On Thu, Jan 15, 2009 at 01:01:54PM +0000, Jamie Lokier wrote: > Dave Kleikamp wrote: > > Is there any reason you couldn't have just changed generic_file_llseek() > > to do this rather than making identical changes to the individual file > > systems. I would think this optimization would be safe for any file > > system. > > Is it safe on 32-bit systems where 64-bit updates are not atomic? Protection of 64-bit updates of i_size where 32-bit systems do not have atomic 64-bit updates is done via i_size_seqcount, which is done automatically as long as the code in question uses i_size_write(), an inline function defined in include/linux/fs.h. > You may say that doing multiple parallel lseek() calls is undefined > behaviour, so it's ok to end up with file position that none of the > individual lseek() calls asked for. > > But if it's undefined behaviour, no programs should be doing parallel > lseek() calls on the same open file, so why optimise it at all? i_mutex gets used for a lot more than protecting i_size updates for lseek(), so removing the need for i_mutex could very well be helpful for non-micro-benchmark tests. As Hirashi-san mentioned in his patch, this could avoid contention between lseek() and write() (possibly via a different file descriptor, so it wouldn't be an undefined race condition) and fsync(). I do agree this is an optimization that should be done in generic_file_llseek(), though. Regards, - Ted -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
