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Date: Sat, 10 Jun 2006 19:31:37 +0200
From: Robert Valentan <rov_200603@solid-soft.at>
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Subject: [linux-lvm] lvcreate - race condition creating snapshots
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To: linux-lvm@redhat.com

I think, there is a race condition in lvcreate while
makeing a new snapshot.

System: Tyan with 2 Opteron 246
        xen 3.0.2-3
        linux 2.6.16.13
        lvm 2.02.07
        3ware-Raid 9550 Raid5

Under xen i was unable to create a snapshot, I got
always "LV system/snap in use: not deactivating".
Without xen, i got occasionally a snapshot.


After i make a "dirty hack":
------ begin ----------
*** old/lvcreate.c      2006-06-10 19:22:37.000000000 +0200
--- lvcreate.c  2006-06-10 19:23:04.000000000 +0200
***************
*** 776,781 ****
--- 776,787 ----
                /* Reset permission after zeroing */
                if (!(lp->permission & LVM_WRITE))
                        lv->status &= ~LVM_WRITE;
+
+                 { struct timeval tv ;
+                   tv.tv_sec=10;
+                   tv.tv_usec=0;
+                   select(2, 0, 0, 0, &tv);
+                 }
                if (!deactivate_lv(cmd, lv)) {
                        log_err("Couldn't deactivate new snapshot.");
                        return 0;
------ end ---------

i got always a snapshot (on all 3 systems!). I think, there is an
other thread which is creating the new lv, and it will take longer as
the lvcreate will "think". I have to less knowledge to make a patch,
which is waiting for the correct event (creating lv complete).

-- 
regards
Robert Valentan