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? >> Anyway, returning to your point about flock(), how would this text >> look for the flock(2) manual page: >> >> NOTES >> Since kernel 2.0, flock() is implemented as a system call in >> its own right rather than being emulated in the GNU C library >> as a call to fcntl(2). This yields classical BSD semantics: >> there is no interaction between the types of lock placed by >> flock() and fcntl(2), and flock() does not detect deadlock. >> (Note, however, that on some modern BSDs, flock() and fcntl(2) >> locks do interact with one another.) >> >> In Linux kernels up to 2.6.11, flock() does not lock files over >> NFS (i.e., the scope of locks was limited to the local system). >> Instead, one could use fcntl(2) byte-range locking, which does >> work over NFS, given a sufficiently recent version of Linux and >> a server which supports locking. Since Linux 2.6.12, NFS >> clients support flock() locks by emulating them as byte-range >> locks on the entire file. This means that fcntl(2) and flock() >> locks do interact with one another over NFS. Since Linux >> 2.6.37, the kernel supports a compatibility mode that allows >> flock() locks (and also fcntl(2) byte region locks) to be >> treated as local; see the discussion of the local_lock option >> in nfs(5). >> ? > > That seems to cover it quite well - thanks. Thanks for checking it. 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
