[PATCH liburing 0/3] Three small liburing patches

[PATCH liburing 0/3] Three small liburing patches

[Bart Van Assche]

Hi Jens,

The changes in this patch series are as follows:
- Enable additional compiler warnings, similar to W=1 for kernel code.
- Simplify the barrier.h header file.
- Convert a macro into an inline function to allow compilers to produce
  better diagnostics.

Please consider this patch series for the official liburing repository.

Thanks,

Bart.

Bart Van Assche (3):
  Makefiles: Enable -Wextra
  src/include/liburing/barrier.h: Use C11 atomics
  Convert __io_uring_get_sqe() from a macro into an inline function

 src/Makefile                   |  3 ++-
 src/include/liburing/barrier.h | 44 ++++++++--------------------------
 src/queue.c                    | 22 +++++++++--------
 test/Makefile                  |  4 +++-
 4 files changed, 27 insertions(+), 46 deletions(-)

[PATCH liburing 1/3] Makefiles: Enable -Wextra

[Bart Van Assche]

Enable -Wextra and disable the unused parameter and different signedness
in comparison warnings.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
 src/Makefile  | 3 ++-
 test/Makefile | 4 +++-
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/src/Makefile b/src/Makefile
index 9bbbacf03d59..3099f7cd51ec 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -4,7 +4,8 @@ libdir ?= $(prefix)/lib
 libdevdir ?= $(prefix)/lib
 
 CFLAGS ?= -g -fomit-frame-pointer -O2
-override CFLAGS += -Wall -Iinclude/ -include ../config-host.h
+override CFLAGS += -Wall -Wextra -Wno-unused-parameter -Wno-sign-compare\
+	-Iinclude/ -include ../config-host.h
 SO_CFLAGS=-shared -fPIC $(CFLAGS)
 L_CFLAGS=$(CFLAGS)
 LINK_FLAGS=
diff --git a/test/Makefile b/test/Makefile
index 93e58f4ba9b0..e103296fabdd 100644
--- a/test/Makefile
+++ b/test/Makefile
@@ -5,7 +5,9 @@ INSTALL=install
 
 CFLAGS ?= -g -O2
 XCFLAGS =
-override CFLAGS += -Wall -D_GNU_SOURCE -L../src/ -I../src/include/ -include ../config-host.h -D__SANE_USERSPACE_TYPES__
+override CFLAGS += -Wall -Wextra -Wno-unused-parameter -Wno-sign-compare\
+	-D_GNU_SOURCE -D__SANE_USERSPACE_TYPES__ -L../src/ \
+	-I../src/include/ -include ../config-host.h
 
 all_targets += poll poll-cancel ring-leak fsync io_uring_setup io_uring_register \
 	       io_uring_enter nop sq-full cq-full 35fa71a030ca-test \

[PATCH liburing 2/3] src/include/liburing/barrier.h: Use C11 atomics

[Bart Van Assche]

Instead of using a combination of open-coding atomic primitives and using
gcc builtins, use C11 atomics for all CPU architectures. Note: despite their
name, atomic_*() operations do not necessarily translate into an atomic
instruction. This patch changes the order of the instructions in e.g.
io_uring_get_sqe() but not the number of instructions generated by gcc 10
on x86_64:

Without this patch:

   0x0000000000000360 <+0>:     mov    0x44(%rdi),%eax
   0x0000000000000363 <+3>:     lea    0x1(%rax),%edx
   0x0000000000000366 <+6>:     mov    (%rdi),%rax
   0x0000000000000369 <+9>:     mov    (%rax),%eax
   0x000000000000036b <+11>:    mov    0x18(%rdi),%rcx
   0x000000000000036f <+15>:    mov    %edx,%esi
   0x0000000000000371 <+17>:    sub    %eax,%esi
   0x0000000000000373 <+19>:    xor    %eax,%eax
   0x0000000000000375 <+21>:    cmp    (%rcx),%esi
   0x0000000000000377 <+23>:    ja     0x38d <io_uring_get_sqe+45>
   0x0000000000000379 <+25>:    mov    0x10(%rdi),%rax
   0x000000000000037d <+29>:    mov    (%rax),%eax
   0x000000000000037f <+31>:    and    0x44(%rdi),%eax
   0x0000000000000382 <+34>:    mov    %edx,0x44(%rdi)
   0x0000000000000385 <+37>:    shl    $0x6,%rax
   0x0000000000000389 <+41>:    add    0x38(%rdi),%rax
   0x000000000000038d <+45>:    retq

With this patch applied:

   0x0000000000000360 <+0>:     mov    0x44(%rdi),%eax
   0x0000000000000363 <+3>:     lea    0x1(%rax),%edx
   0x0000000000000366 <+6>:     mov    (%rdi),%rax
   0x0000000000000369 <+9>:     mov    %edx,%esi
   0x000000000000036b <+11>:    mov    (%rax),%eax
   0x000000000000036d <+13>:    sub    %eax,%esi
   0x000000000000036f <+15>:    xor    %eax,%eax
   0x0000000000000371 <+17>:    mov    0x18(%rdi),%rcx
   0x0000000000000375 <+21>:    cmp    (%rcx),%esi
   0x0000000000000377 <+23>:    ja     0x38d <io_uring_get_sqe+45>
   0x0000000000000379 <+25>:    mov    0x10(%rdi),%rax
   0x000000000000037d <+29>:    mov    (%rax),%eax
   0x000000000000037f <+31>:    and    0x44(%rdi),%eax
   0x0000000000000382 <+34>:    mov    %edx,0x44(%rdi)
   0x0000000000000385 <+37>:    shl    $0x6,%rax
   0x0000000000000389 <+41>:    add    0x38(%rdi),%rax
   0x000000000000038d <+45>:    retq

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
 src/include/liburing/barrier.h | 44 ++++++++--------------------------
 1 file changed, 10 insertions(+), 34 deletions(-)

diff --git a/src/include/liburing/barrier.h b/src/include/liburing/barrier.h
index ad69506bb248..c8aa4210371c 100644
--- a/src/include/liburing/barrier.h
+++ b/src/include/liburing/barrier.h
@@ -2,6 +2,8 @@
 #ifndef LIBURING_BARRIER_H
 #define LIBURING_BARRIER_H
 
+#include <stdatomic.h>
+
 /*
 From the kernel documentation file refcount-vs-atomic.rst:
 
@@ -21,40 +23,14 @@ after the acquire operation executes. This is implemented using
 :c:func:`smp_acquire__after_ctrl_dep`.
 */
 
-/* From tools/include/linux/compiler.h */
-/* Optimization barrier */
-/* The "volatile" is due to gcc bugs */
-#define io_uring_barrier()	__asm__ __volatile__("": : :"memory")
-
-/* From tools/virtio/linux/compiler.h */
-#define IO_URING_WRITE_ONCE(var, val) \
-	(*((volatile __typeof(val) *)(&(var))) = (val))
-#define IO_URING_READ_ONCE(var) (*((volatile __typeof(var) *)(&(var))))
-
+#define IO_URING_WRITE_ONCE(var, val)				\
+	atomic_store_explicit(&(var), (val), memory_order_relaxed)
+#define IO_URING_READ_ONCE(var)					\
+	atomic_load_explicit(&(var), memory_order_relaxed)
 
-#if defined(__x86_64__) || defined(__i386__)
-/* Adapted from arch/x86/include/asm/barrier.h */
-#define io_uring_smp_store_release(p, v)	\
-do {						\
-	io_uring_barrier();			\
-	IO_URING_WRITE_ONCE(*(p), (v));		\
-} while (0)
-
-#define io_uring_smp_load_acquire(p)			\
-({							\
-	__typeof(*p) ___p1 = IO_URING_READ_ONCE(*(p));	\
-	io_uring_barrier();				\
-	___p1;						\
-})
-
-#else /* defined(__x86_64__) || defined(__i386__) */
-/*
- * Add arch appropriate definitions. Use built-in atomic operations for
- * archs we don't have support for.
- */
-#define io_uring_smp_store_release(p, v) \
-	__atomic_store_n(p, v, __ATOMIC_RELEASE)
-#define io_uring_smp_load_acquire(p) __atomic_load_n(p, __ATOMIC_ACQUIRE)
-#endif /* defined(__x86_64__) || defined(__i386__) */
+#define io_uring_smp_store_release(p, v)			\
+	atomic_store_explicit((p), (v), memory_order_release)
+#define io_uring_smp_load_acquire(p)				\
+	atomic_load_explicit((p), memory_order_acquire)
 
 #endif /* defined(LIBURING_BARRIER_H) */

[PATCH liburing 3/3] Convert __io_uring_get_sqe() from a macro into an inline function

[Bart Van Assche]

This patch makes gcc 10 insert one additional assembly instruction in
io_uring_get_sqe(), namely xor %r8d,%r8d. That shouldn't cause any slowdown
since that instruction does not access memory:

   0x0000000000000360 <+0>:     mov    (%rdi),%rax
   0x0000000000000363 <+3>:     xor    %r8d,%r8d
   0x0000000000000366 <+6>:     mov    (%rax),%ecx
   0x0000000000000368 <+8>:     mov    0x44(%rdi),%eax
   0x000000000000036b <+11>:    lea    0x1(%rax),%edx
   0x000000000000036e <+14>:    mov    %edx,%esi
   0x0000000000000370 <+16>:    sub    %ecx,%esi
   0x0000000000000372 <+18>:    mov    0x18(%rdi),%rcx
   0x0000000000000376 <+22>:    cmp    (%rcx),%esi
   0x0000000000000378 <+24>:    ja     0x38e <io_uring_get_sqe+46>
   0x000000000000037a <+26>:    mov    0x10(%rdi),%rcx
   0x000000000000037e <+30>:    and    (%rcx),%eax
   0x0000000000000380 <+32>:    mov    %edx,0x44(%rdi)
   0x0000000000000383 <+35>:    shl    $0x6,%rax
   0x0000000000000387 <+39>:    add    0x38(%rdi),%rax
   0x000000000000038b <+43>:    mov    %rax,%r8
   0x000000000000038e <+46>:    mov    %r8,%rax
   0x0000000000000391 <+49>:    retq

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
 src/queue.c | 22 ++++++++++++----------
 1 file changed, 12 insertions(+), 10 deletions(-)

diff --git a/src/queue.c b/src/queue.c
index 3db52bd17b86..88e0294c19fb 100644
--- a/src/queue.c
+++ b/src/queue.c
@@ -273,16 +273,18 @@ int io_uring_submit_and_wait(struct io_uring *ring, unsigned wait_nr)
 	return __io_uring_submit_and_wait(ring, wait_nr);
 }
 
-#define __io_uring_get_sqe(sq, __head) ({				\
-	unsigned __next = (sq)->sqe_tail + 1;				\
-	struct io_uring_sqe *__sqe = NULL;				\
-									\
-	if (__next - __head <= *(sq)->kring_entries) {			\
-		__sqe = &(sq)->sqes[(sq)->sqe_tail & *(sq)->kring_mask];\
-		(sq)->sqe_tail = __next;				\
-	}								\
-	__sqe;								\
-})
+static inline struct io_uring_sqe *
+__io_uring_get_sqe(struct io_uring_sq *sq, unsigned int __head)
+{
+	unsigned int __next = (sq)->sqe_tail + 1;
+	struct io_uring_sqe *__sqe = NULL;
+
+	if (__next - __head <= *(sq)->kring_entries) {
+		__sqe = &(sq)->sqes[(sq)->sqe_tail & *(sq)->kring_mask];
+		(sq)->sqe_tail = __next;
+	}
+	return __sqe;
+}
 
 /*
  * Return an sqe to fill. Application must later call io_uring_submit()

Re: [PATCH liburing 0/3] Three small liburing patches

[Jens Axboe]

On 6/21/20 2:36 PM, Bart Van Assche wrote:
> Hi Jens,
> 
> The changes in this patch series are as follows:
> - Enable additional compiler warnings, similar to W=1 for kernel code.
> - Simplify the barrier.h header file.
> - Convert a macro into an inline function to allow compilers to produce
>   better diagnostics.
> 
> Please consider this patch series for the official liburing repository.

Thanks, applied.

-- 
Jens Axboe