Date: Fri, 9 Mar 2012 00:39:19 +0200 From: Konstantin Belousov <kostikbel@gmail.com> To: John Baldwin <jhb@freebsd.org> Cc: freebsd-fs@freebsd.org, pho@freebsd.org, fs@freebsd.org Subject: Re: close() of an flock'd file is not atomic Message-ID: <20120308223919.GU75778@deviant.kiev.zoral.com.ua> In-Reply-To: <201203081539.07711.jhb@freebsd.org> References: <201203071318.08241.jhb@freebsd.org> <201203081539.07711.jhb@freebsd.org>
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[-- Attachment #1 --] On Thu, Mar 08, 2012 at 03:39:07PM -0500, John Baldwin wrote: > On Wednesday, March 07, 2012 1:18:07 pm John Baldwin wrote: > > So I ran into this problem at work. Suppose you have a process that opens a > > read-write file descriptor with O_EXLOCK (so it has an flock()). It then > > writes out a binary into that file. Another process wants to execve() the > > file when it is ready, so it opens the file with O_EXLOCK (or O_SHLOCK), and > > will call execve() once it has locked the file. In theory, what should happen > > is that the second process should wait until the first process has finished > > and called close(). In practice what happens is that I occasionally see the > > second process fail with ETXTBUSY. > > > > The bug is that the vn_closefile() does the VOP_ADVLOCK() to unlock the file > > separately from the call to vn_close() which drops the writecount. Thus, the > > second process can do an open() and flock() of the file and subsequently call > > execve() after the first process has done the VOP_ADVLOCK(), but before it > > calls into vn_close(). In fact, since vn_close() requires a write lock on the > > vnode, this turns out to not be too hard to reproduce at all. Below is a > > simple test program that reproduces this constantly. To use, copy /bin/test > > to some other file (e.g. /tmp/foo) and make it writable (chmod a+w), then run > > ./flock_close_race /tmp/foo. > > > > The "fix" I came up with is to defer calling VOP_ADVLOCK() to release the lock > > until after vn_close() executes. However, even with that fix applied, my test > > case still fails. Now it is because open() with a given lock flag is > > non-atomic in that the open(O_RDWR) will call vn_open() and bump v_writecount > > before it blocks on the lock due to O_EXLOCK, so even though the 'exec_child' > > process has the fd locked, the writecount can still be bumped. One gross hack > > would be to defer the bump of the writecount to the caller of vn_open() if the > > caller passes in O_EXLOCK or O_SHLOCK, but that's a really gross kludge, plus > > it doesn't actually work. I ended up moving acquiring the lock into > > vn_open_cred(). The current patch I'm testing has both of these approaches, > > but the first one is #if 0'd out, and the second is #if 1'd. > > > > http://www.freebsd.org/~jhb/patches/flock_open_close.patch > > Based on some feedback from Konstantin, I've fixed some issues in the failure > path handling for VOP_ADVLOCK(). I've also removed the #if 0'd code mentioned > above, so the patch is now the actual change that I'm testing. So far it > handles both my workload at work and my test program without any issues. I think a comment is needed for a reason to call vn_writechk() second time. Could you, please, point me, where the FHASLOCK is set for O_EXLOCK | O_SHLOCK case in the patched kernel ? [-- Attachment #2 --] -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.12 (FreeBSD) iEYEARECAAYFAk9ZNRUACgkQC3+MBN1Mb4g4iQCggp5r8WSezZNIVwxLO9/gp5v0 ZywAoPUpSUbOWJYiX/8EEcLzfhQEgQL7 =aoVo -----END PGP SIGNATURE-----home | help
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