remount a partition to change access permission

[lamle]

Hi all,

I have a partition mounted as read only in my embedded linux machine for safety reason. At some point, I want to write something to it, then I use this command to change the permission (and change back to read only afterward):

mount -o remount,rw path-to-partition

However, there is one thread which is using files in this partition (with mmap function). When this command is executed, the thread will be suspended for a short time, I think because remount option detaches and attaches the partition, hence the thread cannot find it and get suspended.
Is there more efficient way to change partition access permission?

[gaurav-joseph]

check this out

unix.stackexchange.com/questions/30268/mount-device-with-r-w-access-to-specific-user

[lamle]

check this out

unix.stackexchange.com/questions/30268/mount-device-with-r-w-access-to-specific-user

Thank man, unfortunately it does not help
Just to make sure that everyone understand me, I am looking for a solution to change access right on a partition without the need to remount if (this is totally different with chmod thing).

[gaurav-joseph]

you could edit /etc/fstab to do this

UUID=<xxxxx> /media/where ext4 rw,auto,nls=utf8,umask=077,gid=46,uid=1000 0 0

Users are listed in /etc/passwd while groups are listed in /etc/group

you may have to edit this according to your taste

[lamle]

you could edit /etc/fstab to do this

UUID=<xxxxx> /media/where ext4 rw,auto,nls=utf8,umask=077,gid=46,uid=1000 0 0

Users are listed in /etc/passwd while groups are listed in /etc/group

you may have to edit this according to your taste

Hi Joseph,
Thanks for quick reply. However i am afraid I do not understand the point you are trying to help.
As I said this is embedded Linux system and it does not even have /etc/fstab but that was not a problem.
Originally the partition is MOUNTED as read-only. Then at some point (in one thread) I want to change the access permission without remounting because it will affect other threads.

[gaurav-joseph]

sorry my mistake, i forgot the word embedded

it does look pretty difficult

could you specify the linux system specs

[lamle]

Hi,
I am afraid I dont know system information because it has been cleared.
However, I think I dont need the fstab file, all can be done with mount command options.
(problem is which option)

[gaurav-joseph]

the point is with the mount command you will have to do it before the processes begin or the threads are sure to die, you cannot do a remount
the other option is to restart the process after you remount and re-remount

[lamle]

the point is with the mount command you will have to do it before the processes begin or the threads are sure to die

Hi Joseph,
This change permission thing is done in one thread(or process).
The situation is that there are multiple threads running and at some point one thread wants to change permission to rw to write something and change it back to ro. This action must not affect other threads because they are using other resources in the same partition.

[gaurav-joseph]

ah! is est impossible