Feature #5349
Updated by Vadim Mutilin about 10 years ago
*Summary* # 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. *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":https://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git/commit/?id=82eabdf, "83c7c693":https://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git/commit/?id=83c7c693ed3e61535ad6a097ad991a88aafc54b8 "Sample bugfix":https://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git/commit/?id=83c7c693ed3e61535ad6a097ad991a88aafc54b8 *Example* example of bug (2.6.24 -> 2.6.25, drivers/char/specialix.c): <pre><code class="с"> @@ 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); </code></pre>