Patch "powerpc/powernv: Fix race in updating core_idle_state" has been added to the 4.1-stable tree

This is a note to let you know that I've just added the patch titled

powerpc/powernv: Fix race in updating core_idle_state

to the 4.1-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
powerpc-powernv-fix-race-in-updating-core_idle_state.patch
and it can be found in the queue-4.1 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: "Shreyas B. Prabhu" <[email protected]>
Date: Tue, 7 Jul 2015 01:39:23 +0530
Subject: powerpc/powernv: Fix race in updating core_idle_state

From: "Shreyas B. Prabhu" <[email protected]>

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 on-

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

lwarx   r15,0,r14
ori     r15,r15,PNV_CORE_IDLE_LOCK_BIT
stwcx.  r15,0,r14

[Code to restore subcore state]

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

andi. r15,r15,PNV_CORE_IDLE_THREAD_BITS
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.

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

@@ -52,6 +52,22 @@
 	.text
 
 /*
+ * 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
+ */
+
+core_idle_lock_held:
+	HMT_LOW
+3:	lwz	r15,0(r14)
+	andi.   r15,r15,PNV_CORE_IDLE_LOCK_BIT
+	bne	3b
+	HMT_MEDIUM
+	lwarx	r15,0,r14
+	blr
+
+/*
  * Pass requested state in r3:
  *	r3 - PNV_THREAD_NAP/SLEEP/WINKLE
  *

@@ -150,6 +166,10 @@ power7_enter_nap_mode:
 	ld	r14,PACA_CORE_IDLE_STATE_PTR(r13)
 lwarx_loop1:
 	lwarx	r15,0,r14
+
+	andi.   r9,r15,PNV_CORE_IDLE_LOCK_BIT
+	bnel	core_idle_lock_held
+
 	andc	r15,r15,r7			/* Clear thread bit */
 
 	andi.	r15,r15,PNV_CORE_IDLE_THREAD_BITS

@@ -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
 	lbz	r4,PACA_SUBCORE_SIBLING_MASK(r13)

@@ -319,15 +339,6 @@ lwarx_loop2:
 	isync
 	b	common_exit
 
-core_idle_lock_held:
-	HMT_LOW
-core_idle_lock_loop:
-	lwz	r15,0(14)
-	andi.   r9,r15,PNV_CORE_IDLE_LOCK_BIT
-	bne	core_idle_lock_loop
-	HMT_MEDIUM
-	b	lwarx_loop2
-
 first_thread_in_subcore:
 	/* First thread in subcore to wakeup */
 	ori	r15,r15,PNV_CORE_IDLE_LOCK_BIT

Patches currently in stable-queue which might be from [email protected] are

queue-4.1/powerpc-powernv-fix-race-in-updating-core_idle_state.patch