Accessing device information in REMOVE agent

Accessing device information in REMOVE agent

[Andrey Borzenkov]

I'd like to be notified when block device goes away (e.g. USB stick unplugged) 
basically to look if device is in use and possibly initiate clean up. Block 
hotplug currently is passing only DEVPATH; but it alone is not reliable way 
to identify it; device may be used under alias names via symbolic links.

Is it safe to access /sys/$DEVPATH in REMOVE agent? Apparently hotplug is 
called asynchronously i.e. it is possible that /sys entry is already removed?

Would it make sense to add device number? It seems to be natural native "block 
device ID" :)

TIA

-andrey



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Re: Accessing device information in REMOVE agent

[Greg KH]

On Sat, Nov 08, 2003 at 04:02:25PM +0300, Andrey Borzenkov wrote:
> I'd like to be notified when block device goes away (e.g. USB stick unplugged) 
> basically to look if device is in use and possibly initiate clean up. Block 
> hotplug currently is passing only DEVPATH; but it alone is not reliable way 
> to identify it; device may be used under alias names via symbolic links.

What do you mean?  DEVPATH is unique for that point in time.  There are
no alias's in sysfs.

> Is it safe to access /sys/$DEVPATH in REMOVE agent? Apparently hotplug is 
> called asynchronously i.e. it is possible that /sys entry is already removed?

The /sys entry is probably already removed, but if not, it will
disappear any second.  So no, it's not safe to try to access it, as it
will not work.

> Would it make sense to add device number? It seems to be natural native "block 
> device ID" :)

What "device number"?  The major/minor?  Why?  It's about as unique as
DEVPATH is for any point in time.

thanks,

greg k-h


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Re: Accessing device information in REMOVE agent

[Andrey Borzenkov]

On Sunday 09 November 2003 01:25, Greg KH wrote:
> On Sat, Nov 08, 2003 at 04:02:25PM +0300, Andrey Borzenkov wrote:
> > I'd like to be notified when block device goes away (e.g. USB stick
> > unplugged) basically to look if device is in use and possibly initiate
> > clean up. Block hotplug currently is passing only DEVPATH; but it alone
> > is not reliable way to identify it; device may be used under alias names
> > via symbolic links.
>
> What do you mean?  DEVPATH is unique for that point in time.  There are
> no alias's in sysfs.
>

Sorry I had to be more precise.

I'd like to (try to) replace current synchronous media change checks in 
supermount by mounting device on insert and releasing it on remove. For those 
cases when it makes sense of course, USB sticks in the first place.

But users are free to use any names or links for their device names i.e. they 
can do

ln -s sda /de/myflash
mount /dev/myflash

and on remove it is rather hard to match this name against DEVPATH. But I can 
save (major,minor) when mounting and use it to match mounted filesystem on 
remove.

>
> > Would it make sense to add device number? It seems to be natural native
> > "block device ID" :)
>
> What "device number"?  The major/minor?  Why?  It's about as unique as
> DEVPATH is for any point in time.
>

Hmm ... probably I can just as well use device name (meaning genhd->disk_name) 
you are right.

Thank you

-andrey



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Re: Accessing device information in REMOVE agent

[Greg KH]

On Sun, Nov 09, 2003 at 01:06:13PM +0300, Andrey Borzenkov wrote:
> 
> I'd like to (try to) replace current synchronous media change checks in 
> supermount by mounting device on insert and releasing it on remove. For those 
> cases when it makes sense of course, USB sticks in the first place.
> 
> But users are free to use any names or links for their device names i.e. they 
> can do
> 
> ln -s sda /de/myflash
> mount /dev/myflash
> 
> and on remove it is rather hard to match this name against DEVPATH. But I can 
> save (major,minor) when mounting and use it to match mounted filesystem on 
> remove.

You might want to look into what devlabel does, as it sounds like it
does much the same thing of what you are wanting to do.

Good luck,

greg k-h


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