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SubjectRe: [PATCH 2/2] perf/tools: put new buildid locks to use

* Milos Vyletel <milos@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 <milos@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


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