overhaul pthread cancellation

this patch improves the correctness, simplicity, and size of
cancellation-related code. modulo any small errors, it should now be
completely conformant, safe, and resource-leak free.

the notion of entering and exiting cancellation-point context has been
completely eliminated and replaced with alternative syscall assembly
code for cancellable syscalls. the assembly is responsible for setting
up execution context information (stack pointer and address of the
syscall instruction) which the cancellation signal handler can use to
determine whether the interrupted code was in a cancellable state.

these changes eliminate race conditions in the previous generation of
cancellation handling code (whereby a cancellation request received
just prior to the syscall would not be processed, leaving the syscall
to block, potentially indefinitely), and remedy an issue where
non-cancellable syscalls made from signal handlers became cancellable
if the signal handler interrupted a cancellation point.

x86_64 asm is untested and may need a second try to get it right.

8 files changed, 9 insertions, 38 deletions

diff --git a/src/unistd/close.c b/src/unistd/close.c
index f52c0ef..231f79e 100644
--- a/src/unistd/close.c
+++ b/src/unistd/close.c
@@ -4,8 +4,7 @@
 
 int close(int fd)
 {
-	int ret = syscall(SYS_close, fd);
-	CANCELPT_BEGIN;
-	CANCELPT_END;
+	int ret = syscall_cp(SYS_close, fd);
+	if (libc.testcancel) libc.testcancel();
 	return ret;
 }
diff --git a/src/unistd/pause.c b/src/unistd/pause.c
index 57ed25e..f7ed17d 100644
--- a/src/unistd/pause.c
+++ b/src/unistd/pause.c
@@ -4,9 +4,5 @@
 
 int pause(void)
 {
-	int r;
-	CANCELPT_BEGIN;
-	r = syscall(SYS_pause);
-	CANCELPT_END;
-	return r;
+	return syscall_cp(SYS_pause);
 }
diff --git a/src/unistd/pread.c b/src/unistd/pread.c
index 0a04501..1bf0c75 100644
--- a/src/unistd/pread.c
+++ b/src/unistd/pread.c
@@ -4,11 +4,7 @@
 
 ssize_t pread(int fd, void *buf, size_t size, off_t ofs)
 {
-	ssize_t r;
-	CANCELPT_BEGIN;
-	r = syscall(SYS_pread, fd, buf, size, __SYSCALL_LL(ofs));
-	CANCELPT_END;
-	return r;
+	return syscall_cp(SYS_pread, fd, buf, size, __SYSCALL_LL(ofs));
 }
 
 LFS64(pread);
diff --git a/src/unistd/pwrite.c b/src/unistd/pwrite.c
index f878bb6..224eacd 100644
--- a/src/unistd/pwrite.c
+++ b/src/unistd/pwrite.c
@@ -4,11 +4,7 @@
 
 ssize_t pwrite(int fd, const void *buf, size_t size, off_t ofs)
 {
-	ssize_t r;
-	CANCELPT_BEGIN;
-	r = syscall(SYS_pwrite, fd, buf, size, __SYSCALL_LL(ofs));
-	CANCELPT_END;
-	return r;
+	return syscall_cp(SYS_pwrite, fd, buf, size, __SYSCALL_LL(ofs));
 }
 
 LFS64(pwrite);
diff --git a/src/unistd/read.c b/src/unistd/read.c
index 194b389..eb882fc 100644
--- a/src/unistd/read.c
+++ b/src/unistd/read.c
@@ -4,9 +4,5 @@
 
 ssize_t read(int fd, void *buf, size_t count)
 {
-	ssize_t r;
-	CANCELPT_BEGIN;
-	r = syscall(SYS_read, fd, buf, count);
-	CANCELPT_END;
-	return r;
+	return syscall_cp(SYS_read, fd, buf, count);
 }
diff --git a/src/unistd/readv.c b/src/unistd/readv.c
index 9b87728..e45cb48 100644
--- a/src/unistd/readv.c
+++ b/src/unistd/readv.c
@@ -4,9 +4,5 @@
 
 ssize_t readv(int fd, const struct iovec *iov, int count)
 {
-	ssize_t r;
-	CANCELPT_BEGIN;
-	r = syscall(SYS_readv, fd, iov, count);
-	CANCELPT_END;
-	return r;
+	return syscall_cp(SYS_readv, fd, iov, count);
 }
diff --git a/src/unistd/write.c b/src/unistd/write.c
index a8284b3..e2f7e1f 100644
--- a/src/unistd/write.c
+++ b/src/unistd/write.c
@@ -4,9 +4,5 @@
 
 ssize_t write(int fd, const void *buf, size_t count)
 {
-	int r;
-	CANCELPT_BEGIN;
-	r = syscall(SYS_write, fd, buf, count);
-	CANCELPT_END;
-	return r;
+	return syscall_cp(SYS_write, fd, buf, count);
 }
diff --git a/src/unistd/writev.c b/src/unistd/writev.c
index a45afeb..ef300dd 100644
--- a/src/unistd/writev.c
+++ b/src/unistd/writev.c
@@ -4,9 +4,5 @@
 
 ssize_t writev(int fd, const struct iovec *iov, int count)
 {
-	ssize_t r;
-	CANCELPT_BEGIN;
-	r = syscall(SYS_writev, fd, iov, count);
-	CANCELPT_END;
-	return r;
+	return syscall_cp(SYS_writev, fd, iov, count);
 }