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Subject[RFC][PATCH -mm] PM: Fix SMP races in the freezer
Date
Currently, to tell a task that it should go to the refrigerator, we set the
PF_FREEZE flag for it and send a fake signal to it. Unfortunately there are
two SMP-related problems with this approach. First, a task running on another
CPU may be updating its flags while the freezer attempts to set PF_FREEZE
for it and this may leave the task's flags in an inconsistent state. Second,
there is a potential race between freeze_process() and refrigerator() in
which freeze_process() running on one CPU is reading a task's PF_FREEZE
flag while refrigerator() running on another CPU has just set PF_FROZEN
for the same task and attempts to reset PF_FREEZE for it. If the refrigerator
wins the race, freeze_process() will state that PF_FREEZE hasn't been set
for the task and will set it unnecessarily, so the task will go to the
refrigerator once again after it's been thawed.

To solve first of these problems we need to stop using PF_FREEZE to tell
tasks that they should go to the refrigerator. Instead, we can introduce
a special TIF_*** flag and use it for this purpose, since it is allowed to
change the other tasks' TIF_*** flags and there are special calls for it.

To avoid the freeze_process()-refrigerator() race we can make freeze_process()
to always check the task's PF_FROZEN flag after it's read its "freeze" flag.
We should also make sure that refrigerator() will always reset the task's
"freeze" flag after it's set PF_FROZEN for it.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Pavel Machek <pavel@ucw.cz>
---
include/asm-arm/thread_info.h | 2 ++
include/asm-frv/thread_info.h | 2 ++
include/asm-i386/thread_info.h | 2 ++
include/asm-ia64/thread_info.h | 2 ++
include/asm-powerpc/thread_info.h | 2 ++
include/asm-sh/thread_info.h | 2 ++
include/asm-x86_64/thread_info.h | 2 ++
include/linux/freezer.h | 11 ++++++-----
include/linux/sched.h | 1 -
kernel/power/process.c | 17 ++++++++++-------
10 files changed, 30 insertions(+), 13 deletions(-)

Index: linux-2.6.19-rc6-mm2/include/asm-arm/thread_info.h
===================================================================
--- linux-2.6.19-rc6-mm2.orig/include/asm-arm/thread_info.h
+++ linux-2.6.19-rc6-mm2/include/asm-arm/thread_info.h
@@ -137,6 +137,7 @@ extern void iwmmxt_task_switch(struct th
#define TIF_POLLING_NRFLAG 16
#define TIF_USING_IWMMXT 17
#define TIF_MEMDIE 18
+#define TIF_FREEZE 19

#define _TIF_NOTIFY_RESUME (1 << TIF_NOTIFY_RESUME)
#define _TIF_SIGPENDING (1 << TIF_SIGPENDING)
@@ -144,6 +145,7 @@ extern void iwmmxt_task_switch(struct th
#define _TIF_SYSCALL_TRACE (1 << TIF_SYSCALL_TRACE)
#define _TIF_POLLING_NRFLAG (1 << TIF_POLLING_NRFLAG)
#define _TIF_USING_IWMMXT (1 << TIF_USING_IWMMXT)
+#define _TIF_FREEZE (1 << TIF_FREEZE)

/*
* Change these and you break ASM code in entry-common.S
Index: linux-2.6.19-rc6-mm2/include/asm-ia64/thread_info.h
===================================================================
--- linux-2.6.19-rc6-mm2.orig/include/asm-ia64/thread_info.h
+++ linux-2.6.19-rc6-mm2/include/asm-ia64/thread_info.h
@@ -88,6 +88,7 @@ struct thread_info {
#define TIF_MEMDIE 17
#define TIF_MCA_INIT 18 /* this task is processing MCA or INIT */
#define TIF_DB_DISABLED 19 /* debug trap disabled for fsyscall */
+#define TIF_FREEZE 20 /* is freezing for suspend */

#define _TIF_SYSCALL_TRACE (1 << TIF_SYSCALL_TRACE)
#define _TIF_SYSCALL_AUDIT (1 << TIF_SYSCALL_AUDIT)
@@ -98,6 +99,7 @@ struct thread_info {
#define _TIF_POLLING_NRFLAG (1 << TIF_POLLING_NRFLAG)
#define _TIF_MCA_INIT (1 << TIF_MCA_INIT)
#define _TIF_DB_DISABLED (1 << TIF_DB_DISABLED)
+#define _TIF_FREEZE (1 << TIF_FREEZE)

/* "work to do on user-return" bits */
#define TIF_ALLWORK_MASK (_TIF_NOTIFY_RESUME|_TIF_SIGPENDING|_TIF_NEED_RESCHED|_TIF_SYSCALL_TRACE|_TIF_SYSCALL_AUDIT)
Index: linux-2.6.19-rc6-mm2/include/asm-sh/thread_info.h
===================================================================
--- linux-2.6.19-rc6-mm2.orig/include/asm-sh/thread_info.h
+++ linux-2.6.19-rc6-mm2/include/asm-sh/thread_info.h
@@ -106,6 +106,7 @@ static inline struct thread_info *curren
#define TIF_USEDFPU 16 /* FPU was used by this task this quantum (SMP) */
#define TIF_POLLING_NRFLAG 17 /* true if poll_idle() is polling TIF_NEED_RESCHED */
#define TIF_MEMDIE 18
+#define TIF_FREEZE 19

#define _TIF_SYSCALL_TRACE (1<<TIF_SYSCALL_TRACE)
#define _TIF_NOTIFY_RESUME (1<<TIF_NOTIFY_RESUME)
@@ -114,6 +115,7 @@ static inline struct thread_info *curren
#define _TIF_RESTORE_SIGMASK (1<<TIF_RESTORE_SIGMASK)
#define _TIF_USEDFPU (1<<TIF_USEDFPU)
#define _TIF_POLLING_NRFLAG (1<<TIF_POLLING_NRFLAG)
+#define _TIF_FREEZE (1<<TIF_FREEZE)

#define _TIF_WORK_MASK 0x000000FE /* work to do on interrupt/exception return */
#define _TIF_ALLWORK_MASK 0x000000FF /* work to do on any return to u-space */
Index: linux-2.6.19-rc6-mm2/include/asm-frv/thread_info.h
===================================================================
--- linux-2.6.19-rc6-mm2.orig/include/asm-frv/thread_info.h
+++ linux-2.6.19-rc6-mm2/include/asm-frv/thread_info.h
@@ -116,6 +116,7 @@ register struct thread_info *__current_t
#define TIF_RESTORE_SIGMASK 6 /* restore signal mask in do_signal() */
#define TIF_POLLING_NRFLAG 16 /* true if poll_idle() is polling TIF_NEED_RESCHED */
#define TIF_MEMDIE 17 /* OOM killer killed process */
+#define TIF_FREEZE 18 /* freezing for suspend */

#define _TIF_SYSCALL_TRACE (1 << TIF_SYSCALL_TRACE)
#define _TIF_NOTIFY_RESUME (1 << TIF_NOTIFY_RESUME)
@@ -125,6 +126,7 @@ register struct thread_info *__current_t
#define _TIF_IRET (1 << TIF_IRET)
#define _TIF_RESTORE_SIGMASK (1 << TIF_RESTORE_SIGMASK)
#define _TIF_POLLING_NRFLAG (1 << TIF_POLLING_NRFLAG)
+#define _TIF_FREEZE (1 << TIF_FREEZE)

#define _TIF_WORK_MASK 0x0000FFFE /* work to do on interrupt/exception return */
#define _TIF_ALLWORK_MASK 0x0000FFFF /* work to do on any return to u-space */
Index: linux-2.6.19-rc6-mm2/include/asm-i386/thread_info.h
===================================================================
--- linux-2.6.19-rc6-mm2.orig/include/asm-i386/thread_info.h
+++ linux-2.6.19-rc6-mm2/include/asm-i386/thread_info.h
@@ -134,6 +134,7 @@ static inline struct thread_info *curren
#define TIF_MEMDIE 16
#define TIF_DEBUG 17 /* uses debug registers */
#define TIF_IO_BITMAP 18 /* uses I/O bitmap */
+#define TIF_FREEZE 19 /* is freezing for suspend */

#define _TIF_SYSCALL_TRACE (1<<TIF_SYSCALL_TRACE)
#define _TIF_NOTIFY_RESUME (1<<TIF_NOTIFY_RESUME)
@@ -147,6 +148,7 @@ static inline struct thread_info *curren
#define _TIF_RESTORE_SIGMASK (1<<TIF_RESTORE_SIGMASK)
#define _TIF_DEBUG (1<<TIF_DEBUG)
#define _TIF_IO_BITMAP (1<<TIF_IO_BITMAP)
+#define _TIF_FREEZE (1<<TIF_FREEZE)

/* work to do on interrupt/exception return */
#define _TIF_WORK_MASK \
Index: linux-2.6.19-rc6-mm2/include/asm-powerpc/thread_info.h
===================================================================
--- linux-2.6.19-rc6-mm2.orig/include/asm-powerpc/thread_info.h
+++ linux-2.6.19-rc6-mm2/include/asm-powerpc/thread_info.h
@@ -122,6 +122,7 @@ static inline struct thread_info *curren
#define TIF_RESTOREALL 12 /* Restore all regs (implies NOERROR) */
#define TIF_NOERROR 14 /* Force successful syscall return */
#define TIF_RESTORE_SIGMASK 15 /* Restore signal mask in do_signal */
+#define TIF_FREEZE 16 /* Freezing for suspend */

/* as above, but as bit values */
#define _TIF_SYSCALL_TRACE (1<<TIF_SYSCALL_TRACE)
@@ -138,6 +139,7 @@ static inline struct thread_info *curren
#define _TIF_RESTOREALL (1<<TIF_RESTOREALL)
#define _TIF_NOERROR (1<<TIF_NOERROR)
#define _TIF_RESTORE_SIGMASK (1<<TIF_RESTORE_SIGMASK)
+#define _TIF_FREEZE (1<<TIF_FREEZE)
#define _TIF_SYSCALL_T_OR_A (_TIF_SYSCALL_TRACE|_TIF_SYSCALL_AUDIT|_TIF_SECCOMP)

#define _TIF_USER_WORK_MASK (_TIF_NOTIFY_RESUME | _TIF_SIGPENDING | \
Index: linux-2.6.19-rc6-mm2/include/asm-x86_64/thread_info.h
===================================================================
--- linux-2.6.19-rc6-mm2.orig/include/asm-x86_64/thread_info.h
+++ linux-2.6.19-rc6-mm2/include/asm-x86_64/thread_info.h
@@ -122,6 +122,7 @@ static inline struct thread_info *stack_
#define TIF_MEMDIE 20
#define TIF_DEBUG 21 /* uses debug registers */
#define TIF_IO_BITMAP 22 /* uses I/O bitmap */
+#define TIF_FREEZE 23 /* is freezing for suspend */

#define _TIF_SYSCALL_TRACE (1<<TIF_SYSCALL_TRACE)
#define _TIF_NOTIFY_RESUME (1<<TIF_NOTIFY_RESUME)
@@ -137,6 +138,7 @@ static inline struct thread_info *stack_
#define _TIF_ABI_PENDING (1<<TIF_ABI_PENDING)
#define _TIF_DEBUG (1<<TIF_DEBUG)
#define _TIF_IO_BITMAP (1<<TIF_IO_BITMAP)
+#define _TIF_FREEZE (1<<TIF_FREEZE)

/* work to do on interrupt/exception return */
#define _TIF_WORK_MASK \
Index: linux-2.6.19-rc6-mm2/include/linux/freezer.h
===================================================================
--- linux-2.6.19-rc6-mm2.orig/include/linux/freezer.h
+++ linux-2.6.19-rc6-mm2/include/linux/freezer.h
@@ -14,16 +14,15 @@ static inline int frozen(struct task_str
*/
static inline int freezing(struct task_struct *p)
{
- return p->flags & PF_FREEZE;
+ return test_tsk_thread_flag(p, TIF_FREEZE);
}

/*
* Request that a process be frozen
- * FIXME: SMP problem. We may not modify other process' flags!
*/
static inline void freeze(struct task_struct *p)
{
- p->flags |= PF_FREEZE;
+ set_tsk_thread_flag(p, TIF_FREEZE);
}

/*
@@ -31,7 +30,7 @@ static inline void freeze(struct task_st
*/
static inline void do_not_freeze(struct task_struct *p)
{
- p->flags &= ~PF_FREEZE;
+ clear_tsk_thread_flag(p, TIF_FREEZE);
}

/*
@@ -52,7 +51,9 @@ static inline int thaw_process(struct ta
*/
static inline void frozen_process(struct task_struct *p)
{
- p->flags = (p->flags & ~PF_FREEZE) | PF_FROZEN;
+ p->flags |= PF_FROZEN;
+ wmb();
+ clear_tsk_thread_flag(p, TIF_FREEZE);
}

extern void refrigerator(void);
Index: linux-2.6.19-rc6-mm2/include/linux/sched.h
===================================================================
--- linux-2.6.19-rc6-mm2.orig/include/linux/sched.h
+++ linux-2.6.19-rc6-mm2/include/linux/sched.h
@@ -1161,7 +1161,6 @@ static inline void put_task_struct(struc
#define PF_MEMALLOC 0x00000800 /* Allocating memory */
#define PF_FLUSHER 0x00001000 /* responsible for disk writeback */
#define PF_USED_MATH 0x00002000 /* if unset the fpu must be initialized before use */
-#define PF_FREEZE 0x00004000 /* this task is being frozen for suspend now */
#define PF_NOFREEZE 0x00008000 /* this thread should not be frozen */
#define PF_FROZEN 0x00010000 /* frozen for system suspend */
#define PF_FSTRANS 0x00020000 /* inside a filesystem transaction */
Index: linux-2.6.19-rc6-mm2/kernel/power/process.c
===================================================================
--- linux-2.6.19-rc6-mm2.orig/kernel/power/process.c
+++ linux-2.6.19-rc6-mm2/kernel/power/process.c
@@ -60,13 +60,16 @@ static inline void freeze_process(struct
unsigned long flags;

if (!freezing(p)) {
- if (p->state == TASK_STOPPED)
- force_sig_specific(SIGSTOP, p);
-
- freeze(p);
- spin_lock_irqsave(&p->sighand->siglock, flags);
- signal_wake_up(p, p->state == TASK_STOPPED);
- spin_unlock_irqrestore(&p->sighand->siglock, flags);
+ rmb();
+ if (!frozen(p)) {
+ if (p->state == TASK_STOPPED)
+ force_sig_specific(SIGSTOP, p);
+
+ freeze(p);
+ spin_lock_irqsave(&p->sighand->siglock, flags);
+ signal_wake_up(p, p->state == TASK_STOPPED);
+ spin_unlock_irqrestore(&p->sighand->siglock, flags);
+ }
}
}

-
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