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out a real bug. During suspend and resume the tlb_flush tracepoint is called when the CPU is going offline. As the CPU has been noted as offline, RCU is ignoring that CPU, which means that it can not use RCU protected locks. When tracepoints are activated, they require RCU locking, and if RCU is ignoring a CPU that runs a tracepoint, there is a chance that the tracepoint could cause corruption. The solution was to change the tracepoint into a TRACE_EVENT_CONDITION() which allows us to check a condition to determine if the tracepoint should be called or not. If the condition is not met, the rcu protected code will not be executed. By adding the condition "cpu_online(smp_processor_id())", this will prevent the RCU protected code from being executed if the CPU is marked offline. After adding this, another bug was discovered. As RCU checks rcu callers, if a rcu call is not done, there is no check (obviously). We found that tracepoints could be added in RCU ignored locations and not have lockdep complain until the tracepoint is activated. This missed places where tracepoints were added in places they should not have been. To fix this, code was added in 3.18 that if lockdep is enabled, any tracepoint will still call the rcu checks even if the tracepoint is not enabled. The bug here, is that the check does not take the CONDITION into account. As the condition may prevent tracepoints from being activated in RCU ignored areas (as the one patch does), we get false positives when we enable lockdep and hit a tracepoint that the condition prevents it from being called in a RCU ignored location. The fix for this is to add the CONDITION to the rcu checks, even if the tracepoint is not enabled. -----BEGIN PGP SIGNATURE----- Version: GnuPG v1 iQEcBAABAgAGBQJU1rQfAAoJEEjnJuOKh9ld19UH/juFLZFjpYBgtRmbCZa/54Zk i2Fa3U8jQe8MHHEYOjCLT9MQTYo/42btmJhr7kWKtIoUgDEli4lkOpbs+H0qar5y Vv9+1cLeNFQzgIE3nwV7cjAw7Jufoyzd1lstDqIQvcmzZnQ5sNyyVeigMcxGv8Ls 4FyqzG6zCVgiDL4LyYNHdNcMr6qLs3KTFDEqp+kQreeO7R1r3ZEpq3JoWaEUgoPP qrYv/rqVosLBUGA0pd7RmiGOxhjeKm15qz1GkiPeeus6DDWC6bvPC8cAc/FfkXH0 hYpoQghSZVnXGy0LzVsd44gj7tYx1FHEpYy1s8G6d5WJcNOGZ6OoZOdOZMyjPVw= =PeL2 -----END PGP SIGNATURE----- Merge tag 'trace-fixes-v3.19-rc7' of git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace Pull ftrace fixes from Steven Rostedt: "During testing Sedat Dilek hit a "suspicious RCU usage" splat that pointed out a real bug. During suspend and resume the tlb_flush tracepoint is called when the CPU is going offline. As the CPU has been noted as offline, RCU is ignoring that CPU, which means that it can not use RCU protected locks. When tracepoints are activated, they require RCU locking, and if RCU is ignoring a CPU that runs a tracepoint, there is a chance that the tracepoint could cause corruption. The solution was to change the tracepoint into a TRACE_EVENT_CONDITION() which allows us to check a condition to determine if the tracepoint should be called or not. If the condition is not met, the rcu protected code will not be executed. By adding the condition "cpu_online(smp_processor_id())", this will prevent the RCU protected code from being executed if the CPU is marked offline. After adding this, another bug was discovered. As RCU checks rcu callers, if a rcu call is not done, there is no check (obviously). We found that tracepoints could be added in RCU ignored locations and not have lockdep complain until the tracepoint is activated. This missed places where tracepoints were added in places they should not have been. To fix this, code was added in 3.18 that if lockdep is enabled, any tracepoint will still call the rcu checks even if the tracepoint is not enabled. The bug here, is that the check does not take the CONDITION into account. As the condition may prevent tracepoints from being activated in RCU ignored areas (as the one patch does), we get false positives when we enable lockdep and hit a tracepoint that the condition prevents it from being called in a RCU ignored location. The fix for this is to add the CONDITION to the rcu checks, even if the tracepoint is not enabled" * tag 'trace-fixes-v3.19-rc7' of git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace: x86/tlb/trace: Do not trace on CPU that is offline tracing: Add condition check to RCU lockdep checks |
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