From dba68bf98fc708cea4c478278c889fc7ad802b00 Mon Sep 17 00:00:00 2001
From: Rich Felker <dalias@aerifal.cx>
Date: Sat, 30 Jul 2011 08:02:14 -0400
Subject: 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             |  3 ++-
 src/internal/pthread_impl.h     |  2 ++
 src/internal/stdio_impl.h       |  9 ++++-----
 src/stdio/__fdopen.c            |  8 +++++---
 src/stdio/__lockfile.c          | 26 ++++++++++++--------------
 src/stdio/fclose.c              |  2 +-
 src/stdio/fflush.c              | 10 +++++-----
 src/stdio/fgetc.c               |  5 +++--
 src/stdio/fgetwc.c              |  1 -
 src/stdio/flockfile.c           |  6 ++++--
 src/stdio/fputc.c               |  5 +++--
 src/stdio/ftell.c               |  6 ++----
 src/stdio/ftrylockfile.c        |  3 ++-
 src/stdio/funlockfile.c         |  2 +-
 src/stdio/stderr.c              |  2 ++
 src/stdio/stdin.c               |  2 ++
 src/stdio/stdout.c              |  5 ++---
 src/thread/pthread_create.c     | 16 ++++++++++++++++
 src/thread/pthread_key_create.c |  2 +-
 src/thread/pthread_self.c       |  2 ++
 20 files changed, 71 insertions(+), 46 deletions(-)

diff --git a/src/internal/libc.h b/src/internal/libc.h
index 906de2ca..929ff97a 100644
--- a/src/internal/libc.h
+++ b/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)
 
diff --git a/src/internal/pthread_impl.h b/src/internal/pthread_impl.h
index e6089f02..03af4c12 100644
--- a/src/internal/pthread_impl.h
+++ b/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 *);
diff --git a/src/internal/stdio_impl.h b/src/internal/stdio_impl.h
index 76b58be9..c5f45eb1 100644
--- a/src/internal/stdio_impl.h
+++ b/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)
diff --git a/src/stdio/__fdopen.c b/src/stdio/__fdopen.c
index b13318e5..07f966a5 100644
--- a/src/stdio/__fdopen.c
+++ b/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;
diff --git a/src/stdio/__lockfile.c b/src/stdio/__lockfile.c
index 66a4d26d..6ebf6202 100644
--- a/src/stdio/__lockfile.c
+++ b/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;
+	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;
+}
 
-	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;
+void __unlockfile(FILE *f)
+{
+	a_store(&f->lock, 0);
+	if (f->waiters) __wake(&f->lock, 1, 1);
 }
diff --git a/src/stdio/fclose.c b/src/stdio/fclose.c
index ee772fbb..373a2c76 100644
--- a/src/stdio/fclose.c
+++ b/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();
 	}
 
diff --git a/src/stdio/fflush.c b/src/stdio/fflush.c
index cdbd39bc..4c1647b7 100644
--- a/src/stdio/fflush.c
+++ b/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);
diff --git a/src/stdio/fgetc.c b/src/stdio/fgetc.c
index da638684..4d8aca37 100644
--- a/src/stdio/fgetc.c
+++ b/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;
 }
 
diff --git a/src/stdio/fgetwc.c b/src/stdio/fgetwc.c
index 5e420594..6f9f9ec2 100644
--- a/src/stdio/fgetwc.c
+++ b/src/stdio/fgetwc.c
@@ -34,7 +34,6 @@ wint_t __fgetwc_unlocked(FILE *f)
 		if (l == -1) return WEOF;
 	}
 
-	FUNLOCK(f);
 	return wc;
 }
 
diff --git a/src/stdio/flockfile.c b/src/stdio/flockfile.c
index 0d4c92c2..a196c1ef 100644
--- a/src/stdio/flockfile.c
+++ b/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);
+	}
 }
diff --git a/src/stdio/fputc.c b/src/stdio/fputc.c
index 98d0a20a..6a144a54 100644
--- a/src/stdio/fputc.c
+++ b/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;
 }
 
diff --git a/src/stdio/ftell.c b/src/stdio/ftell.c
index aa1f5381..3904a1d8 100644
--- a/src/stdio/ftell.c
+++ b/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);
 }
diff --git a/src/stdio/ftrylockfile.c b/src/stdio/ftrylockfile.c
index 0b0e44aa..725c4c3e 100644
--- a/src/stdio/ftrylockfile.c
+++ b/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;
diff --git a/src/stdio/funlockfile.c b/src/stdio/funlockfile.c
index d69f68ee..f8a2a071 100644
--- a/src/stdio/funlockfile.c
+++ b/src/stdio/funlockfile.c
@@ -3,5 +3,5 @@
 
 void funlockfile(FILE *f)
 {
-	FUNLOCK(f);
+	if (!--f->lockcount) __unlockfile(f);
 }
diff --git a/src/stdio/stderr.c b/src/stdio/stderr.c
index 9a70700c..3fd8f81d 100644
--- a/src/stdio/stderr.c
+++ b/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;
diff --git a/src/stdio/stdin.c b/src/stdio/stdin.c
index 51c99232..476dc708 100644
--- a/src/stdio/stdin.c
+++ b/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;
diff --git a/src/stdio/stdout.c b/src/stdio/stdout.c
index 552d729e..3855dd0b 100644
--- a/src/stdio/stdout.c
+++ b/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;
diff --git a/src/thread/pthread_create.c b/src/thread/pthread_create.c
index 856015ff..adef510c 100644
--- a/src/thread/pthread_create.c
+++ b/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;
 	}
diff --git a/src/thread/pthread_key_create.c b/src/thread/pthread_key_create.c
index c9ca48ab..e51cb023 100644
--- a/src/thread/pthread_key_create.c
+++ b/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)) {
diff --git a/src/thread/pthread_self.c b/src/thread/pthread_self.c
index 55d20c9f..9f885d94 100644
--- a/src/thread/pthread_self.c
+++ b/src/thread/pthread_self.c
@@ -35,3 +35,5 @@ pthread_t pthread_self()
 	}
 	return __pthread_self();
 }
+
+weak_alias(pthread_self, __pthread_self_init);
-- 
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