Messages in this thread | | | Date | Thu, 14 May 2015 19:38:08 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH 2/2] perf/tools: put new buildid locks to use |
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* 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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