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Date: Tue, 16 Oct 2012 10:13:02 +1100 From: Dave Chinner <david@...morbit.com> To: Juliusz Chroboczek <jch@....jussieu.fr> Cc: linux-kernel@...r.kernel.org Subject: Re: Write is not atomic? On Mon, Oct 15, 2012 at 11:36:15PM +0200, Juliusz Chroboczek wrote: > Hi, > > The Linux manual page for write(2) says: > > The adjustment of the file offset and the write operation are > performed as an atomic step. That's wrong. The file offset update is not synchronised at all with the write, and for a shared fd the update will race. > This is apparently an extension to POSIX, which says > > This volume of IEEE Std 1003.1-2001 does not specify behavior of > concurrent writes to a file from multiple processes. Applications > should use some form of concurrency control. This is how Linux behaves. > The following fragment of code > > int fd; > fd = open("exemple", O_CREAT | O_WRONLY | O_TRUNC, 0666); > fork(); > write(fd, "Ouille", 6); > close(fd); > > produces "OuilleOuille", as expected, on ext4 on two machines running > Linux 3.2 AMD64. However, over XFS on an old Pentium III at 500 MHz > running 2.6.32, it produces just "Ouille" roughly once in three times. ext4, on 3.6: $ for i in `seq 0 10000`; do ./a.out ; cat /mnt/scratch/foo ; echo ; done | sort |uniq -c 39 Ouille 9962 OuilleOuille $ XFS, on the same kernel, hardware and block device: $ for i in `seq 0 10000`; do ./a.out ; cat /mnt/scratch/foo ; echo ; done | sort |uniq -c 40 Ouille 9961 OuilleOuille $ So both filesystems behave according to the POSIX definition of concurrent writes.... Cheers, Dave. -- Dave Chinner david@...morbit.com -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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