race condition on looking up inodes

race condition on looking up inodes

[warren]

Hi,
    I had post a simillar message before.
    Thanks for the replay from Albert D. Cahalan. But i found some results
confusing me.
    For example,  process 1 and process 2 run concurrently and execute the
following system calls.

    rename("/usr/hybrid/cfg/data","/usr/mytemp/data1"); /*for process 1*/

   ----------------------------------------------------------------

  rename("/usr/mytemp/data1","/usr/test");/* for process 2*/

  ----------------------------------------------------------------
  It is possible that context switch happens when process 1 is look ing up
the inode for "/usr/mytemp/data1"  or the inode for "/usr/hybrid/cfg/data".

 It will result in diffrent behaviour for process 2 and confuses the
application.
  If so,how does Linux solve?

  Thanks.


    Warren

Re: race condition on looking up inodes

[Alexander Viro]

On Mon, 9 Apr 2001, warren wrote:

> 
> 
> Hi,
>     I had post a simillar message before.
>     Thanks for the replay from Albert D. Cahalan. But i found some results
> confusing me.
>     For example,  process 1 and process 2 run concurrently and execute the
> following system calls.
> 
>     rename("/usr/hybrid/cfg/data","/usr/mytemp/data1"); /*for process 1*/
> 
>    ----------------------------------------------------------------
> 
>   rename("/usr/mytemp/data1","/usr/test");/* for process 2*/

>   ----------------------------------------------------------------
>   It is possible that context switch happens when process 1 is look ing up
> the inode for "/usr/mytemp/data1"  or the inode for "/usr/hybrid/cfg/data".

>  It will result in diffrent behaviour for process 2 and confuses the
> application.

>   If so,how does Linux solve?

Solves what, precisely? Result depends on the order of these calls. If
you don't provide any serialization - you get timing-dependent results
you were asking for. What's the problem and what behaviour do you expect?

Besides, what's the difference caused by the moment of context switch?

Re: race condition on looking up inodes

[Bernd Eckenfels]

In article <000201c0c0a4$eb5c7b10$321ea8c0@saturn> you wrote:
>    rename("/usr/hybrid/cfg/data","/usr/mytemp/data1"); /*for process 1*/
>  rename("/usr/mytemp/data1","/usr/test");/* for process 2*/

Rename syscall is expected to be atomic on unixoid systems. And I dont know of
a case where a problem is, besides if you use some network file system, where
nobody can realy gurantee anything (well kidding, but it is harder than on a
local one).

The second rename may see the result of the first rename or the original state
before the first rename. It will not see any half-state or locked nodes.

Greetings
Bernd