Messages in this thread Patch in this message | | | Date | Wed, 11 Oct 2017 13:50:20 +0200 | From | Borislav Petkov <> | Subject | Re: x86/mce: suspicious RCU usage in 4.13.4 |
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Ok,
here's a second attempt at a more rigorous simplification: RCU stuff is gone and only a single loop scans through the elements.
--- diff --git a/arch/x86/kernel/cpu/mcheck/dev-mcelog.c b/arch/x86/kernel/cpu/mcheck/dev-mcelog.c index 10cec43aac38..1e1c6d22c93e 100644 --- a/arch/x86/kernel/cpu/mcheck/dev-mcelog.c +++ b/arch/x86/kernel/cpu/mcheck/dev-mcelog.c @@ -24,14 +24,6 @@ static DEFINE_MUTEX(mce_chrdev_read_mutex); static char mce_helper[128]; static char *mce_helper_argv[2] = { mce_helper, NULL }; -#define mce_log_get_idx_check(p) \ -({ \ - RCU_LOCKDEP_WARN(!rcu_read_lock_sched_held() && \ - !lockdep_is_held(&mce_chrdev_read_mutex), \ - "suspicious mce_log_get_idx_check() usage"); \ - smp_load_acquire(&(p)); \ -}) - /* * Lockless MCE logging infrastructure. * This avoids deadlocks on printk locks without having to break locks. Also @@ -53,43 +45,36 @@ static int dev_mce_log(struct notifier_block *nb, unsigned long val, void *data) { struct mce *mce = (struct mce *)data; - unsigned int next, entry; - - wmb(); - for (;;) { - entry = mce_log_get_idx_check(mcelog.next); - for (;;) { - - /* - * When the buffer fills up discard new entries. - * Assume that the earlier errors are the more - * interesting ones: - */ - if (entry >= MCE_LOG_LEN) { - set_bit(MCE_OVERFLOW, - (unsigned long *)&mcelog.flags); - return NOTIFY_OK; - } - /* Old left over entry. Skip: */ - if (mcelog.entry[entry].finished) { - entry++; - continue; - } - break; - } - smp_rmb(); - next = entry + 1; - if (cmpxchg(&mcelog.next, entry, next) == entry) - break; + unsigned int entry; + + mutex_lock(&mce_chrdev_read_mutex); + + for (entry = mcelog.next; entry < MCE_LOG_LEN; entry++) { + /* Old left over entry. Skip: */ + if (mcelog.entry[entry].finished) + continue; + } + + /* + * When the buffer fills up discard new entries. Assume that the + * earlier errors are the more interesting ones: + */ + if (entry >= MCE_LOG_LEN) { + set_bit(MCE_OVERFLOW, (unsigned long *)&mcelog.flags); + goto unlock; } + + mcelog.next = entry + 1; + memcpy(mcelog.entry + entry, mce, sizeof(struct mce)); - wmb(); mcelog.entry[entry].finished = 1; - wmb(); /* wake processes polling /dev/mcelog */ wake_up_interruptible(&mce_chrdev_wait); +unlock: + mutex_unlock(&mce_chrdev_read_mutex); + return NOTIFY_OK; } @@ -247,7 +232,7 @@ static ssize_t mce_chrdev_read(struct file *filp, char __user *ubuf, goto out; } - next = mce_log_get_idx_check(mcelog.next); + next = mcelog.next; /* Only supports full reads right now */ err = -EINVAL; @@ -281,8 +266,6 @@ static ssize_t mce_chrdev_read(struct file *filp, char __user *ubuf, next = cmpxchg(&mcelog.next, prev, 0); } while (next != prev); - synchronize_sched(); - /* * Collect entries that were still getting written before the * synchronize. -- Regards/Gruss, Boris.
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