linux/include
Linus Torvalds 386afc9114 spinlocks and preemption points need to be at least compiler barriers
In UP and non-preempt respectively, the spinlocks and preemption
disable/enable points are stubbed out entirely, because there is no
regular code that can ever hit the kind of concurrency they are meant to
protect against.

However, while there is no regular code that can cause scheduling, we
_do_ end up having some exceptional (literally!) code that can do so,
and that we need to make sure does not ever get moved into the critical
region by the compiler.

In particular, get_user() and put_user() is generally implemented as
inline asm statements (even if the inline asm may then make a call
instruction to call out-of-line), and can obviously cause a page fault
and IO as a result.  If that inline asm has been scheduled into the
middle of a preemption-safe (or spinlock-protected) code region, we
obviously lose.

Now, admittedly this is *very* unlikely to actually ever happen, and
we've not seen examples of actual bugs related to this.  But partly
exactly because it's so hard to trigger and the resulting bug is so
subtle, we should be extra careful to get this right.

So make sure that even when preemption is disabled, and we don't have to
generate any actual *code* to explicitly tell the system that we are in
a preemption-disabled region, we need to at least tell the compiler not
to move things around the critical region.

This patch grew out of the same discussion that caused commits
79e5f05edc ("ARC: Add implicit compiler barrier to raw_local_irq*
functions") and 3e2e0d2c22 ("tile: comment assumption about
__insn_mtspr for <asm/irqflags.h>") to come about.

Note for stable: use discretion when/if applying this.  As mentioned,
this bug may never have actually bitten anybody, and gcc may never have
done the required code motion for it to possibly ever trigger in
practice.

Cc: stable@vger.kernel.org
Cc: Steven Rostedt <srostedt@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2013-04-09 10:48:33 -07:00
..
acpi Fixes: 2013-03-12 20:25:53 -07:00
asm-generic asm-generic: move cmpxchg*_local defs to cmpxchg.h 2013-03-13 06:11:05 +01:00
clocksource ImgTec Meta architecture changes for v3.9-rc1 2013-03-03 12:06:09 -08:00
crypto
drm drm/radeon: add Richland pci ids 2013-03-15 18:47:19 -04:00
keys
linux spinlocks and preemption points need to be at least compiler barriers 2013-04-09 10:48:33 -07:00
math-emu
media [media] media: ov7670: Add possibility to disable pixclk during hblank 2013-02-08 14:35:06 -02:00
memory
misc
net ipv4: Fix ip-header identification for gso packets. 2013-03-26 13:50:05 -04:00
pcmcia
ras edac: add support for error type "Info" 2013-02-21 14:16:27 -03:00
rdma IB/core: Add "type 2" memory windows support 2013-02-21 11:51:45 -08:00
rxrpc
scsi libfc, fcoe, bnx2fc: Split fc_disc_init into fc_disc_{init, config} 2013-03-25 16:03:03 -07:00
sound Merge remote-tracking branch 'asoc/fix/max98090' into asoc-next 2013-03-26 14:08:03 +00:00
target target: Rename spc_get_write_same_sectors -> sbc_get_write_same_sectors 2013-02-23 12:46:14 -08:00
trace Various bug fixes for ext4. The most important is a fix for the new 2013-03-02 19:33:21 -08:00
uapi net: fix *_DIAG_MAX constants 2013-03-21 12:36:33 -04:00
video ARM: at91: fix LCD-wiring mode 2013-03-13 11:05:04 +01:00
xen for-linus-20130331 2013-03-31 11:38:59 -07:00
Kbuild