Re: [PATCH] GFS2: minor readability improvement in gfs2_mount()

Alexey Khoroshilov
2013-06-07T06:49:57Z

Hi Steve,

On 06/06/2013 01:24 PM, Steven Whitehouse wrote:
> Hi,
>
> On Thu, 2013-06-06 at 01:29 +0400, Alexey Khoroshilov wrote:
>> The patch makes usage pattern of IS_ERR-PTR_ERR more typical.
>>
>> Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
>> ---
>> fs/gfs2/ops_fstype.c | 5 +++--
>> 1 file changed, 3 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/gfs2/ops_fstype.c b/fs/gfs2/ops_fstype.c
>> index 0262c19..0d30e8e 100644
>> --- a/fs/gfs2/ops_fstype.c
>> +++ b/fs/gfs2/ops_fstype.c
>> @@ -1313,9 +1313,10 @@ static struct dentry *gfs2_mount(struct file_system_type *fs_type, int flags,
>> }
>> s = sget(fs_type, test_gfs2_super, set_gfs2_super, flags, bdev);
>> mutex_unlock(&bdev->bd_fsfreeze_mutex);
>> - error = PTR_ERR(s);
>> - if (IS_ERR(s))
>> + if (IS_ERR(s)) {
>> + error = PTR_ERR(s);
>> goto error_bdev;
>> + }
> I'm not sure what you are getting at here. Why add a jump into the fast
> path?
My experiments with gcc 4.7.3 on x86-64 do not show any difference in
code generated with patch and without it.
227a: e8 00 00 00 00 callq 227f <gfs2_mount+0xb1>
227b: R_X86_64_PC32 sget-0x4
227f: 4c 89 f7 mov %r14,%rdi
2282: 49 89 c4 mov %rax,%r12
2285: e8 00 00 00 00 callq 228a <gfs2_mount+0xbc>
2286: R_X86_64_PC32 mutex_unlock-0x4
228a: 49 81 fc 00 f0 ff ff cmp $0xfffffffffffff000,%r12
2291: 0f 87 2a 01 00 00 ja 23c1 <gfs2_mount+0x1f3>
2297: 49 83 7c 24 60 00 cmpq $0x0,0x60(%r12)
229d: 74 0b je 22aa <gfs2_mount+0xdc>
229f: 44 89 ee mov %r13d,%esi
22a2: 48 89 df mov %rbx,%rdi
22a5: e8 00 00 00 00 callq 22aa <gfs2_mount+0xdc>
22a6: R_X86_64_PC32 blkdev_put-0x4

It looks like 'error = PTR_ERR(s);' assignment is just ignored because s and error share the same register %r12.

So the only point of the patch is to improve readability.

--
Best regards,
Alexey