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powerpc/64: Fix race condition in setting lock bit in idle/wakeup code 

This fixes a race condition where one thread that is entering or
leaving a power-saving state can inadvertently ignore the lock bit
that was set by another thread, and potentially also clear it.
The core_idle_lock_held function is called when the lock bit is
seen to be set.  It polls the lock bit until it is clear, then
does a lwarx to load the word containing the lock bit and thread
idle bits so it can be updated.  However, it is possible that the
value loaded with the lwarx has the lock bit set, even though an
immediately preceding lwz loaded a value with the lock bit clear.
If this happens then we go ahead and update the word despite the
lock bit being set, and when called from pnv_enter_arch207_idle_mode,
we will subsequently clear the lock bit.

No identifiable misbehaviour has been attributed to this race.

This fixes it by checking the lock bit in the value loaded by the
lwarx.  If it is set then we just go back and keep on polling.

Fixes: b32aadc1a8 ("powerpc/powernv: Fix race in updating core_idle_state")
Cc: stable@vger.kernel.org # v4.2+
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
This commit is contained in:
Paul Mackerras 2016-10-21 20:04:17 +11:00 committed by Michael Ellerman
parent 56c46222af
commit 09b7e37b18
1 changed files with 3 additions and 0 deletions
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3
arch/powerpc/kernel/idle_book3s.S
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@ -90,6 +90,7 @@ ALT_FTR_SECTION_END_IFSET(CPU_FTR_ARCH_300)
* Threads will spin in HMT_LOW until the lock bit is cleared.
* r14 - pointer to core_idle_state
* r15 - used to load contents of core_idle_state
* r9 - used as a temporary variable
*/
core_idle_lock_held:
@ -99,6 +100,8 @@ core_idle_lock_held:
bne 3b
HMT_MEDIUM
lwarx r15,0,r14
andi. r9,r15,PNV_CORE_IDLE_LOCK_BIT
bne core_idle_lock_held
blr
/*