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

[Milos Vyletel <milos@redhat.com>]

On Thu, May 14, 2015 at 07:38:08PM +0200, Ingo Molnar wrote:

> 
> * 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. I'll take a look at this later this week.

Milos