diff options
authorRich Felker <>2020-05-21 23:32:45 -0400
committerRich Felker <>2020-05-22 17:39:57 -0400
commite01b5939b38aea5ecbe41670643199825874b26c (patch)
parent4d5aa20a94a2d3fae3e69289dc23ecafbd0c16c4 (diff)
don't use libc.threads_minus_1 as relaxed atomic for skipping locks
after all but the last thread exits, the next thread to observe libc.threads_minus_1==0 and conclude that it can skip locking fails to synchronize with any changes to memory that were made by the last-exiting thread. this can produce data races. on some archs, at least x86, memory synchronization is unlikely to be a problem; however, with the inline locks in malloc, skipping the lock also eliminated the compiler barrier, and caused code that needed to re-check chunk in-use bits after obtaining the lock to reuse a stale value, possibly from before the process became single-threaded. this in turn produced corruption of the heap state. some uses of libc.threads_minus_1 remain, especially for allocation of new TLS in the dynamic linker; otherwise, it could be removed entirely. it's made non-volatile to reflect that the remaining accesses are only made under lock on the thread list. instead of libc.threads_minus_1, libc.threaded is now used for skipping locks. the difference is that libc.threaded is permanently true once an additional thread has been created. this will produce some performance regression in processes that are mostly single-threaded but occasionally creating threads. in the future it may be possible to bring back the full lock-skipping, but more care needs to be taken to produce a safe design.
3 files changed, 3 insertions, 3 deletions
diff --git a/src/internal/libc.h b/src/internal/libc.h
index ac97dc7e..c0614852 100644
--- a/src/internal/libc.h
+++ b/src/internal/libc.h
@@ -21,7 +21,7 @@ struct __libc {
int can_do_threads;
int threaded;
int secure;
- volatile int threads_minus_1;
+ int threads_minus_1;
size_t *auxv;
struct tls_module *tls_head;
size_t tls_size, tls_align, tls_cnt;
diff --git a/src/malloc/malloc.c b/src/malloc/malloc.c
index 96982596..2553a62e 100644
--- a/src/malloc/malloc.c
+++ b/src/malloc/malloc.c
@@ -26,7 +26,7 @@ int __malloc_replaced;
static inline void lock(volatile int *lk)
- if (libc.threads_minus_1)
+ if (libc.threaded)
while(a_swap(lk, 1)) __wait(lk, lk+1, 1, 1);
diff --git a/src/thread/__lock.c b/src/thread/__lock.c
index 45557c88..5b9b144e 100644
--- a/src/thread/__lock.c
+++ b/src/thread/__lock.c
@@ -18,7 +18,7 @@
void __lock(volatile int *l)
- if (!libc.threads_minus_1) return;
+ if (!libc.threaded) return;
/* fast path: INT_MIN for the lock, +1 for the congestion */
int current = a_cas(l, 0, INT_MIN + 1);
if (!current) return;