[PATCH] PM: Fix SMP races in the freezer

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>
Cc: Russell King <rmk@arm.linux.org.uk>
Cc: David Howells <dhowells@redhat.com>
Cc: Andi Kleen <ak@muc.de>
Cc: "Luck, Tony" <tony.luck@intel.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Paul Mundt <lethal@linux-sh.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

10 files changed, 30 insertions, 13 deletions

diff --git a/include/asm-arm/thread_info.h b/include/asm-arm/thread_info.h
index d9b8bddc8732..5014794f9eb3 100644
--- a/include/asm-arm/thread_info.h
+++ b/include/asm-arm/thread_info.h
@@ -147,6 +147,7 @@ extern void iwmmxt_task_switch(struct thread_info *);
 #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)
@@ -154,6 +155,7 @@ extern void iwmmxt_task_switch(struct thread_info *);
 #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
diff --git a/include/asm-frv/thread_info.h b/include/asm-frv/thread_info.h
index d66c48e6ef14..d881f518e6a9 100644
--- a/include/asm-frv/thread_info.h
+++ b/include/asm-frv/thread_info.h
@@ -116,6 +116,7 @@ register struct thread_info *__current_thread_info asm("gr15");
 #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_thread_info asm("gr15");
 #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 */
diff --git a/include/asm-i386/thread_info.h b/include/asm-i386/thread_info.h
index 46d32ad92082..4b187bb377b4 100644
--- a/include/asm-i386/thread_info.h
+++ b/include/asm-i386/thread_info.h
@@ -134,6 +134,7 @@ static inline struct thread_info *current_thread_info(void)
 #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 *current_thread_info(void)
 #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 \
diff --git a/include/asm-ia64/thread_info.h b/include/asm-ia64/thread_info.h
index 8adcde0934ca..9b505b25544f 100644
--- a/include/asm-ia64/thread_info.h
+++ b/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)
diff --git a/include/asm-powerpc/thread_info.h b/include/asm-powerpc/thread_info.h
index d339e2e88b11..3f32ca8bfec9 100644
--- a/include/asm-powerpc/thread_info.h
+++ b/include/asm-powerpc/thread_info.h
@@ -122,6 +122,7 @@ static inline struct thread_info *current_thread_info(void)
 #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 *current_thread_info(void)
 #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 | \
diff --git a/include/asm-sh/thread_info.h b/include/asm-sh/thread_info.h
index 0c01dc550819..879f741105db 100644
--- a/include/asm-sh/thread_info.h
+++ b/include/asm-sh/thread_info.h
@@ -106,6 +106,7 @@ static inline struct thread_info *current_thread_info(void)
 #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 *current_thread_info(void)
 #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 */
diff --git a/include/asm-x86_64/thread_info.h b/include/asm-x86_64/thread_info.h
index 787a08114b48..74a6c74397f7 100644
--- a/include/asm-x86_64/thread_info.h
+++ b/include/asm-x86_64/thread_info.h
@@ -122,6 +122,7 @@ static inline struct thread_info *stack_thread_info(void)
 #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_thread_info(void)
 #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 \
diff --git a/include/linux/freezer.h b/include/linux/freezer.h
index 393063096134..5e75e26d4787 100644
--- a/include/linux/freezer.h
+++ b/include/linux/freezer.h
@@ -16,16 +16,15 @@ static inline int frozen(struct task_struct *p)
  */
 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);
 }
 
 /*
@@ -33,7 +32,7 @@ static inline void freeze(struct task_struct *p)
  */
 static inline void do_not_freeze(struct task_struct *p)
 {
-        p->flags &= ~PF_FREEZE;
+        clear_tsk_thread_flag(p, TIF_FREEZE);
 }
 
 /*
@@ -54,7 +53,9 @@ static inline int thaw_process(struct task_struct *p)
  */
 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);
diff --git a/include/linux/sched.h b/include/linux/sched.h
index ea92e5c89089..446373535190 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1144,7 +1144,6 @@ static inline void put_task_struct(struct task_struct *t)
 #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 */
diff --git a/kernel/power/process.c b/kernel/power/process.c
index b9a32860bef3..6d566bf7085c 100644
--- a/kernel/power/process.c
+++ b/kernel/power/process.c
@@ -60,13 +60,16 @@ static inline void freeze_process(struct task_struct *p)
         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);
+                }
         }
 }