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ntp: keep kernel RX timestamping permanently enabled on Linux

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The Linux kernel has a counter for sockets using kernel RX timestamping
and timestamps (all) received packets only when it is not zero. However,
this counter is updated asynchronously from setsockopt(). If there are
currently no other sockets using the timestamping, it is possible that a
fast server response is received before the kernel timestamping is
actually enabled after setting the socket option and sending a request.

Open a dummy socket on start to make sure there is always at least one
timestamping socket to avoid the race condition.
This commit is contained in:
Miroslav Lichvar 2018-02-14 10:11:19 +01:00
parent b563048ee2
commit 0df8328ceb
1 changed files with 35 additions and 0 deletions
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35
ntp_io_linux.c
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@ -107,6 +107,12 @@ static SCH_TimeoutID resume_timeout_id;
#define RESUME_TIMEOUT 200.0e-6
/* Unbound socket keeping the kernel RX timestamping permanently enabled
in order to avoid a race condition between receiving a server response
and the kernel actually starting to timestamp received packets after
enabling the timestamping and sending a request */
static int dummy_rxts_socket;
#define INVALID_SOCK_FD -3
/* ================================================== */
@ -316,6 +322,29 @@ check_timestamping_option(int option)
/* ================================================== */
static int
open_dummy_socket(void)
{
int sock_fd, events = 0;
if ((sock_fd = socket(AF_INET, SOCK_DGRAM, 0)) < 0
#ifdef FEAT_IPV6
&& (sock_fd = socket(AF_INET6, SOCK_DGRAM, 0)) < 0
#endif
)
return INVALID_SOCK_FD;
if (!NIO_Linux_SetTimestampSocketOptions(sock_fd, 1, &events)) {
close(sock_fd);
return INVALID_SOCK_FD;
}
UTI_FdSetCloexec(sock_fd);
return sock_fd;
}
/* ================================================== */
void
NIO_Linux_Initialise(void)
{
@ -368,6 +397,9 @@ NIO_Linux_Initialise(void)
monitored_socket = INVALID_SOCK_FD;
suspended_socket = INVALID_SOCK_FD;
/* Open a socket to keep the kernel RX timestamping permanently enabled */
dummy_rxts_socket = open_dummy_socket();
}
/* ================================================== */
@ -378,6 +410,9 @@ NIO_Linux_Finalise(void)
struct Interface *iface;
unsigned int i;
if (dummy_rxts_socket != INVALID_SOCK_FD)
close(dummy_rxts_socket);
for (i = 0; i < ARR_GetSize(interfaces); i++) {
iface = ARR_GetElement(interfaces, i);
HCL_DestroyInstance(iface->clock);