18790, 18795, 18796, 18803, 18820, 18823, 18824, 18825, 18857, 18863,
18870, 18872, 18873, 18875, 18887, 18921, 18951, 18952, 18956, 18961,
18966, 18967, 18969, 18970, 18977, 18980, 18981, 18985, 19003, 19007,
- 19012, 19016, 19018, 19032, 19046, 19049, 19050, 19059, 19071, 19076,
- 19077, 19078, 19079, 19085, 19086, 19088, 19094, 19095, 19124, 19125
+ 19012, 19016, 19018, 19032, 19046, 19049, 19050, 19059, 19071, 19074,
+ 19076, 19077, 19078, 19079, 19085, 19086, 19088, 19094, 19095, 19124,
+ 19125
* The obsolete header <regexp.h> has been removed. Programs that require
this header must be updated to use <regex.h> instead.
#include "ifreq.h"
#include "res_hconf.h"
#include <wchar.h>
+#include <atomic.h>
#if IS_IN (libc)
# define fgets_unlocked __fgets_unlocked
{
#if defined SIOCGIFCONF && defined SIOCGIFNETMASK
int i, j;
- /* Number of interfaces. */
+ /* Number of interfaces. Also serves as a flag for the
+ double-checked locking idiom. */
static int num_ifs = -1;
- /* We need to protect the dynamic buffer handling. */
+ /* Local copy of num_ifs, for non-atomic access. */
+ int num_ifs_local;
+ /* We need to protect the dynamic buffer handling. The lock is only
+ acquired during initialization. Afterwards, a positive num_ifs
+ value indicates completed initialization. */
__libc_lock_define_initialized (static, lock);
/* Only reorder if we're supposed to. */
if (hp->h_addrtype != AF_INET)
return;
- if (num_ifs <= 0)
+ /* This load synchronizes with the release MO store in the
+ initialization block below. */
+ num_ifs_local = atomic_load_acquire (&num_ifs);
+ if (num_ifs_local <= 0)
{
struct ifreq *ifr, *cur_ifr;
int sd, num, i;
/* Get lock. */
__libc_lock_lock (lock);
- /* Recheck, somebody else might have done the work by now. */
- if (num_ifs <= 0)
+ /* Recheck, somebody else might have done the work by now. No
+ ordering is required for the load because we have the lock,
+ and num_ifs is only updated under the lock. Also see (3) in
+ the analysis below. */
+ num_ifs_local = atomic_load_relaxed (&num_ifs);
+ if (num_ifs_local <= 0)
{
+ /* This is the only block which writes to num_ifs. It can
+ be executed several times (sequentially) if
+ initialization does not yield any interfaces, and num_ifs
+ remains zero. However, once we stored a positive value
+ in num_ifs below, this block cannot be entered again due
+ to the condition above. */
int new_num_ifs = 0;
/* Get a list of interfaces. */
/* Release lock, preserve error value, and close socket. */
errno = save;
- num_ifs = new_num_ifs;
+ /* Advertise successful initialization if new_num_ifs is
+ positive (and no updates to ifaddrs are permitted after
+ that). Otherwise, num_ifs remains unchanged, at zero.
+ This store synchronizes with the initial acquire MO
+ load. */
+ atomic_store_release (&num_ifs, new_num_ifs);
+ /* Keep the local copy current, to save another load. */
+ num_ifs_local = new_num_ifs;
}
__libc_lock_unlock (lock);
__close (sd);
}
- if (num_ifs == 0)
+ /* num_ifs_local cannot be negative because the if statement above
+ covered this case. It can still be zero if we just performed
+ initialization, but could not find any interfaces. */
+ if (num_ifs_local == 0)
return;
+ /* The code below accesses ifaddrs, so we need to ensure that the
+ initialization happens-before this point.
+
+ The actual initialization is sequenced-before the release store
+ to num_ifs, and sequenced-before the end of the critical section.
+
+ This means there are three possible executions:
+
+ (1) The thread that initialized the data also uses it, so
+ sequenced-before is sufficient to ensure happens-before.
+
+ (2) The release MO store of num_ifs synchronizes-with the acquire
+ MO load, and the acquire MO load is sequenced before the use
+ of the initialized data below.
+
+ (3) We enter the critical section, and the relaxed MO load of
+ num_ifs yields a positive value. The write to ifaddrs is
+ sequenced-before leaving the critical section. Leaving the
+ critical section happens-before we entered the critical
+ section ourselves, which means that the write to ifaddrs
+ happens-before this point.
+
+ Consequently, all potential writes to ifaddrs (and the data it
+ points to) happens-before this point. */
+
/* Find an address for which we have a direct connection. */
for (i = 0; hp->h_addr_list[i]; ++i)
{
struct in_addr *haddr = (struct in_addr *) hp->h_addr_list[i];
- for (j = 0; j < num_ifs; ++j)
+ for (j = 0; j < num_ifs_local; ++j)
{
u_int32_t if_addr = ifaddrs[j].u.ipv4.addr;
u_int32_t if_netmask = ifaddrs[j].u.ipv4.mask;
projects / thirdparty / glibc.git / commitdiff
