linux-kernel - Re: [PATCH] gpio/gpio-sysfs: Try to export busy GPIO line leads to wrong GPIO line exporting

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Message-ID: <4EC1A0BE.3000901@solonet.org.ua>
Date:	Tue, 15 Nov 2011 01:14:06 +0200
From:	Denis Kuzmenko <linux@...onet.org.ua>
To:	Stephen Warren <swarren@...dia.com>
CC:	Grant Likely <grant.likely@...retlab.ca>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] gpio/gpio-sysfs: Try to export busy GPIO line leads to
 wrong GPIO line exporting

On 11/14/2011 10:45 PM, Stephen Warren wrote:
> Denis Kuzmenko wrote at Saturday, November 12, 2011 6:31 PM:
>>
>> diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
>> index a971e3d..ccec497 100644
>> --- a/drivers/gpio/gpiolib.c
>> +++ b/drivers/gpio/gpiolib.c
>> @@ -633,7 +633,7 @@ static ssize_t export_store(struct class *class,
>>  done:
>>  	if (status)
>>  		pr_debug("%s: status %d\n", __func__, status);
>> -	return status ? : len;
>> +	return len;
>>  }
> 
> I assume that when the error occurs, status is negative. Is it some
> special value like EINTR, EAGAIN? I'm surprised that the retried write
> is smaller than the whole original buffer.
> 
> What's actually retrying the failed write? Is it user-space in response
> to the previous failed write, in a (mistaken?) attempt to handle shorter-
> than-expected-writes? You could confirm this with strace.
> 
> If the patch above really is correct, there are other places it'd be
> needed; it looks like e.g. drivers/video/backlight/backlight.c would
> have the same issue if you gave too-large integers to its sysfs files.
> 

Hi,

Looks like userspace doesn't retries write. Here is trace:

open("/sys/class/gpio/export", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 3
ioctl(3, SNDCTL_TMR_TIMEBASE or TCGETS, 0xbef76c64) = -1 ENOTTY (Inappropriate ioctl for device)
brk(0x30e000)                           = 0x30e000
write(3, "37\n", 3)                     = -1 EBUSY (Device or resource busy)
close(3)                                = 0
exit(0)                                 = ?

this was made by simple self-written program:
	fopen("/sys/class/gpio/export", "w"); fprintf(fp, "37\n");


additional kernel-trace (WARN_ON(0 > status)) was also added before function return:

WARNING: at drivers/gpio/gpiolib.c:637 export_store+0xa0/0xb4()
Modules linked in:
[<c0028064>] (unwind_backtrace+0x0/0xf0) from [<c0039234>] (warn_slowpath_common+0x48/0x60)
[<c0039234>] (warn_slowpath_common+0x48/0x60) from [<c0039268>] (warn_slowpath_null+0x1c/0x24)
[<c0039268>] (warn_slowpath_null+0x1c/0x24) from [<c0182da8>] (export_store+0xa0/0xb4)
[<c0182da8>] (export_store+0xa0/0xb4) from [<c01b17e0>] (class_attr_store+0x1c/0x28)
[<c01b17e0>] (class_attr_store+0x1c/0x28) from [<c00db9ac>] (sysfs_write_file+0xfc/0x180)
[<c00db9ac>] (sysfs_write_file+0xfc/0x180) from [<c0093304>] (vfs_write+0xac/0x13c)
[<c0093304>] (vfs_write+0xac/0x13c) from [<c0093578>] (sys_write+0x40/0x6c)
[<c0093578>] (sys_write+0x40/0x6c) from [<c0023ffc>] (__sys_trace_return+0x0/0x24)
---[ end trace 8141a98436c712ec ]---

I'll try digg into functions in backtrace and find retry if any.

Best regards, Denis Kuzmenko.
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