1], 4) = 0
> 19118 bind(515, {sa_family=AF_INET6, sin6_port=htons(31050), 
> inet_pton(AF_INET6, "::", &sin6_addr), sin6_flowinfo=0, 
> sin6_scope_id=0}, 28) = 0
> 19118 recvmsg(515, 0xb74ccd64, 0)       = -1 EAGAIN (Resource 
> temporarily unavailable)
> 19118 write(7, "\3\2\0\0\375\377\377\377"..., 8) = 8
> 19118 gettimeofday({1262638342, 857733}, NULL) = 0
> 19118 gettimeofday({1262638342, 857733}, NULL) = 0
> 19118 sendmsg(515, {msg_name(28)={sa_family=AF_INET6, 
> sin6_port=htons(53), inet_pton(AF_INET6, "2001:418:3f4::5", &sin6_addr), 
> sin6_flowinfo=0, sin6_scope_id=0}, 
> msg_iov(1)=[{"\252B$\0\0\1\0\1\0\0\0\0\ninfogratis\3org\0\0\6\0\1\300"..., 
> 77}], msg_controllen=0, msg_flags=0}, 0) = -1 ENETUNREACH (Network is 
> unreachable)
> 19118 socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP) = 6
> 19118 fcntl64(6, F_DUPFD, 512)          = 516
> 19118 close(6)                          = 0
> 19118 fcntl64(516, F_GETFL)             = 0x2 (flags O_RDWR)
> 19118 fcntl64(516, F_SETFL, O_RDWR|O_NONBLOCK) = 0
> 19118 setsockopt(516, SOL_SOCKET, SO_TIMESTAMP, [1], 4) = 0
> 19118 setsockopt(516, SOL_IP, IP_MTU_DISCOVER, [0], 4) = 0
> 19118 getsockopt(516, SOL_SOCKET, SO_RCVBUF, [111616], [4]) = 0
> 19118 bind(516, {sa_family=AF_INET, sin_port=htons(62848), 
> sin_addr=inet_addr("0.0.0.0")}, 16) = 0
> 19118 read(9, 0xb74ccc68, 40)           = -1 EAGAIN (Resource 
> temporarily unavailable)
> 19118 read(9, 0xb74ccc68, 40)           = -1 EAGAIN (Resource 
> temporarily unavailable)
> 19118 read(9, 0xb74ccc68, 40)           = -1 EAGAIN (Resource 
> temporarily unavailable)
> 19118 read(9, 0xb74ccc68, 40)           = -1 EAGAIN (Resource 
> temporarily unavailable)
> 19118 read(9, 0xb74ccc68, 40)           = -1 EAGAIN (Resource 
> temporarily unavailable)
> 19118 read(9, 0xb74ccc68, 40)           = -1 EAGAIN (Resource 
> temporarily unavailable)
> 19118 read(9, 0xb74ccc68, 40)           = -1 EAGAIN (Resource 
> temporarily unavailable)
> 19118 read(9, 0xb74ccc68, 40)           = -1 EAGAIN (Resource 
> temporarily unavailable)
> 19118 read(9, 0xb74ccc68, 40)           = -1 EAGAIN (Resource 
> temporarily unavailable)
> 19118 read(9, 0xb74ccc68, 40)           = -1 EAGAIN (Resource 
> temporarily unavailable)
> 19118 read(9, 0xb74ccc68, 40)           = -1 EAGAIN (Resource 
> temporarily unavailable)
> 19118 read(9, 0xb74ccc68, 40)           = -1 EAGAIN (Resource 
> temporarily unavailable)
> 19118 read(9, 0xb74ccc68, 40)           = -1 EAGAIN (Resource 
> temporarily unavailable)
> 19118 recvmsg(516, 0xb74ccd64, 0)       = -1 EAGAIN (Resource 
> temporarily unavailable)
> 19118 write(7, "\4\2\0\0\375\377\377\377"..., 8) = 8
> 19118 gettimeofday({1262638342, 857733}, NULL) = 0
> 19118 gettimeofday({1262638342, 857733}, NULL) = 0
> 19118 sendmsg(516, {msg_name(16)={sa_family=AF_INET, sin_port=htons(53), 
> sin_addr=inet_addr("66.252.5.14")}, 
> msg_iov(1)=[{"=\310$\0\0\1\0\1\0\0\0\0\ninfogratis\3org\0\0\6\0\1\300"..., 
> 77}], msg_controllen=0, msg_flags=0}, 0) = 77
> 19118 gettimeofday({1262638342, 857733}, NULL) = 0
> 19118 gettimeofday({1262638342, 857733}, NULL) = 0
> 19118 write(7, "\3\2\0\0\373\377\377\377"..., 8) = 8
> 19118 gettimeofday({1262638342, 857733}, NULL) = 0
> 19118 futex(0xb74d2048, FUTEX_WAIT_PRIVATE, 87, NULL <unfinished ...>
> 19119 <... futex resumed> )             = 1
> 19119 futex(0xb74d4044, FUTEX_WAIT_PRIVATE, 147, {0, 499999763}) = -1 
> EAGAIN (Resource temporarily unavailable)
> 19119 gettimeofday({1262638342, 857733}, NULL) = 0
> 19119 futex(0xb74d4010, FUTEX_WAKE_PRIVATE, 1) = 0
> 19119 clock_gettime(CLOCK_REALTIME, {1262638342, 857733237}) = 0
> 19119 futex(0xb74d4044, FUTEX_WAIT_PRIVATE, 149, {14, 999999763} 
> <unfinished ...>
> 19120 <... epoll_wait resumed> {{EPOLLIN, {u32=5, 
> u64=13240600445115695109}}}, 64, -1) = 1
> 19120 read(5, "\3\2\0\0\375\377\377\377"..., 8) = 8
> 19120 epoll_ctl(8, EPOLL_CTL_DEL, 515, {EPOLLIN, {u32=515, u64=515}}) = 
> -1 ENOENT (No such file or directory)
> 19120 epoll_ctl(8, EPOLL_CTL_DEL, 515, {EPOLLOUT, {u32=515, u64=515}}) = 
> -1 ENOENT (No such file or directory)
now we attempt to remove fd 515 from the epoll set (twice), but it was
never added!  Maybe it was never added because the sendmsg call on this
fd failed above.

> 19120 read(5, "\4\2\0\0\375\377\377\377"..., 8) = 8
> 19120 epoll_ctl(8, EPOLL_CTL_ADD, 516, {EPOLLIN, {u32=516, 
> u64=2216203125252}}) = 0
> 19120 read(5, "\3\2\0\0\373\377\377\377"..., 8) = 8
> 19120 epoll_ctl(8, EPOLL_CTL_DEL, 515, {EPOLLIN, {u32=515, u64=515}}) = 
> -1 ENOENT (No such file or directory)
> 19120 epoll_ctl(8, EPOLL_CTL_DEL, 515, {EPOLLOUT, {u32=515, u64=515}}) = 
> -1 ENOENT (No such file or directory)
> 19120 close(515)                        = 0
> 19120 read(5, 0xb64cb28c, 8)            = -1 EAGAIN (Resource 
> temporarily unavailable)
> 19120 epoll_wait(8, {{EPOLLIN, {u32=516, u64=2216203125252}}}, 64, -1) = 1
> 19120 futex(0xb74d2048, FUTEX_WAKE_OP_PRIVATE, 1, 1, 0xb74d2044, 
> {FUTEX_OP_SET, 0, FUTEX_OP_CMP_GT, 1} <unfinished ...>
> 19118 <... futex resumed> )             = 0
> 19118 futex(0xb74d2010, FUTEX_WAIT_PRIVATE, 2, NULL <unfinished ...>
> 19120 <... futex resumed> )             = 1
> 19120 futex(0xb74d2010, FUTEX_WAKE_PRIVATE, 1 <unfinished ...>
> 19118 <... futex resumed> )             = 0
> 19118 futex(0xb74d2010, FUTEX_WAKE_PRIVATE, 1) = 0
> 19118 gettimeofday({1262638342, 945690}, NULL) = 0
> 19118 futex(0xb5b63430, FUTEX_WAIT_PRIVATE, 2, NULL <unfinished ...>
> 19120 <... futex resumed> )             = 1
> 19120 futex(0xb5b63430, FUTEX_WAKE_PRIVATE, 1 <unfinished ...>
> 19118 <... futex resumed> )             = 0
> 19118 recvmsg(516, {msg_name(16)={sa_family=AF_INET, sin_port=htons(53), 
> sin_addr=inet_addr("66.252.5.14")}, 
> msg_iov(1)=[{"=\310\244\0\0\1\0\0\0\0\0\0\ninfogratis\3org\0\0\6\0\1\276"..., 
> 4096}], msg_controllen=20, {cmsg_len=20, cmsg_level=SOL_SOCKET, 
> cmsg_type=0x1d /* SCM_??? */, ...}, msg_flags=0}, 0) = 32
> 19118 futex(0xb5b63430, FUTEX_WAKE_PRIVATE, 1) = 0
> 19118 recvmsg(516, 0xb74ccf54, 0)       = -1 EAGAIN (Resource 
> temporarily unavailable)
> 19118 write(7, "\4\2\0\0\375\377\377\377"..., 8) = 8
> 19118 gettimeofday({1262638342, 945690}, NULL) = 0
> 19118 futex(0xb74d4044, FUTEX_WAKE_OP_PRIVATE, 1, 1, 0xb74d4040, 
> {FUTEX_OP_SET, 0, FUTEX_OP_CMP_GT, 1}) = 1
> 19118 gettimeofday({1262638342, 945690}, NULL) = 0
> 19118 futex(0xb8e61e30, FUTEX_WAIT_PRIVATE, 2, NULL <unfinished ...>
> 19119 <... futex resumed> )             = 0
> 19119 gettimeofday({1262638342, 945690}, NULL) = 0
> 19119 futex(0xb74d4010, FUTEX_WAKE_PRIVATE, 1) = 0
> 19119 clock_gettime(CLOCK_REALTIME, {1262638342, 945690526}) = 0
> 19119 futex(0xb74d4044, FUTEX_WAIT_PRIVATE, 151, {1199, 68451474} 
> <unfinished ...>
> 19120 <... futex resumed> )             = 1
> 19120 futex(0xb8e61e30, FUTEX_WAKE_PRIVATE, 1 <unfinished ...>
> 19118 <... futex resumed> )             = 0
> 19118 futex(0xb8e61e30, FUTEX_WAKE_PRIVATE, 1) = 0
> 19118 write(7, "\4\2\0\0\373\377\377\377"..., 8) = 8
> 19118 futex(0xb74d2048, FUTEX_WAIT_PRIVATE, 89, NULL <unfinished ...>
> 19120 <... futex resumed> )             = 1
> 19120 epoll_ctl(8, EPOLL_CTL_DEL, 516, {EPOLLIN, {u32=516, u64=516}}) = 0
> 19120 epoll_wait(8, {{EPOLLIN, {u32=5, u64=13240600445115695109}}}, 64, 
> -1) = 1
> 19120 read(5, "\4\2\0\0\375\377\377\377"..., 8) = 8
> 19120 epoll_ctl(8, EPOLL_CTL_DEL, 516, {EPOLLIN, {u32=516, u64=516}}) = 
> -1 ENOENT (No such file or directory)
> 19120 epoll_ctl(8, EPOLL_CTL_DEL, 516, {EPOLLOUT, {u32=516, u64=516}}) = 
> -1 ENOENT (No such file or directory)
> 19120 read(5, "\4\2\0\0\373\377\377\377"..., 8) = 8
> 19120 epoll_ctl(8, EPOLL_CTL_DEL, 516, {EPOLLIN, {u32=516, u64=516}}) = 
> -1 ENOENT (No such file or directory)
> 19120 epoll_ctl(8, EPOLL_CTL_DEL, 516, {EPOLLOUT, {u32=516, u64=516}}) = 
> -1 ENOENT (No such file or directory)
> 19120 close(516)                        = 0
> 19120 read(5, 0xb64cb28c, 8)            = -1 EAGAIN (Resource 
> temporarily unavailable)
> 19120 epoll_wait(8,
> 
> 

Resource temporarily unavailable is okay, it means that no more can be
read from the file descriptor for now, so try again later.  It would be
interesting to see what fd 5, 7 and 9 are for.

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