pstore: Avoid deadlock in panic and emergency-restart path

commit 9f244e9cfd70c7c0f82d3c92ce772ab2a92d9f64 upstream.

[Issue]

When pstore is in panic and emergency-restart paths, it may be blocked
in those paths because it simply takes spin_lock.

This is an example scenario which pstore may hang up in a panic path:

 - cpuA grabs psinfo->buf_lock
 - cpuB panics and calls smp_send_stop
 - smp_send_stop sends IRQ to cpuA
 - after 1 second, cpuB gives up on cpuA and sends an NMI instead
 - cpuA is now in an NMI handler while still holding buf_lock
 - cpuB is deadlocked

This case may happen if a firmware has a bug and
cpuA is stuck talking with it more than one second.

Also, this is a similar scenario in an emergency-restart path:

 - cpuA grabs psinfo->buf_lock and stucks in a firmware
 - cpuB kicks emergency-restart via either sysrq-b or hangcheck timer.
   And then, cpuB is deadlocked by taking psinfo->buf_lock again.

[Solution]

This patch avoids the deadlocking issues in both panic and emergency_restart
paths by introducing a function, is_non_blocking_path(), to check if a cpu
can be blocked in current path.

With this patch, pstore is not blocked even if another cpu has
taken a spin_lock, in those paths by changing from spin_lock_irqsave
to spin_trylock_irqsave.

In addition, according to a comment of emergency_restart() in kernel/sys.c,
spin_lock shouldn't be taken in an emergency_restart path to avoid
deadlock. This patch fits the comment below.

<snip>
/**
 *      emergency_restart - reboot the system
 *
 *      Without shutting down any hardware or taking any locks
 *      reboot the system.  This is called when we know we are in
 *      trouble so this is our best effort to reboot.  This is
 *      safe to call in interrupt context.
 */
void emergency_restart(void)
<snip>

Signed-off-by: Seiji Aguchi <seiji.aguchi@hds.com>
Acked-by: Don Zickus <dzickus@redhat.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>
[bwh: Backported to 3.2:
 - Adjust context
 - Add #include <linux/kmsg_dump.h>]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/fs/pstore/platform.c b/fs/pstore/platform.c
index 57bbf90..45d18d1 100644 (file)
--- a/fs/pstore/platform.c
+++ b/fs/pstore/platform.c
@@ -72,6 +72,27 @@ static char *reason_str[] = {
        "Oops", "Panic", "Kexec", "Restart", "Halt", "Poweroff", "Emergency"
 };
 
+bool pstore_cannot_block_path(enum kmsg_dump_reason reason)
+{
+       /*
+        * In case of NMI path, pstore shouldn't be blocked
+        * regardless of reason.
+        */
+       if (in_nmi())
+               return true;
+
+       switch (reason) {
+       /* In panic case, other cpus are stopped by smp_send_stop(). */
+       case KMSG_DUMP_PANIC:
+       /* Emergency restart shouldn't be blocked by spin lock. */
+       case KMSG_DUMP_EMERG:
+               return true;
+       default:
+               return false;
+       }
+}
+EXPORT_SYMBOL_GPL(pstore_cannot_block_path);
+
 /*
  * callback from kmsg_dump. (s2,l2) has the most recently
  * written bytes, older bytes are in (s1,l1). Save as much
@@ -97,10 +118,12 @@ static void pstore_dump(struct kmsg_dumper *dumper,
        else
                why = "Unknown";
 
-       if (in_nmi()) {
-               is_locked = spin_trylock(&psinfo->buf_lock);
-               if (!is_locked)
-                       pr_err("pstore dump routine blocked in NMI, may corrupt error record\n");
+       if (pstore_cannot_block_path(reason)) {
+               is_locked = spin_trylock_irqsave(&psinfo->buf_lock, flags);
+               if (!is_locked) {
+                       pr_err("pstore dump routine blocked in %s path, may corrupt error record\n"
+                                      , in_nmi() ? "NMI" : why);
+               }
        } else
                spin_lock_irqsave(&psinfo->buf_lock, flags);
        oopscount++;
@@ -131,9 +154,9 @@ static void pstore_dump(struct kmsg_dumper *dumper,
                total += l1_cpy + l2_cpy;
                part++;
        }
-       if (in_nmi()) {
+       if (pstore_cannot_block_path(reason)) {
                if (is_locked)
-                       spin_unlock(&psinfo->buf_lock);
+                       spin_unlock_irqrestore(&psinfo->buf_lock, flags);
        } else
                spin_unlock_irqrestore(&psinfo->buf_lock, flags);
 }

diff --git a/include/linux/pstore.h b/include/linux/pstore.h
index 2ca8cde..9b16969 100644 (file)
--- a/include/linux/pstore.h
+++ b/include/linux/pstore.h
@@ -22,6 +22,8 @@
 #ifndef _LINUX_PSTORE_H
 #define _LINUX_PSTORE_H
 
+#include <linux/kmsg_dump.h>
+
 /* types */
 enum pstore_type_id {
        PSTORE_TYPE_DMESG       = 0,
@@ -50,6 +52,7 @@ struct pstore_info {
 
 #ifdef CONFIG_PSTORE
 extern int pstore_register(struct pstore_info *);
+extern bool pstore_cannot_block_path(enum kmsg_dump_reason reason);
 extern int pstore_write(enum pstore_type_id type, char *buf, size_t size);
 #else
 static inline int
@@ -57,6 +60,11 @@ pstore_register(struct pstore_info *psi)
 {
        return -ENODEV;
 }
+static inline bool
+pstore_cannot_block_path(enum kmsg_dump_reason reason)
+{
+       return false;
+}
 static inline int
 pstore_write(enum pstore_type_id type, char *buf, size_t size)
 {