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Re: [PATCH] Avoid reading errno in syscall implementations

From: "H.J. Lu" <hjl dot tools at gmail dot com>
To: Joseph Myers <joseph at codesourcery dot com>
Cc: GNU C Library <libc-alpha at sourceware dot org>
Date: Mon, 19 Oct 2015 10:49:17 -0700
Subject: Re: [PATCH] Avoid reading errno in syscall implementations
Authentication-results: sourceware.org; auth=none
References: <20151019164832 dot GA28619 at intel dot com> <alpine dot DEB dot 2 dot 10 dot 1510191653500 dot 23235 at digraph dot polyomino dot org dot uk>

On Mon, Oct 19, 2015 at 10:00 AM, Joseph Myers <joseph@codesourcery.com> wrote:
> On Mon, 19 Oct 2015, H.J. Lu wrote:
>
>> Reading errno is expensive for x86 PIC.  With INTERNAL_SYSCALL,
>> INTERNAL_SYSCALL_ERROR_P, INTERNAL_SYSCALL_ERRNO and
>> INLINE_SYSCALL_ERROR_RETURN_VALUE, we can avoid reading errno.
>
> I don't follow how this patch works.  How do you ensure that in the cases
> where there is an error that is not ENOSYS, errno does get set as it would
> have been before?
>
>> -  int res = INLINE_SYSCALL (eventfd2, 2, count, flags);
>>  # ifndef __ASSUME_EVENTFD2
>> -  if (res != -1 || errno != ENOSYS)
>> -# endif
>> +  INTERNAL_SYSCALL_DECL (err);
>> +  int res = INTERNAL_SYSCALL (eventfd2, err, 2, count, flags);
>> +  if (!__glibc_unlikely (INTERNAL_SYSCALL_ERROR_P (res, err))
>> +      || INTERNAL_SYSCALL_ERRNO (res, err) != ENOSYS)
>>      return res;
>
> E.g. this appears to be semantically different from the previous code -
> the previous code would have set errno here from a non-ENOSYS error, and
> the new code wouldn't.
>
> (__ASSUME_EVENTFD2 is always defined except on alpha.  Presumably your "no
> code changes" was because code paths used on x86_64 didn't get changed by
> the patch.)

Here is the updated patch.  Only getrlimit64.os and setrlimit64.os
are changed on i686.  OK for master?

Thanks.


-- 
H.J.

From 6f6e3ee567cbff328df83dcf9ee0a5459746fce0 Mon Sep 17 00:00:00 2001
From: "H.J. Lu" <hjl.tools@gmail.com>
Date: Fri, 21 Aug 2015 14:46:05 -0700
Subject: [PATCH] Avoid reading errno in syscall implementations

Reading errno is expensive for x86 PIC.  With INTERNAL_SYSCALL,
INTERNAL_SYSCALL_ERROR_P, INTERNAL_SYSCALL_ERRNO and
INLINE_SYSCALL_ERROR_RETURN_VALUE, we can avoid reading errno.

There are no code changes on x86-64.  On i686, libc.so sizes in bytes
show:

        text	   data	    bss	    dec
after  1748495	  11380	  11132	  1771007
before 1748403	  11380	  11132	  1770915

	* sysdeps/unix/sysv/linux/eventfd.c (eventfd): Use
	INTERNAL_SYSCALL, INTERNAL_SYSCALL_ERROR_P and
	INTERNAL_SYSCALL_ERRNO to avoid reading errno.
	* sysdeps/unix/sysv/linux/fstatfs64.c (__fstatfs64): Likewise.
	* sysdeps/unix/sysv/linux/getrlimit64.c (__getrlimit64):
	Likewise.
	* sysdeps/unix/sysv/linux/setrlimit64.c (setrlimit64):
	Likewise.
	* sysdeps/unix/sysv/linux/signalfd.c (signalfd): Likewise.
	* sysdeps/unix/sysv/linux/statfs64.c (__statfs64): Likewise.
---
 sysdeps/unix/sysv/linux/eventfd.c     | 11 ++++++++---
 sysdeps/unix/sysv/linux/fstatfs64.c   | 13 +++++++++----
 sysdeps/unix/sysv/linux/getrlimit64.c |  8 ++++++--
 sysdeps/unix/sysv/linux/lxstat64.c    |  7 +++++--
 sysdeps/unix/sysv/linux/setrlimit64.c |  8 ++++++--
 sysdeps/unix/sysv/linux/signalfd.c    | 12 +++++++++---
 sysdeps/unix/sysv/linux/statfs64.c    | 14 ++++++++++----
 7 files changed, 53 insertions(+), 20 deletions(-)

diff --git a/sysdeps/unix/sysv/linux/eventfd.c b/sysdeps/unix/sysv/linux/eventfd.c
index efce282..3a91781 100644
--- a/sysdeps/unix/sysv/linux/eventfd.c
+++ b/sysdeps/unix/sysv/linux/eventfd.c
@@ -25,11 +25,16 @@ int
 eventfd (unsigned int count, int flags)
 {
 #ifdef __NR_eventfd2
-  int res = INLINE_SYSCALL (eventfd2, 2, count, flags);
 # ifndef __ASSUME_EVENTFD2
-  if (res != -1 || errno != ENOSYS)
-# endif
+  INTERNAL_SYSCALL_DECL (err);
+  int res = INTERNAL_SYSCALL (eventfd2, err, 2, count, flags);
+  if (!__glibc_unlikely (INTERNAL_SYSCALL_ERROR_P (res, err)))
     return res;
+  else if (INTERNAL_SYSCALL_ERRNO (res, err) != ENOSYS)
+    return INLINE_SYSCALL_ERROR_RETURN_VALUE (INTERNAL_SYSCALL_ERRNO (res, err));
+# else
+  return INLINE_SYSCALL (eventfd2, 2, count, flags);
+# endif
 #endif
 
 #ifndef __ASSUME_EVENTFD2
diff --git a/sysdeps/unix/sysv/linux/fstatfs64.c b/sysdeps/unix/sysv/linux/fstatfs64.c
index af83830..819b623 100644
--- a/sysdeps/unix/sysv/linux/fstatfs64.c
+++ b/sysdeps/unix/sysv/linux/fstatfs64.c
@@ -35,12 +35,17 @@ __fstatfs64 (int fd, struct statfs64 *buf)
   if (! __no_statfs64)
 # endif
     {
-      int result = INLINE_SYSCALL (fstatfs64, 3, fd, sizeof (*buf), buf);
-
 # if __ASSUME_STATFS64 == 0
-      if (result == 0 || errno != ENOSYS)
-# endif
+      INTERNAL_SYSCALL_DECL (err);
+      int result = INTERNAL_SYSCALL (fstatfs64, err, 3, fd,
+				     sizeof (*buf), buf);
+      if (!__glibc_unlikely (INTERNAL_SYSCALL_ERROR_P (result, err)))
 	return result;
+      else if (INTERNAL_SYSCALL_ERRNO (result, err) != ENOSYS)
+	return INLINE_SYSCALL_ERROR_RETURN_VALUE (INTERNAL_SYSCALL_ERRNO (result, err));
+# else
+      return INLINE_SYSCALL (fstatfs64, 3, fd, sizeof (*buf), buf);
+# endif
 
 # if __ASSUME_STATFS64 == 0
       __no_statfs64 = 1;
diff --git a/sysdeps/unix/sysv/linux/getrlimit64.c b/sysdeps/unix/sysv/linux/getrlimit64.c
index 100ba62..bc36c14 100644
--- a/sysdeps/unix/sysv/linux/getrlimit64.c
+++ b/sysdeps/unix/sysv/linux/getrlimit64.c
@@ -30,9 +30,13 @@ __getrlimit64 (enum __rlimit_resource resource, struct rlimit64 *rlimits)
   return INLINE_SYSCALL (prlimit64, 4, 0, resource, NULL, rlimits);
 #else
 # ifdef __NR_prlimit64
-  int res = INLINE_SYSCALL (prlimit64, 4, 0, resource, NULL, rlimits);
-  if (res == 0 || errno != ENOSYS)
+  INTERNAL_SYSCALL_DECL (err);
+  int res = INTERNAL_SYSCALL (prlimit64, err, 4, 0, resource, NULL,
+			      rlimits);
+  if (!__glibc_unlikely (INTERNAL_SYSCALL_ERROR_P (res, err)))
     return res;
+  else if (INTERNAL_SYSCALL_ERRNO (res, err) != ENOSYS)
+    return INLINE_SYSCALL_ERROR_RETURN_VALUE (INTERNAL_SYSCALL_ERRNO (res, err));
 # endif
   struct rlimit rlimits32;
 
diff --git a/sysdeps/unix/sysv/linux/lxstat64.c b/sysdeps/unix/sysv/linux/lxstat64.c
index 5d0c051..63959cf 100644
--- a/sysdeps/unix/sysv/linux/lxstat64.c
+++ b/sysdeps/unix/sysv/linux/lxstat64.c
@@ -30,8 +30,11 @@
 int
 ___lxstat64 (int vers, const char *name, struct stat64 *buf)
 {
-  int result;
-  result = INLINE_SYSCALL (lstat64, 2, name, buf);
+  INTERNAL_SYSCALL_DECL (err);
+  int result = INTERNAL_SYSCALL (lstat64, err, 2, name, buf);
+  if (__glibc_unlikely (INTERNAL_SYSCALL_ERROR_P (result, err)))
+    return INLINE_SYSCALL_ERROR_RETURN_VALUE (INTERNAL_SYSCALL_ERRNO (result,
+								      err));
 #if defined _HAVE_STAT64___ST_INO && __ASSUME_ST_INO_64_BIT == 0
   if (__builtin_expect (!result, 1) && buf->__st_ino != (__ino_t) buf->st_ino)
     buf->st_ino = buf->__st_ino;
diff --git a/sysdeps/unix/sysv/linux/setrlimit64.c b/sysdeps/unix/sysv/linux/setrlimit64.c
index 5b1f657..b13014a 100644
--- a/sysdeps/unix/sysv/linux/setrlimit64.c
+++ b/sysdeps/unix/sysv/linux/setrlimit64.c
@@ -31,9 +31,13 @@ setrlimit64 (enum __rlimit_resource resource, const struct rlimit64 *rlimits)
   return INLINE_SYSCALL (prlimit64, 4, 0, resource, rlimits, NULL);
 #else
 # ifdef __NR_prlimit64
-  int res = INLINE_SYSCALL (prlimit64, 4, 0, resource, rlimits, NULL);
-  if (res == 0 || errno != ENOSYS)
+  INTERNAL_SYSCALL_DECL (err);
+  int res = INTERNAL_SYSCALL (prlimit64, err, 4, 0, resource, rlimits,
+			      NULL);
+  if (!__glibc_unlikely (INTERNAL_SYSCALL_ERROR_P (res, err)))
     return res;
+  else if (INTERNAL_SYSCALL_ERRNO (res, err) != ENOSYS)
+    return INLINE_SYSCALL_ERROR_RETURN_VALUE (INTERNAL_SYSCALL_ERRNO (res, err));
 # endif
   struct rlimit rlimits32;
 
diff --git a/sysdeps/unix/sysv/linux/signalfd.c b/sysdeps/unix/sysv/linux/signalfd.c
index 8573450..95a9707 100644
--- a/sysdeps/unix/sysv/linux/signalfd.c
+++ b/sysdeps/unix/sysv/linux/signalfd.c
@@ -26,11 +26,17 @@ int
 signalfd (int fd, const sigset_t *mask, int flags)
 {
 #ifdef __NR_signalfd4
-  int res = INLINE_SYSCALL (signalfd4, 4, fd, mask, _NSIG / 8, flags);
 # ifndef __ASSUME_SIGNALFD4
-  if (res != -1 || errno != ENOSYS)
-# endif
+  INTERNAL_SYSCALL_DECL (err);
+  int res = INTERNAL_SYSCALL (signalfd4, err, 4, fd, mask, _NSIG / 8,
+			      flags);
+  if (!__glibc_unlikely (INTERNAL_SYSCALL_ERROR_P (res, err)))
     return res;
+  else if (INTERNAL_SYSCALL_ERRNO (res, err) != ENOSYS)
+    return INLINE_SYSCALL_ERROR_RETURN_VALUE (INTERNAL_SYSCALL_ERRNO (res, err));
+# else
+  return INLINE_SYSCALL (signalfd4, 4, fd, mask, _NSIG / 8, flags);
+# endif
 #endif
 
 #ifndef __ASSUME_SIGNALFD4
diff --git a/sysdeps/unix/sysv/linux/statfs64.c b/sysdeps/unix/sysv/linux/statfs64.c
index ac5c33f..469eda1 100644
--- a/sysdeps/unix/sysv/linux/statfs64.c
+++ b/sysdeps/unix/sysv/linux/statfs64.c
@@ -37,12 +37,18 @@ __statfs64 (const char *file, struct statfs64 *buf)
   if (! __no_statfs64)
 # endif
     {
-      int result = INLINE_SYSCALL (statfs64, 3, file, sizeof (*buf), buf);
-
 # if __ASSUME_STATFS64 == 0
-      if (result == 0 || errno != ENOSYS)
-# endif
+      INTERNAL_SYSCALL_DECL (err);
+      int result = INTERNAL_SYSCALL (statfs64, err, 3, file,
+				     sizeof (*buf), buf);
+
+      if (!__glibc_unlikely (INTERNAL_SYSCALL_ERROR_P (result, err)))
 	return result;
+      else if (INTERNAL_SYSCALL_ERRNO (result, err) != ENOSYS)
+	return INLINE_SYSCALL_ERROR_RETURN_VALUE (INTERNAL_SYSCALL_ERRNO (result, err));
+# else
+      return INLINE_SYSCALL (statfs64, 3, file, sizeof (*buf), buf);
+# endif
 
 # if __ASSUME_STATFS64 == 0
       __no_statfs64 = 1;
-- 
2.4.3

References:
- [PATCH] Avoid reading errno in syscall implementations
  - From: H.J. Lu
- Re: [PATCH] Avoid reading errno in syscall implementations
  - From: Joseph Myers

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