[PATCH 4.1 003/123] powerpc/powernv: Fix race in updating core_idle_state

From: Greg Kroah-Hartman
Date: Sat Aug 08 2015 - 18:13:01 EST

4.1-stable review patch. If anyone has any objections, please let me know.


From: "Shreyas B. Prabhu" <shreyas@xxxxxxxxxxxxxxxxxx>

commit b32aadc1a8ed84afbe924cd2ced31cd6a2e67074 upstream.

core_idle_state is maintained for each core. It uses 0-7 bits to track
whether a thread in the core has entered fastsleep or winkle. 8th bit is
used as a lock bit.
The lock bit is set in these 2 scenarios-
- The thread is first in subcore to wakeup from sleep/winkle.
- If its the last thread in the core about to enter sleep/winkle

While the lock bit is set, if any other thread in the core wakes up, it
loops until the lock bit is cleared before proceeding in the wakeup
path. This helps prevent race conditions w.r.t fastsleep workaround and
prevents threads from switching to process context before core/subcore
resources are restored.

But, in the path to sleep/winkle entry, we currently don't check for
lock-bit. This exposes us to following race when running with subcore

First thread in the subcorea Another thread in the same
waking up core entering sleep/winkle

lwarx r15,0,r14
stwcx. r15,0,r14
[Code to restore subcore state]

lwarx r15,0,r14
[clear thread bit]
stwcx. r15,0,r14

stw r15,0(r14)

Here, after the thread entering sleep clears its thread bit in
core_idle_state, the value is overwritten by the thread waking up.
In such cases when the core enters fastsleep, code mistakes an idle
thread as running. Because of this, the first thread waking up from
fastsleep which is supposed to resync timebase skips it. So we can
end up having a core with stale timebase value.

This patch fixes the above race by looping on the lock bit even while
entering the idle states.

Signed-off-by: Shreyas B. Prabhu <shreyas@xxxxxxxxxxxxxxxxxx>
Fixes: 7b54e9f213f76 'powernv/powerpc: Add winkle support for offline cpus'
Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

arch/powerpc/kernel/idle_power7.S | 31 +++++++++++++++++++++----------
1 file changed, 21 insertions(+), 10 deletions(-)

--- a/arch/powerpc/kernel/idle_power7.S
+++ b/arch/powerpc/kernel/idle_power7.S
@@ -52,6 +52,22 @@

+ * Used by threads when the lock bit of core_idle_state is set.
+ * 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
+ */
+3: lwz r15,0(r14)
+ andi. r15,r15,PNV_CORE_IDLE_LOCK_BIT
+ bne 3b
+ lwarx r15,0,r14
+ blr
* Pass requested state in r3:
@@ -150,6 +166,10 @@ power7_enter_nap_mode:
lwarx r15,0,r14
+ andi. r9,r15,PNV_CORE_IDLE_LOCK_BIT
+ bnel core_idle_lock_held
andc r15,r15,r7 /* Clear thread bit */

@@ -294,7 +314,7 @@ lwarx_loop2:
* workaround undo code or resyncing timebase or restoring context
* In either case loop until the lock bit is cleared.
- bne core_idle_lock_held
+ bnel core_idle_lock_held

cmpwi cr2,r15,0
@@ -319,15 +339,6 @@ lwarx_loop2:
b common_exit

- lwz r15,0(14)
- andi. r9,r15,PNV_CORE_IDLE_LOCK_BIT
- bne core_idle_lock_loop
- b lwarx_loop2
/* First thread in subcore to wakeup */

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