On Tue, Oct 30, 2012 at 12:11 AM, Philip Martin <philip.mar...@wandisco.com>wrote:
> Stefan Fuhrmann <stefan.fuhrm...@wandisco.com> writes: > > > On Mon, Oct 29, 2012 at 10:46 PM, Philip Martin > > <philip.mar...@wandisco.com>wrote: > > > >> Philip Martin <philip.mar...@wandisco.com> writes: > >> > >> > Philip Martin <philip.mar...@wandisco.com> writes: > >> > > >> >> I can't see any order in which we can do attach/create that doesn't > have > >> >> a similar race. I think the best solution is a short loop trying > >> >> attach-create a few times before giving up. > >> > > >> > I've committed a loop in r1403463. That doesn't fix the race but it > is > >> > now very unlikely to fail. > >> > > > > The creation code is protected by a repo-global lock/unlock pair. > > So, in theory, there should be no race condition. > > Which lock and where? Does this lock out other processes? > Lines 266 to 292 implement the lock. It first takes out the process-local and and then the global lock (a repo-local file lock). L416 acquires the lock in svn_atomic_namespace__create and L453 releases it. >> I've just observed the same failure with the looping code. I'm not sure > >> what is wrong. I suppose there is a window during the creation process > >> where the file exists, so the create fails, but the memory is not yet > >> ready, so the attach also fails. If one process is in this state > >> another process might loop around 10 times and have both create and > >> attach fail. Perhaps a short and/or random delay would help? > >> > > > > It's on my TODO list to identify the root cause of this issue. > > I think it must be the window between > > apr_file_open( APR_EXCL ) > > and > > mmap( MAP_SHARED ) > > in apr_shm_create. During that period any other process will see both > apr_shm_create and apr_shm_attach fail. But that would imply that your > process lock isn't working. > It is well possible that the locking logic is faulty. Maybe, there should be a regression test that tries concurrent initialization. -- Stefan^2. -- Certified & Supported Apache Subversion Downloads: * http://www.wandisco.com/subversion/download *
