The patch titled
Subject: printk: fix spinlock deadlock in printk reenty
has been added to the -mm tree. Its filename is
printk-fix-spinlock-deadlock-in-printk-reenty.patch
This patch should soon appear at
http://ozlabs.org/~akpm/mmots/broken-out/printk-fix-spinlock-deadlock-in-printk-reenty.patch
and later at
http://ozlabs.org/~akpm/mmotm/broken-out/printk-fix-spinlock-deadlock-in-printk-reenty.patch
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------------------------------------------------------
From: Jinling Ke <kejinling@xxxxxxxxxx>
Subject: printk: fix spinlock deadlock in printk reenty
When Oops in printk, printk will call zap_locks() to reinitialize spinlock
to prevent deadlock. In arm, arm64, x86 or other architecture smp cpu,
race condition will occur in printk spinlock logbuf_lock and then it will
result other cpu that is waiting printk spinlock in deadlock(in function
raw_spin_lock). Because the cpus deadlock, you can see the error printk
log:
"SMP: failed to stop secondary CPUs"
In arm, arm64, x86 or other architecture, spinlock variable is divided
into 2 parts, for example they are 'owner' and 'next' in arm. When get a
spinlock, the 'next' part will add 1 and wait 'next' being equal to
'owner'. However, at this moment, the 'next' part is local variable, but
'owner' part value is get from global variable logbuf_lock.
However,raw_spin_lock_init(&logbuf_lock) will set 'owner' part and 'next'
part to zero, the result is that cpu deadlock in function raw_spin_lock(
while loop in function arch_spin_lock ).
struct of arm spinlock
union {
u32 slock;
struct __raw_tickets {
u16 owner;
u16 next;
} tickets;
};
} arch_spinlock_t;
static inline void arch_spin_lock(arch_spinlock_t *lock)
{...
<--- At the moment, other cpu call zap_locks()->spin_lock_init(),
<--- set the 'owner' part to zero, but lockval.tickets.next is a
<--- local variable
while (lockval.tickets.next != lockval.tickets.owner) {
lockval.tickets.owner = ACCESS_ONCE(lock->tickets.owner);
}
...
}
The solution is that in zap_locks(), replace
raw_spin_lock_init(&logbuf_lock) with raw_spin_unlock(&logbuf_lock), to
let the spinlock stay in unlocked state.
Link: http://lkml.kernel.org/r/1480490119-63559-1-git-send-email-linyongting@xxxxxxxxxx
Signed-off-by: Jinling Ke <kejinling@xxxxxxxxxx>
Cc: Petr Mladek <pmladek@xxxxxxxx>
Cc: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: Thierry Reding <treding@xxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxx>
Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
kernel/printk/printk.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff -puN kernel/printk/printk.c~printk-fix-spinlock-deadlock-in-printk-reenty kernel/printk/printk.c
--- a/kernel/printk/printk.c~printk-fix-spinlock-deadlock-in-printk-reenty
+++ a/kernel/printk/printk.c
@@ -1603,7 +1603,7 @@ static void zap_locks(void)
debug_locks_off();
/* If a crash is occurring, make sure we can't deadlock */
- raw_spin_lock_init(&logbuf_lock);
+ raw_spin_unlock(&logbuf_lock);
/* And make sure that we print immediately */
sema_init(&console_sem, 1);
}
_
Patches currently in -mm which might be from kejinling@xxxxxxxxxx are
printk-fix-spinlock-deadlock-in-printk-reenty.patch
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