Date: Sun, 3 Feb 2013 05:30:01 GMT From: Giorgos Keramidas <keramida@FreeBSD.org> To: freebsd-bugs@FreeBSD.org Subject: Re: kern/175674: sem_open() should use O_EXLOCK with open() instead of a separate flock() call Message-ID: <201302030530.r135U1HP013561@freefall.freebsd.org>
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The following reply was made to PR kern/175674; it has been noted by GNATS.
From: Giorgos Keramidas <keramida@FreeBSD.org>
To: Jukka Ukkonen <jau@iki.fi>
Cc: freebsd-gnats-submit@FreeBSD.org, jilles@FreeBSD.org,
davidxu@FreeBSD.org
Subject: Re: kern/175674: sem_open() should use O_EXLOCK with open() instead
of a separate flock() call
Date: Sun, 3 Feb 2013 06:25:25 +0100
On 2013-01-29 18:03, Jukka Ukkonen <jau@iki.fi> wrote:
> >Number: 175674
> >Category: kern
> >Synopsis: sem_open() should use O_EXLOCK with open() instead of a separate flock() call
> >Environment:
> FreeBSD sleipnir 9.1-STABLE FreeBSD 9.1-STABLE #2 r246056M: Tue Jan 29 07:33:01 EET 2013 root@sleipnir:/usr/obj/usr/src/sys/Sleipnir amd64
> >Description:
> sem_open() is calling flock() to set a lock on a newly created file descriptor.
> That is pointless. The open() call a few lines before the flock() could, and
> in my opinion should, be done with the O_EXLOCK flag set.
It's also a bit safer to obtain the exclusive lock atomically before
open() returns. Waiting for open() to complete and then calling flock()
has a race condition.
Jilles and David, do you think this patch looks ok for libc?
> Patch attached with submission follows:
>
> --- lib/libc/gen/sem_new.c.flock 2012-11-09 18:50:05.000000000 +0200
> +++ lib/libc/gen/sem_new.c 2012-11-09 18:44:59.000000000 +0200
> @@ -198,11 +198,13 @@
> goto error;
> }
>
> - fd = _open(path, flags|O_RDWR|O_CLOEXEC, mode);
> + fd = _open(path, flags|O_RDWR|O_CLOEXEC|O_EXLOCK, mode);
> if (fd == -1)
> goto error;
> +#if 0
> if (flock(fd, LOCK_EX) == -1)
> goto error;
> +#endif
> if (_fstat(fd, &sb)) {
> flock(fd, LOCK_UN);
> goto error;
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