* Milos Vyletel <mi...@redhat.com> wrote:
> On Thu, May 14, 2015 at 01:38:21PM +0200, Jiri Olsa wrote:
>
> > On Thu, May 14, 2015 at 12:40:59PM +0200, Ingo Molnar wrote:
> > >
> > > * Milos Vyletel <mi...@redhat.com> wrote:
> > >
> > > > Use new read/write locks when accesing buildid directory on places where
> > > > we may race if multiple instances are run simultaneously.
> > >
> > > Dunno, this will create locking interaction between multiple instances
> > > of perf - hanging each other, etc.
> > >
> > > And it seems unnecessary: the buildid hierarchy is already spread out.
> > > What kind of races might there be?
> >
> > there was just recently one fixed by commit:
> > 0635b0f71424 perf tools: Fix race in build_id_cache__add_s()
> >
> > havent checked the final patch yet, but the idea is to
> > protect us from similar bugs
>
> right. on top of race with EEXIST couple more are possible (EMLINK,
> ENOSPC, EDQUOT, ENOMEM... the only way to prevent them all is to
> lock this kind of operations and make sure we run one at a time.
Yeah, so the race pointed out in 0635b0f71424 can be (and should be)
fixed without locking:
- first create the file under a process-private name under
~/.debug/tmp/ if the target does not exist yet
- then fully fill it in with content
- then link(2) it to the public target name, which VFS operation is
atomic and may fail safely: at which point it got already created
by someone else.
- finally unlink() the private instance name and the target will now
be the only instance left: either created by us, or by some other
perf instance in the rare racy case.
Since all of ~/.debug is on the same filesystem this should work fine.
Beyond avoiding locking this approach has another advantage: it's
transaction safe, so a crashed/interrupted perf instance won't corrupt
the debug database, it will only put fully constructed files into the
public build-id namespace. It at most leaves a stale private file
around in ~/.debug/tmp/.
Really, we should be following the example of Git, which is using a
similar append-mostly flow to handle data, and generally avoids file
locking as much as possible - which is a whole new can of worms.
Thanks,
Ingo
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
