open_dev_excl in reshape_container() fails

Hi,

I've got a problem using open_dev_excl() in reshape_container(), it fails. This functions goes down
and calls open() with flags O_DIRECT| O_RDWR| O_EXCL.
It fails in reshape_container(), ealier in Grow_reshape() exactly the same test call to open_dev_excl() succeed. 
Between test open and open in reshape_container() there is no other open() performed (or other i/o operation that can cause problem).
It not depends on mdmon action because it is the same for raid5 and raid0.
Function open_dev() works, and I'm using it as workaround.

In man pages for open() is told that O_EXCL flag should be used with O_CREAT flag only.
I've read in manual for open() also: "If O_EXCL is set and O_CREAT is not set, the result is undefined"
This can explain my experiences, about open_dev_excl() results.

Please let me know if above is right? What tell you your tests?

If mdadm needs some locking mechanism, probably we should use F_SETLK/F_GETLK i.e.:
    int fd = open("/dev/name", <flags>);

   /* To lock the file/device */
   
    struct flock fl = { F_WRLCK, SEEK_SET, 0, 0, 0 };
    fl.l_pid = getpid();
    fcntl(ld, F_SETLK, &fl);

    ...

    / * To unlock the file/device */

    struct flock fl = { F_UNLCK, SEEK_SET, 0, 0, 0 };
    fl.l_pid = getpid();
    fcntl(ld, F_SETLK, &fl);
    
    ...

    close(fd);

Please tell me your opinion, because I think we have got a problem.

BR
Adam


-------------------------------------------------------------
Intel Technology Poland Sp. z o.o.
email: adam.kwolek@intel.com    
phone: +48 58 766 1773


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