Messages in this thread | | | Date | Wed, 3 Jun 2015 13:27:11 +0200 | From | Milos Vyletel <> | Subject | Re: [PATCH 2/2] perf/tools: put new buildid locks to use |
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On Wed, Jun 03, 2015 at 01:21:41PM +0200, Ingo Molnar wrote:
> > * Milos Vyletel <milos@redhat.com> wrote: > > > 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/. > > > > > > > Ingo, > > > > I finally found some time to make this change. While going over the code I've > > noticed one thing that would make concurrent creation even easier to solve. > > Instead of copying the file to temp file what about simply opening file with > > O_CREAT|O_EXCL? creat itself > > > > "creat() is equivalent to open() with flags equal to O_CREAT|O_WRONLY|O_TRUNC." > > > > addition of O_EXCL would > > > > "Ensure that this call creates the file: if this flag is specified in > > conjunction with O_CREAT, and pathname already exists, then open() will fail." > > > > This we would prevent truncation of already linked file in case link() races as > > in 0635b0f71424. What do you think? > > But it would not prevent the problem of creating a not yet fully constructed file > - which some other tool invocation could attempt to parse in an incomplete > fashion. > > Using create+link+unlink avoids that race, the files in the publicly visible > namespace will always be fully constructed by the time they are made visible > (atomically). >
Got it. Will use the approach proposed by you.
Milos
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