x86_64: support poll() on /dev/mcelog

Background:
 /dev/mcelog is typically polled manually.  This is less than optimal for
 situations where accurate accounting of MCEs is important.  Calling
 poll() on /dev/mcelog does not work.

Description:
 This patch adds support for poll() to /dev/mcelog.  This results in
 immediate wakeup of user apps whenever the poller finds MCEs.  Because
 the exception handler can not take any locks, it can not call the wakeup
 itself.  Instead, it uses a thread_info flag (TIF_MCE_NOTIFY) which is
 caught at the next return from interrupt or exit from idle, calling the
 mce_user_notify() routine.  This patch also disables the "fake panic"
 path of the mce_panic(), because it results in printk()s in the exception
 handler and crashy systems.

 This patch also does some small cleanup for essentially unused variables,
 and moves the user notification into the body of the poller, so it is
 only called once per poll, rather than once per CPU.

Result:
 Applications can now poll() on /dev/mcelog.  When an error is logged
 (whether through the poller or through an exception) the applications are
 woken up promptly.  This should not affect any previous behaviors.  If no
 MCEs are being logged, there is no overhead.

Alternatives:
 I considered simply supporting poll() through the poller and not using
 TIF_MCE_NOTIFY at all.  However, the time between an uncorrectable error
 happening and the user application being notified is *the*most* critical
 window for us.  Many uncorrectable errors can be logged to the network if
 given a chance.

 I also considered doing the MCE poll directly from the idle notifier, but
 decided that was overkill.

Testing:
 I used an error-injecting DIMM to create lots of correctable DRAM errors
 and verified that my user app is woken up in sync with the polling interval.
 I also used the northbridge to inject uncorrectable ECC errors, and
 verified (printk() to the rescue) that the notify routine is called and the
 user app does wake up.  I built with PREEMPT on and off, and verified
 that my machine survives MCEs.

[wli@holomorphy.com: build fix]
Signed-off-by: Tim Hockin <thockin@google.com>
Signed-off-by: William Irwin <bill.irwin@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 4 insertions, 0 deletions

diff --git a/include/asm-x86_64/mce.h b/include/asm-x86_64/mce.h
index 177e92b4019..556be5563e3 100644
--- a/include/asm-x86_64/mce.h
+++ b/include/asm-x86_64/mce.h
@@ -105,6 +105,8 @@ extern atomic_t mce_entry;
 
 extern void do_machine_check(struct pt_regs *, long);
 
+extern int mce_notify_user(void);
+
 #endif
 
 #endif
diff --git a/include/asm-x86_64/thread_info.h b/include/asm-x86_64/thread_info.h
index 10bb5a8ed68..33c72ef15a0 100644
--- a/include/asm-x86_64/thread_info.h
+++ b/include/asm-x86_64/thread_info.h
@@ -115,6 +115,7 @@ static inline struct thread_info *stack_thread_info(void)
 #define TIF_SYSCALL_AUDIT	7	/* syscall auditing active */
 #define TIF_SECCOMP		8	/* secure computing */
 #define TIF_RESTORE_SIGMASK	9	/* restore signal mask in do_signal */
+#define TIF_MCE_NOTIFY		10	/* notify userspace of an MCE */
 /* 16 free */
 #define TIF_IA32		17	/* 32bit process */ 
 #define TIF_FORK		18	/* ret_from_fork */
@@ -133,6 +134,7 @@ static inline struct thread_info *stack_thread_info(void)
 #define _TIF_SYSCALL_AUDIT	(1<<TIF_SYSCALL_AUDIT)
 #define _TIF_SECCOMP		(1<<TIF_SECCOMP)
 #define _TIF_RESTORE_SIGMASK	(1<<TIF_RESTORE_SIGMASK)
+#define _TIF_MCE_NOTIFY		(1<<TIF_MCE_NOTIFY)
 #define _TIF_IA32		(1<<TIF_IA32)
 #define _TIF_FORK		(1<<TIF_FORK)
 #define _TIF_ABI_PENDING	(1<<TIF_ABI_PENDING)