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

[Ingo Molnar]

* Milos Vyletel <milos@xxxxxxxxxx> 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@xxxxxxxxxx> 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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