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Date: Fri, 10 Oct 2008 21:17:59 +0200
From: dcg <diegocalleja@...il.com>
To: Nick Piggin <nickpiggin@...oo.com.au>
Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>,
Hisashi Hifumi <hifumi.hisashi@....ntt.co.jp>,
torvalds@...ux-foundation.org,
Andrew Morton <akpm@...ux-foundation.org>,
Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
linux-fsdevel@...r.kernel.org,
"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
Theodore Ts'o <tytso@....edu>
Subject: [PATCH] Document file->f_pos as thread-unsafe
El Fri, 10 Oct 2008 13:25:15 +1100, Nick Piggin <nickpiggin@...oo.com.au> escribió:
> 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@...il.com>
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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