* Milos Vyletel <[email protected]> wrote:
> On Thu, May 14, 2015 at 07:38:08PM +0200, Ingo Molnar wrote:
>
> >
> > * Milos Vyletel <[email protected]> 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 <[email protected]> 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).
Thanks,
Ingo
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