On 04/27/2014 11:28 PM, NeilBrown wrote: > On Sun, 27 Apr 2014 13:11:33 +0200 "Michael Kerrisk (man-pages)" > <mtk.manpages@xxxxxxxxx> wrote: > >> On Sun, Apr 27, 2014 at 12:04 PM, NeilBrown <neilb@xxxxxxx> wrote: >>> On Sun, 27 Apr 2014 11:16:02 +0200 "Michael Kerrisk (man-pages)" >>> <mtk.manpages@xxxxxxxxx> wrote: >>> >>>> [Trimming some folk from CC, and adding various NFS people] >>>> >>>> On 04/27/2014 06:51 AM, NeilBrown wrote: >>>> >>>> [...] >>>> >>>>> Note to Michael: The text >>>>> flock() does not lock files over NFS. >>>>> in flock(2) is no longer accurate. The reality is ... complex. >>>>> See nfs(5), and search for "local_lock". >>>> >>>> Ahhh -- I see: >>>> http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=5eebde23223aeb0ad2d9e3be6590ff8bbfab0fc2 >>>> >>>> Thanks for the heads up. >>>> >>>> Just in general, it would be great if the flock(2) and fcntl(2) man pages >>>> contained correct details for NFS, of course. So, for example, if there >>>> are any current gotchas for NFS and fcntl() byte-range locking, I'd like >>>> to add those to the fcntl(2) man page. >>> >>> The only peculiarities I can think of are: >>> - With NFS, locking or unlocking a region forces a flush of any cached data >>> for that file (or maybe for the region of the file). I'm not sure if this >>> is worth mentioning. >> >> I agree that it's probably not necessary to mention. >> >>> - With NFSv4 the client can lose a lock if it is out of contact with the >>> server for a period of time. When this happens, any IO to the file by a >>> process which "thinks" it holds a lock will fail until that process closes >>> and re-opens the file. >>> This behaviour is since 3.12. Prior to that the client might lose and >>> regain the lock without ever knowing thus potentially risking corruption >>> (but only if client and server lost contact for an extended period). >> >> Do you have a pointer for that commit to 3.12? >> > > http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=ef1820f9be27b6ad158f433ab38002ab8131db4d > > did most of the work while the subsequent commit > > http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=f6de7a39c181dfb8a2c534661a53c73afb3081cd > > changed some details, added some documentation, and inverted the default > behaviour. Thanks for that detail. What do you think of the following text for the fcntl(2) man page: Before Linux 3.12, if an NFS client is out of contact with the server for a period of time, it might lose and regain a lock without ever being aware of the fact. This scenario poten‐ tially risks data corruption, since another process might acquire a lock in the intervening period and perform file I/O. Since Linux 3.12, if the client loses contact with the server, any I/O to the file by a process which "thinks" it holds a lock will fail until that process closes and reopens the file. A kernel parameter, nfs.recover_lost_locks, can be set to 1 to obtain the pre-3.12 behavior, whereby the client will attempt to recover lost locks when contact is reestablished with the server. Because of the attendant risk of data corruption, this parameter defaults to 0 (disabled). ? Cheers, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/ -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
