From 182dd4b277177e8465ad11cd9f85f282946b5578 Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> Date: Tue, 22 Nov 2011 10:55:12 -0800 Subject: [PATCH] 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> --- Documentation/atomic_ops.txt | 87 ++++++++++++++++++++++++++++++++++++ 1 file changed, 87 insertions(+) diff --git a/Documentation/atomic_ops.txt b/Documentation/atomic_ops.txt index 3bd585b449270..27f2b21a9d5cd 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. -- GitLab