[PATCH] Document file->f_pos as thread-unsafe

[dcg]

El Fri, 10 Oct 2008 13:25:15 +1100, Nick Piggin <nickpiggin@xxxxxxxxxxxx> escribiÃ:

> Note that I don't think we'd want to explicitly guarantee that it is atomic
> on 64-bit machines either. It does happen to be, but I don't think we want
> anybody to rely on that...

Easily fixable. Performance testing of this new version didn't find
regressions.




Document that it is not safe to play with file->f_pos from multiple processes
if they aren't synchronizing themselves.

Signed-off-by: Diego Calleja <diegocg@xxxxxxxxx>

Index: 2.6/include/linux/fs.h
===================================================================
--- 2.6.orig/include/linux/fs.h	2008-10-10 21:07:29.000000000 +0200
+++ 2.6/include/linux/fs.h	2008-10-10 21:08:25.000000000 +0200
@@ -821,6 +821,17 @@
 	atomic_long_t		f_count;
 	unsigned int 		f_flags;
 	mode_t			f_mode;
+	/*
+	 * Linux does NOT guarantee atomic reading/writing to file->f_pos in
+	 * multithreaded apps. There're several reasons for this behaviour:
+	 *  - Specifications don't say it must be implemented that way.
+	 *  - This behaviour is part of the Linux semantics.
+	 *  - Any application that does multithreaded access to file->f_pos
+	 *    should be doing its own locking: the processes should synchronize
+	 *    themselves when accessing a file descriptor. If an application
+	 *    doesn't do that, its file descriptor handling is buggy anyway and
+	 *    must be fixed to do its accesses to file->f_pos properly.
+	 */
 	loff_t			f_pos;
 	struct fown_struct	f_owner;
 	unsigned int		f_uid, f_gid;

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