[PATCH] small change to fget()

[Eric Dumazet]

Hello all

This one liner patch permits to save 2 instructions in the fget() 
function, at least on x86_64 SMP (only one ref to current->files)

diff -Nru linux-2.6.2/fs/file_table.c linux-2.6.2-custom/fs/file_table.c
--- linux-2.6.2/fs/file_table.c     2004-02-04 17:59:43.000000000 +0100
+++ linux-2.6.2-custom/fs/file_table.c        2004-03-29 
11:26:42.170555000 +0200
@@ -198,7 +198,7 @@
       struct files_struct *files = current->files;

       spin_lock(&files->file_lock);
-       file = fcheck(fd);
+       file = fcheck_files(files,fd) ;
       if (file)
               get_file(file);
       spin_unlock(&files->file_lock);


The assembly code was (oprofile data included) :

ffffffff8016c3d0 <fget>: /* fget total: 921168  0.1624 */
51191  0.0090 :ffffffff8016c3d0:       mov    %gs:0x0,%rax
76621  0.0135 :ffffffff8016c3d9:       mov    0x7e8(%rax),%rsi
121592  0.0214 :ffffffff8016c3e0:       lock decb 0x4(%rsi)
11278  0.0020 :ffffffff8016c3e4:       js     ffffffff8016c5e8 
<.text.lock.file_table+0x20>
122737  0.0216 :ffffffff8016c3ea:       mov    %gs:0x0,%rax
174870  0.0308 :ffffffff8016c3f3:       mov    0x7e8(%rax),%rcx
 2274 4.0e-04 :ffffffff8016c3fa:       xor    %eax,%eax
77418  0.0137 :ffffffff8016c3fc:       cmp    0x8(%rcx),%edi
 3417 6.0e-04 :ffffffff8016c3ff:       jae    ffffffff8016c40b <fget+0x3b>
 8184  0.0014 :ffffffff8016c401:       mov    0x18(%rcx),%rax
37585  0.0066 :ffffffff8016c405:       mov    %edi,%edx
 7357  0.0013 :ffffffff8016c407:       mov    (%rax,%rdx,8),%rax
 5817  0.0010 :ffffffff8016c40b:       test   %rax,%rax
57910  0.0102 :ffffffff8016c40e:       je     ffffffff8016c414 <fget+0x44>
 3401 6.0e-04 :ffffffff8016c410:       lock incl 0x28(%rax)
 7729  0.0014 :ffffffff8016c414:       movb   $0x1,0x4(%rsi)
151787  0.0268 :ffffffff8016c418:       retq



And it is now :

380:   65 48 8b 04 25 00 00    mov    %gs:0x0,%rax
387:   00 00
389:   48 8b 88 e8 07 00 00    mov    0x7e8(%rax),%rcx
390:   f0 fe 49 04             lock decb 0x4(%rcx)
394:   0f 88 ee 01 00 00       js     588 <.text.lock.file_table+0x20>
39a:   31 c0                   xor    %eax,%eax
39c:   3b 79 08                cmp    0x8(%rcx),%edi
39f:   73 0a                   jae    3ab <fget+0x2b>
3a1:   48 8b 41 18             mov    0x18(%rcx),%rax
3a5:   89 fa                   mov    %edi,%edx
3a7:   48 8b 04 d0             mov    (%rax,%rdx,8),%rax
3ab:   48 85 c0                test   %rax,%rax
3ae:   74 04                   je     3b4 <fget+0x34>
3b0:   f0 ff 40 28             lock incl 0x28(%rax)
3b4:   c6 41 04 01             movb   $0x1,0x4(%rcx)
3b8:   c3                      retq


Thanks

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