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authorPaul E. McKenney <paulmck@linux.vnet.ibm.com>2011-11-22 10:55:12 -0800
committerPaul E. McKenney <paulmck@linux.vnet.ibm.com>2011-12-11 10:31:58 -0800
commit182dd4b277177e8465ad11cd9f85f282946b5578 (patch)
tree3fa8d555651b04ba39b871c8ffb7cb9cb2c6f512 /Documentation/atomic_ops.txt
parent1268fbc746ea1cd279886a740dcbad4ba5232225 (diff)
downloadlinux-linaro-stable-182dd4b277177e8465ad11cd9f85f282946b5578.tar.gz
doc: Add load/store guarantees to Documentation/atomic-ops.txt
An IRC discussion uncovered many conflicting opinions on what types of data may be atomically loaded and stored. This commit therefore calls this out the official set: pointers, longs, ints, and chars (but not shorts). This commit also gives some examples of compiler mischief that can thwart atomicity. Please note that this discussion is relevant to !SMP kernels if CONFIG_PREEMPT=y: preemption can cause almost as much trouble as can SMP. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Richard Henderson <rth@twiddle.net> Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru> Cc: Matt Turner <mattst88@gmail.com> Cc: Russell King <linux@arm.linux.org.uk> Cc: Haavard Skinnemoen <hskinnemoen@gmail.com> Cc: Hans-Christian Egtvedt <egtvedt@samfundet.no> Cc: Mike Frysinger <vapier@gentoo.org> Cc: Mikael Starvik <starvik@axis.com> Cc: Jesper Nilsson <jesper.nilsson@axis.com> Cc: David Howells <dhowells@redhat.com> Cc: Yoshinori Sato <ysato@users.sourceforge.jp> Cc: Richard Kuo <rkuo@codeaurora.org> Cc: Jes Sorensen <jes@sgi.com> Cc: Hirokazu Takata <takata@linux-m32r.org> Cc: Geert Uytterhoeven <geert@linux-m68k.org> Cc: Michal Simek <monstr@monstr.eu> Cc: Ralf Baechle <ralf@linux-mips.org> Cc: Koichi Yasutake <yasutake.koichi@jp.panasonic.com> Cc: Jonas Bonn <jonas@southpole.se> Cc: Kyle McMartin <kyle@mcmartin.ca> Cc: Helge Deller <deller@gmx.de> Cc: "James E.J. Bottomley" <jejb@parisc-linux.org> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org> Cc: Paul Mackerras <paulus@samba.org> Cc: Martin Schwidefsky <schwidefsky@de.ibm.com> Cc: Heiko Carstens <heiko.carstens@de.ibm.com> Cc: Chen Liqin <liqin.chen@sunplusct.com> Cc: Lennox Wu <lennox.wu@gmail.com> Cc: Paul Mundt <lethal@linux-sh.org> Cc: "David S. Miller" <davem@davemloft.net> Cc: Chris Metcalf <cmetcalf@tilera.com> Cc: Jeff Dike <jdike@addtoit.com> Cc: Richard Weinberger <richard@nod.at> Cc: Guan Xuetao <gxt@mprc.pku.edu.cn> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Ingo Molnar <mingo@redhat.com> Cc: "H. Peter Anvin" <hpa@zytor.com> Cc: Chris Zankel <chris@zankel.net>
Diffstat (limited to 'Documentation/atomic_ops.txt')
-rw-r--r--Documentation/atomic_ops.txt87
1 files changed, 87 insertions, 0 deletions
diff --git a/Documentation/atomic_ops.txt b/Documentation/atomic_ops.txt
index 3bd585b44927..27f2b21a9d5c 100644
--- a/Documentation/atomic_ops.txt
+++ b/Documentation/atomic_ops.txt
@@ -84,6 +84,93 @@ compiler optimizes the section accessing atomic_t variables.
*** YOU HAVE BEEN WARNED! ***
+Properly aligned pointers, longs, ints, and chars (and unsigned
+equivalents) may be atomically loaded from and stored to in the same
+sense as described for atomic_read() and atomic_set(). The ACCESS_ONCE()
+macro should be used to prevent the compiler from using optimizations
+that might otherwise optimize accesses out of existence on the one hand,
+or that might create unsolicited accesses on the other.
+
+For example consider the following code:
+
+ while (a > 0)
+ do_something();
+
+If the compiler can prove that do_something() does not store to the
+variable a, then the compiler is within its rights transforming this to
+the following:
+
+ tmp = a;
+ if (a > 0)
+ for (;;)
+ do_something();
+
+If you don't want the compiler to do this (and you probably don't), then
+you should use something like the following:
+
+ while (ACCESS_ONCE(a) < 0)
+ do_something();
+
+Alternatively, you could place a barrier() call in the loop.
+
+For another example, consider the following code:
+
+ tmp_a = a;
+ do_something_with(tmp_a);
+ do_something_else_with(tmp_a);
+
+If the compiler can prove that do_something_with() does not store to the
+variable a, then the compiler is within its rights to manufacture an
+additional load as follows:
+
+ tmp_a = a;
+ do_something_with(tmp_a);
+ tmp_a = a;
+ do_something_else_with(tmp_a);
+
+This could fatally confuse your code if it expected the same value
+to be passed to do_something_with() and do_something_else_with().
+
+The compiler would be likely to manufacture this additional load if
+do_something_with() was an inline function that made very heavy use
+of registers: reloading from variable a could save a flush to the
+stack and later reload. To prevent the compiler from attacking your
+code in this manner, write the following:
+
+ tmp_a = ACCESS_ONCE(a);
+ do_something_with(tmp_a);
+ do_something_else_with(tmp_a);
+
+For a final example, consider the following code, assuming that the
+variable a is set at boot time before the second CPU is brought online
+and never changed later, so that memory barriers are not needed:
+
+ if (a)
+ b = 9;
+ else
+ b = 42;
+
+The compiler is within its rights to manufacture an additional store
+by transforming the above code into the following:
+
+ b = 42;
+ if (a)
+ b = 9;
+
+This could come as a fatal surprise to other code running concurrently
+that expected b to never have the value 42 if a was zero. To prevent
+the compiler from doing this, write something like:
+
+ if (a)
+ ACCESS_ONCE(b) = 9;
+ else
+ ACCESS_ONCE(b) = 42;
+
+Don't even -think- about doing this without proper use of memory barriers,
+locks, or atomic operations if variable a can change at runtime!
+
+*** WARNING: ACCESS_ONCE() DOES NOT IMPLY A BARRIER! ***
+
Now, we move onto the atomic operation interfaces typically implemented with
the help of assembly code.