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authorPetr Mladek <pmladek@suse.com>2016-12-14 15:05:55 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2016-12-14 16:04:08 -0800
commitd5d8d3d0d4adcc3aec6e2e0fb656165014a712b7 (patch)
tree090aa03f64ff9535310f073d3574fb692f720fa9 /usr/Kconfig
parentd1bd8ead126668a2d6c42d97cc3664e95b3fa1dc (diff)
kdb: properly synchronize vkdb_printf() calls with other CPUs
kdb_printf_lock does not prevent other CPUs from entering the critical section because it is ignored when KDB_STATE_PRINTF_LOCK is set. The problematic situation might look like: CPU0 CPU1 vkdb_printf() if (!KDB_STATE(PRINTF_LOCK)) KDB_STATE_SET(PRINTF_LOCK); spin_lock_irqsave(&kdb_printf_lock, flags); vkdb_printf() if (!KDB_STATE(PRINTF_LOCK)) BANG: The PRINTF_LOCK state is set and CPU1 is entering the critical section without spinning on the lock. The problem is that the code tries to implement locking using two state variables that are not handled atomically. Well, we need a custom locking because we want to allow reentering the critical section on the very same CPU. Let's use solution from Petr Zijlstra that was proposed for a similar scenario, see https://lkml.kernel.org/r/20161018171513.734367391@infradead.org This patch uses the same trick with cmpxchg(). The only difference is that we want to handle only recursion from the same context and therefore we disable interrupts. In addition, KDB_STATE_PRINTF_LOCK is removed. In fact, we are not able to set it a non-racy way. Link: http://lkml.kernel.org/r/1480412276-16690-3-git-send-email-pmladek@suse.com Signed-off-by: Petr Mladek <pmladek@suse.com> Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org> Cc: Jason Wessel <jason.wessel@windriver.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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