diff options
| author | John Ogness | 2021-06-17 11:56:51 +0206 |
|---|---|---|
| committer | Petr Mladek | 2021-06-22 09:57:15 +0200 |
| commit | 3342aa8e6b4f6e3f1521e9b4cf5cfe50dbc37774 (patch) | |
| tree | 40f06e36bcbedf876fa6d3b8b1ebb2713ee7aec7 /kernel/printk | |
| parent | 766c268bc6d39b8124e50d075a36b8a3305bc8e2 (diff) | |
printk: fix cpu lock ordering
The cpu lock implementation uses a full memory barrier to take
the lock, but no memory barriers when releasing the lock. This
means that changes performed by a lock owner may not be seen by
the next lock owner. This may have been "good enough" for use
by dump_stack() as a serialization mechanism, but it is not
enough to provide proper protection for a critical section.
Correct this problem by using acquire/release memory barriers
for lock/unlock, respectively.
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20210617095051.4808-3-john.ogness@linutronix.de
Diffstat (limited to 'kernel/printk')
| -rw-r--r-- | kernel/printk/printk.c | 53 |
1 files changed, 50 insertions, 3 deletions
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index 9dfad0efb67f..142a58d124d9 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -3568,10 +3568,33 @@ int __printk_cpu_trylock(void) cpu = smp_processor_id(); - old = atomic_cmpxchg(&printk_cpulock_owner, -1, cpu); + /* + * Guarantee loads and stores from this CPU when it is the lock owner + * are _not_ visible to the previous lock owner. This pairs with + * __printk_cpu_unlock:B. + * + * Memory barrier involvement: + * + * If __printk_cpu_trylock:A reads from __printk_cpu_unlock:B, then + * __printk_cpu_unlock:A can never read from __printk_cpu_trylock:B. + * + * Relies on: + * + * RELEASE from __printk_cpu_unlock:A to __printk_cpu_unlock:B + * of the previous CPU + * matching + * ACQUIRE from __printk_cpu_trylock:A to __printk_cpu_trylock:B + * of this CPU + */ + old = atomic_cmpxchg_acquire(&printk_cpulock_owner, -1, + cpu); /* LMM(__printk_cpu_trylock:A) */ if (old == -1) { - /* This CPU is now the owner. */ + /* + * This CPU is now the owner and begins loading/storing + * data: LMM(__printk_cpu_trylock:B) + */ return 1; + } else if (old == cpu) { /* This CPU is already the owner. */ atomic_inc(&printk_cpulock_nested); @@ -3596,7 +3619,31 @@ void __printk_cpu_unlock(void) return; } - atomic_set(&printk_cpulock_owner, -1); + /* + * This CPU is finished loading/storing data: + * LMM(__printk_cpu_unlock:A) + */ + + /* + * Guarantee loads and stores from this CPU when it was the + * lock owner are visible to the next lock owner. This pairs + * with __printk_cpu_trylock:A. + * + * Memory barrier involvement: + * + * If __printk_cpu_trylock:A reads from __printk_cpu_unlock:B, + * then __printk_cpu_trylock:B reads from __printk_cpu_unlock:A. + * + * Relies on: + * + * RELEASE from __printk_cpu_unlock:A to __printk_cpu_unlock:B + * of this CPU + * matching + * ACQUIRE from __printk_cpu_trylock:A to __printk_cpu_trylock:B + * of the next CPU + */ + atomic_set_release(&printk_cpulock_owner, + -1); /* LMM(__printk_cpu_unlock:B) */ } EXPORT_SYMBOL(__printk_cpu_unlock); #endif /* CONFIG_SMP */ |