path: root/src/thread/synccall.c
diff options
authorRich Felker <>2014-07-05 23:29:55 -0400
committerRich Felker <>2014-07-05 23:29:55 -0400
commit83dc6eb087633abcf5608ad651d3b525ca2ec35e (patch)
tree1bed298fdfcf4167d63fed54d055a3c66af4f0f0 /src/thread/synccall.c
parent4c48501ee2a022a0dd207a2db4d346a00f9927a1 (diff)
eliminate use of cached pid from thread structure
the main motivation for this change is to remove the assumption that the tid of the main thread is also the pid of the process. (the value returned by the set_tid_address syscall was used to fill both fields despite it semantically being the tid.) this is historically and presently true on linux and unlikely to change, but it conceivably could be false on other systems that otherwise reproduce the linux syscall api/abi. only a few parts of the code were actually still using the cached pid. in a couple places (aio and synccall) it was a minor optimization to avoid a syscall. caching could be reintroduced, but lazily as part of the public getpid function rather than at program startup, if it's deemed important for performance later. in other places (cancellation and pthread_kill) the pid was completely unnecessary; the tkill syscall can be used instead of tgkill. this is actually a rather subtle issue, since tgkill is supposedly a solution to race conditions that can affect use of tkill. however, as documented in the commit message for commit 7779dbd2663269b465951189b4f43e70839bc073, tgkill does not actually solve this race; it just limits it to happening within one process rather than between processes. we use a lock that avoids the race in pthread_kill, and the use in the cancellation signal handler is self-targeted and thus not subject to tid reuse races, so both are safe regardless of which syscall (tgkill or tkill) is used.
Diffstat (limited to 'src/thread/synccall.c')
1 files changed, 3 insertions, 5 deletions
diff --git a/src/thread/synccall.c b/src/thread/synccall.c
index a21578dc..c4149904 100644
--- a/src/thread/synccall.c
+++ b/src/thread/synccall.c
@@ -1,5 +1,6 @@
#include "pthread_impl.h"
#include <semaphore.h>
+#include <unistd.h>
static struct chain {
struct chain *next;
@@ -13,12 +14,11 @@ static sem_t chainlock, chaindone;
static void handler(int sig, siginfo_t *si, void *ctx)
struct chain ch;
- pthread_t self = __pthread_self();
int old_errno = errno;
if (chainlen == libc.threads_minus_1) return;
- sigqueue(self->pid, SIGSYNCCALL, (union sigval){0});
+ sigqueue(getpid(), SIGSYNCCALL, (union sigval){0});
sem_init(&ch.sem, 0, 0);
sem_init(&ch.sem2, 0, 0);
@@ -39,7 +39,6 @@ static void handler(int sig, siginfo_t *si, void *ctx)
void __synccall(void (*func)(void *), void *ctx)
- pthread_t self;
struct sigaction sa;
struct chain *next;
sigset_t oldmask;
@@ -65,8 +64,7 @@ void __synccall(void (*func)(void *), void *ctx)
__libc_sigaction(SIGSYNCCALL, &sa, 0);
- self = __pthread_self();
- sigqueue(self->pid, SIGSYNCCALL, (union sigval){0});
+ sigqueue(getpid(), SIGSYNCCALL, (union sigval){0});
while (sem_wait(&chaindone));
sa.sa_flags = 0;