Linux Driver Verification » Linux Kernel Safety RuleDB

Summary

1. It’s not allowed to acquire spin_lock twice.
2. It’s not allowed to release not acquired spin_lock.
3. At the end all spin_lock should be released.

Description

Unlike other operating systems spin-locks in Linux are not recursive. So if a thread tries to acquire a lock it has already acquired then this thread begins a periodical check and waits till it releases the lock. This leads to a deadlocks. The situation is not often in its explicit form, but it is widely spread in an implicit form when some function (external function usually) tries to acquire a lock that has already been acquired by the module.

It’s not allowed to acquire spin_lock twice. It’s not allowed to release not acquired spin_lock. At the end all spin_lock should be released. It’s not allowed to re-release a lock by spin_unlock/spin_unlock_irqrestore functions.

Links

Sample bugfixes 82eabdf, 83c7c693

Example

example of bug (2.6.24 -> 2.6.25, drivers/char/specialix.c):

    @@ 2109  
    sx_out(bp, CD186x_CAR, port_No(port));  
    /* releasing a lock */  
    spin_unlock_irqrestore(&bp->lock, flags);   
    if (I_IXOFF(tty)) {  
        /* in some cases double unlock is possible */  
        spin_unlock_irqrestore(&bp->lock, flags);  
        sx_wait_CCR(bp);  
        spin_lock_irqsave(&bp->lock, flags);  
        sx_out(bp, CD186x_CCR, CCR_SSCH2);