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Date: Mon, 3 Mar 2014 13:56:31 -0800 From: Linus Torvalds <torvalds@...ux-foundation.org> To: Al Viro <viro@...iv.linux.org.uk> Cc: "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>, lkml <linux-kernel@...r.kernel.org>, Miklos Szeredi <miklos@...redi.hu>, "Theodore T'so" <tytso@....edu>, Christoph Hellwig <hch@....de>, Chris Mason <clm@...com>, Dave Chinner <david@...morbit.com>, Linux-Fsdevel <linux-fsdevel@...r.kernel.org>, "J. Bruce Fields" <bfields@...i.umich.edu>, Yongzhi Pan <panyongzhi@...il.com> Subject: Re: Update of file offset on write() etc. is non-atomic with I/O On Mon, Mar 3, 2014 at 1:45 PM, Al Viro <viro@...iv.linux.org.uk> wrote: > > Um... That's odd - we *could* get there with f.need_put and > file_count(file) equal to 1, but why would we want to take > f_pos_lock in that case? Because that means that the file table is shared among threads. So another thread can access the struct file pointer and do a concurrent read() or write() on it, and so we need to lock f_pos. Basically, there are two cases: - duplicated file pointers due to fork(). That's the "file_count()" test. Yes, this will trigger even if they didn't fork, just dup'ed the file descriptor. We have no way of telling the difference, though. - concurrent access due to duplicated "struct file_table" pointers. This is the "need_put" test, since __fget_light() will have tested the proper files->count already. Both need f_pos_lock. Linus -- 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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