Date: Fri, 16 Oct 2009 15:51:35 -0400 From: John Baldwin <jhb@freebsd.org> To: src-committers@freebsd.org Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org Subject: Re: svn commit: r198174 - head/sys/nfsclient Message-ID: <200910161551.35583.jhb@freebsd.org> In-Reply-To: <200910161930.n9GJUnHD011723@svn.freebsd.org> References: <200910161930.n9GJUnHD011723@svn.freebsd.org>
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On Friday 16 October 2009 3:30:49 pm John Baldwin wrote: > Author: jhb > Date: Fri Oct 16 19:30:48 2009 > New Revision: 198174 > URL: http://svn.freebsd.org/changeset/base/198174 > > Log: > Close a race with caching of -ve name lookups in the NFS client. > Specifically, clients only trust -ve cache entries while the directory > remains unchanged and discard any -ve cache entries for a directory when > they notice that the modification time of a directory entry changes. The > race involves two concurrent lookups as follows: > - Thread A does a lookup for file 'foo' which sends a lookup RPC to the > server. The lookup fails and the server replies. > - The 'foo' file is created (either by the same client or a different > client) updating the modification time on the parent directory of 'foo'. > - Thread B does a lookup for a different file 'bar' which updates the > cached attributes of the parent directory of 'foo' to reflect the new > modification time after 'foo' was created. > - Thread A finally resumes execution to parse the reply from the NFS > server. It adds a -ve cache entry and sets the cached value of the > directory's modification time that is used for invalidating -ve cached > lookups to the new modification time set by thread B. Slight correction, the file has to be created on a different client since the local client would exclusively lock the directory vnode while it created the file avoiding this race. I created the file on a separate client when I reproduced this during testing (I used a hack to force a 5 second sleep when adding -ve cache entries to let thread B do its lookup and update the mtime before thread A tried to add its -ve cache entry to "force" the race). I have seen this in practice in a couple of cases at work where an ls of a directory would show a file that was added, but attempting to open the file itself or even ls the individual file would fail with ENOENT. -- John Baldwin
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