add proper fuxed-based locking for stdio

previously, stdio used spinlocks, which would be unacceptable if we
ever add support for thread priorities, and which yielded
pathologically bad performance if an application attempted to use
flockfile on a key file as a major/primary locking mechanism.

i had held off on making this change for fear that it would hurt
performance in the non-threaded case, but actually support for
recursive locking had already inflicted that cost. by having the
internal locking functions store a flag indicating whether they need
to perform unlocking, rather than using the actual recursive lock
counter, i was able to combine the conditionals at unlock time,
eliminating any additional cost, and also avoid a nasty corner case
where a huge number of calls to ftrylockfile could cause deadlock
later at the point of internal locking.

this commit also fixes some issues with usage of pthread_self
conflicting with __attribute__((const)) which resulted in crashes with
some compiler versions/optimizations, mainly in flockfile prior to
pthread_create.

src/internal/libc.h

@@ -38,7 +38,8 @@ extern struct __libc *__libc_loc(void) __attribute__((const));
 
 /* Designed to avoid any overhead in non-threaded processes */
 void __lock(volatile int *);
-void __lockfile(FILE *);
+int __lockfile(FILE *);
+void __unlockfile(FILE *);
 #define LOCK(x) (libc.threads_minus_1 ? (__lock(x),1) : ((void)(x),1))
 #define UNLOCK(x) (*(volatile int *)(x)=0)
 

src/internal/pthread_impl.h

@@ -87,6 +87,8 @@ struct __timer {
 #define SIGTIMER_SET ((sigset_t *)(unsigned long [1+(sizeof(long)==4)]){ \
 	 0x80000000 })
 
+pthread_t __pthread_self_init(void);
+
 int __set_thread_area(void *);
 int __libc_sigaction(int, const struct sigaction *, struct sigaction *);
 int __libc_sigprocmask(int, const sigset_t *, sigset_t *);

src/internal/stdio_impl.h

@@ -24,8 +24,8 @@
 
 #define UNGET 8
 
-#define FLOCK(f) ((libc.threads_minus_1 && (f)->lock>=0) ? (__lockfile((f)),0) : 0)
-#define FUNLOCK(f) ((f)->lockcount && (--(f)->lockcount || ((f)->lock=0)))
+#define FLOCK(f) int __need_unlock = ((f)->lock>=0 ? __lockfile((f)) : 0)
+#define FUNLOCK(f) if (__need_unlock) __unlockfile((f)); else
 
 #define F_PERM 1
 #define F_NORD 4
@@ -49,12 +49,12 @@ struct __FILE_s {
 	FILE *prev, *next;
 	int fd;
 	int pipe_pid;
-	long dummy2;
+	long lockcount;
 	short dummy3;
 	signed char mode;
 	signed char lbf;
 	int lock;
-	int lockcount;
+	int waiters;
 	void *cookie;
 	off_t off;
 	int (*flush)(FILE *);
@@ -86,7 +86,6 @@ FILE *__fdopen(int, const char *);
 
 #define OFLLOCK() LOCK(&libc.ofl_lock)
 #define OFLUNLOCK() UNLOCK(&libc.ofl_lock)
-#define ofl_head (libc.ofl_head)
 
 #define feof(f) ((f)->flags & F_EOF)
 #define ferror(f) ((f)->flags & F_ERR)

src/stdio/__fdopen.c

@@ -39,11 +39,13 @@ FILE *__fdopen(int fd, const char *mode)
 	f->seek = __stdio_seek;
 	f->close = __stdio_close;
 
+	if (!libc.threaded) f->lock = -1;
+
 	/* Add new FILE to open file list */
 	OFLLOCK();
-	f->next = ofl_head;
-	if (ofl_head) ofl_head->prev = f;
-	ofl_head = f;
+	f->next = libc.ofl_head;
+	if (libc.ofl_head) libc.ofl_head->prev = f;
+	libc.ofl_head = f;
 	OFLUNLOCK();
 
 	return f;

src/stdio/__lockfile.c

@@ -1,20 +1,18 @@
 #include "stdio_impl.h"
 #include "pthread_impl.h"
 
-void __lockfile(FILE *f)
+int __lockfile(FILE *f)
 {
-	int spins=10000;
-	int tid;
-
-	if (f->lock < 0) return;
-	tid = __pthread_self()->tid;
-	if (f->lock == tid) {
-		while (f->lockcount == INT_MAX);
-		f->lockcount++;
-		return;
-	}
-	while (a_cas(&f->lock, 0, tid))
-		if (spins) spins--, a_spin();
-		else __syscall(SYS_sched_yield);
-	f->lockcount = 1;
+	int owner, tid = __pthread_self()->tid;
+	if (f->lock == tid)
+		return 0;
+	while ((owner = a_cas(&f->lock, 0, tid)))
+		__wait(&f->lock, &f->waiters, owner, 1);
+	return f->lockcount = 1;
+}
+
+void __unlockfile(FILE *f)
+{
+	a_store(&f->lock, 0);
+	if (f->waiters) __wake(&f->lock, 1, 1);
 }

src/stdio/fclose.c

@@ -9,7 +9,7 @@ int fclose(FILE *f)
 		OFLLOCK();
 		if (f->prev) f->prev->next = f->next;
 		if (f->next) f->next->prev = f->prev;
-		if (ofl_head == f) ofl_head = f->next;
+		if (libc.ofl_head == f) libc.ofl_head = f->next;
 		OFLUNLOCK();
 	}
 

src/stdio/fflush.c

@@ -22,8 +22,8 @@ static int __fflush_unlocked(FILE *f)
 }
 
 /* stdout.c will override this if linked */
-static FILE *const __dummy = 0;
-weak_alias(__dummy, __stdout_to_flush);
+static FILE *const dummy = 0;
+weak_alias(dummy, __stdout_used);
 
 int fflush(FILE *f)
 {
@@ -37,13 +37,13 @@ int fflush(FILE *f)
 		return r;
 	}
 
-	r = __stdout_to_flush ? fflush(__stdout_to_flush) : 0;
+	r = __stdout_used ? fflush(__stdout_used) : 0;
 
 	OFLLOCK();
-	for (f=ofl_head; f; f=next) {
+	for (f=libc.ofl_head; f; f=next) {
 		FLOCK(f);
 		//OFLUNLOCK();
-		r |= __fflush_unlocked(f);
+		if (f->wpos > f->wbase) r |= __fflush_unlocked(f);
 		//OFLLOCK();
 		next = f->next;
 		FUNLOCK(f);

src/stdio/fgetc.c

@@ -3,9 +3,10 @@
 int fgetc(FILE *f)
 {
 	int c;
-	FLOCK(f);
+	if (f->lock < 0 || !__lockfile(f))
+		return getc_unlocked(f);
 	c = getc_unlocked(f);
-	FUNLOCK(f);
+	__unlockfile(f);
 	return c;
 }
 

src/stdio/fgetwc.c

@@ -34,7 +34,6 @@ wint_t __fgetwc_unlocked(FILE *f)
 		if (l == -1) return WEOF;
 	}
 
-	FUNLOCK(f);
 	return wc;
 }
 

src/stdio/flockfile.c

@@ -3,6 +3,8 @@
 
 void flockfile(FILE *f)
 {
-	if (!libc.threaded) pthread_self();
-	__lockfile(f);
+	while (ftrylockfile(f)) {
+		int owner = f->lock;
+		if (owner) __wait(&f->lock, &f->waiters, owner, 1);
+	}
 }

src/stdio/fputc.c

@@ -2,9 +2,10 @@
 
 int fputc(int c, FILE *f)
 {
-	FLOCK(f);
+	if (f->lock < 0 || !__lockfile(f))
+		return putc_unlocked(c, f);
 	c = putc_unlocked(c, f);
-	FUNLOCK(f);
+	__unlockfile(f);
 	return c;
 }
 

src/stdio/ftell.c

@@ -3,10 +3,8 @@
 off_t __ftello_unlocked(FILE *f)
 {
 	off_t pos = f->seek(f, 0, SEEK_CUR);
-	if (pos < 0) {
-		FUNLOCK(f);
-		return pos;
-	}
+	if (pos < 0) return pos;
+
 	/* Adjust for data in buffer. */
 	return pos - (f->rend - f->rpos) + (f->wpos - f->wbase);
 }

src/stdio/ftrylockfile.c

@@ -5,11 +5,12 @@ int ftrylockfile(FILE *f)
 {
 	int tid = pthread_self()->tid;
 	if (f->lock == tid) {
-		if (f->lockcount == INT_MAX)
+		if (f->lockcount == LONG_MAX)
 			return -1;
 		f->lockcount++;
 		return 0;
 	}
+	if (f->lock < 0) f->lock = 0;
 	if (f->lock || a_cas(&f->lock, 0, tid))
 		return -1;
 	f->lockcount = 1;

src/stdio/funlockfile.c

@@ -3,5 +3,5 @@
 
 void funlockfile(FILE *f)
 {
-	FUNLOCK(f);
+	if (!--f->lockcount) __unlockfile(f);
 }

src/stdio/stderr.c

@@ -10,5 +10,7 @@ static FILE f = {
 	.write = __stdio_write,
 	.seek = __stdio_seek,
 	.close = __stdio_close,
+	.lock = -1,
 };
 FILE *const stderr = &f;
+FILE *const __stderr_used = &f;

src/stdio/stdin.c

@@ -9,5 +9,7 @@ static FILE f = {
 	.read = __stdio_read,
 	.seek = __stdio_seek,
 	.close = __stdio_close,
+	.lock = -1,
 };
 FILE *const stdin = &f;
+FILE *const __stdin_used = &f;

src/stdio/stdout.c

@@ -10,8 +10,7 @@ static FILE f = {
 	.write = __stdout_write,
 	.seek = __stdio_seek,
 	.close = __stdio_close,
+	.lock = -1,
 };
 FILE *const stdout = &f;
-
-/* overrides symbol in fflush.c, used for flushing NULL */
-FILE *const __stdout_to_flush = &f;
+FILE *const __stdout_used = &f;

src/thread/pthread_create.c

@@ -1,4 +1,5 @@
 #include "pthread_impl.h"
+#include "stdio_impl.h"
 
 static void dummy_0()
 {
@@ -60,6 +61,16 @@ int __uniclone(void *, int (*)(), void *);
 static const size_t dummy = 0;
 weak_alias(dummy, __pthread_tsd_size);
 
+static FILE *const dummy_file = 0;
+weak_alias(dummy_file, __stdin_used);
+weak_alias(dummy_file, __stdout_used);
+weak_alias(dummy_file, __stderr_used);
+
+static void init_file_lock(FILE *f)
+{
+	if (f && f->lock<0) f->lock = 0;
+}
+
 int pthread_create(pthread_t *res, const pthread_attr_t *attr, void *(*entry)(void *), void *arg)
 {
 	int ret;
@@ -70,6 +81,11 @@ int pthread_create(pthread_t *res, const pthread_attr_t *attr, void *(*entry)(vo
 
 	if (!self) return ENOSYS;
 	if (!libc.threaded) {
+		for (FILE *f=libc.ofl_head; f; f=f->next)
+			init_file_lock(f);
+		init_file_lock(__stdin_used);
+		init_file_lock(__stdout_used);
+		init_file_lock(__stderr_used);
 		__syscall(SYS_rt_sigprocmask, SIG_UNBLOCK, SIGPT_SET, 0, 8);
 		libc.threaded = 1;
 	}

src/thread/pthread_key_create.c

@@ -14,7 +14,7 @@ int pthread_key_create(pthread_key_t *k, void (*dtor)(void *))
 	unsigned i = (uintptr_t)&k / 16 % PTHREAD_KEYS_MAX;
 	unsigned j = i;
 
-	pthread_self();
+	__pthread_self_init();
 	if (!dtor) dtor = nodtor;
 	do {
 		if (!a_cas_p(keys+j, 0, dtor)) {

src/thread/pthread_self.c

@@ -35,3 +35,5 @@ pthread_t pthread_self()
 	}
 	return __pthread_self();
 }
+
+weak_alias(pthread_self, __pthread_self_init);