diff options
Diffstat (limited to 'Documentation/RCU/NMI-RCU.txt')
| -rw-r--r-- | Documentation/RCU/NMI-RCU.txt | 45 |
1 files changed, 25 insertions, 20 deletions
diff --git a/Documentation/RCU/NMI-RCU.txt b/Documentation/RCU/NMI-RCU.txt index a6d32e65d22..687777f83b2 100644 --- a/Documentation/RCU/NMI-RCU.txt +++ b/Documentation/RCU/NMI-RCU.txt @@ -5,8 +5,8 @@ Although RCU is usually used to protect read-mostly data structures, it is possible to use RCU to provide dynamic non-maskable interrupt handlers, as well as dynamic irq handlers. This document describes how to do this, drawing loosely from Zwane Mwaikambo's NMI-timer -work in "arch/i386/oprofile/nmi_timer_int.c" and in -"arch/i386/kernel/traps.c". +work in "arch/x86/oprofile/nmi_timer_int.c" and in +"arch/x86/kernel/traps.c". The relevant pieces of code are listed below, each followed by a brief explanation. @@ -34,7 +34,7 @@ NMI handler. cpu = smp_processor_id(); ++nmi_count(cpu); - if (!rcu_dereference(nmi_callback)(regs, cpu)) + if (!rcu_dereference_sched(nmi_callback)(regs, cpu)) default_do_nmi(regs); nmi_exit(); @@ -47,12 +47,13 @@ function pointer. If this handler returns zero, do_nmi() invokes the default_do_nmi() function to handle a machine-specific NMI. Finally, preemption is restored. -Strictly speaking, rcu_dereference() is not needed, since this code runs -only on i386, which does not need rcu_dereference() anyway. However, -it is a good documentation aid, particularly for anyone attempting to -do something similar on Alpha. +In theory, rcu_dereference_sched() is not needed, since this code runs +only on i386, which in theory does not need rcu_dereference_sched() +anyway. However, in practice it is a good documentation aid, particularly +for anyone attempting to do something similar on Alpha or on systems +with aggressive optimizing compilers. -Quick Quiz: Why might the rcu_dereference() be necessary on Alpha, +Quick Quiz: Why might the rcu_dereference_sched() be necessary on Alpha, given that the code referenced by the pointer is read-only? @@ -94,22 +95,26 @@ not to return until all ongoing NMI handlers exit. It is therefore safe to free up the handler's data as soon as synchronize_sched() returns. Important note: for this to work, the architecture in question must -invoke irq_enter() and irq_exit() on NMI entry and exit, respectively. +invoke nmi_enter() and nmi_exit() on NMI entry and exit, respectively. Answer to Quick Quiz - Why might the rcu_dereference() be necessary on Alpha, given + Why might the rcu_dereference_sched() be necessary on Alpha, given that the code referenced by the pointer is read-only? Answer: The caller to set_nmi_callback() might well have - initialized some data that is to be used by the - new NMI handler. In this case, the rcu_dereference() - would be needed, because otherwise a CPU that received - an NMI just after the new handler was set might see - the pointer to the new NMI handler, but the old - pre-initialized version of the handler's data. - - More important, the rcu_dereference() makes it clear - to someone reading the code that the pointer is being - protected by RCU. + initialized some data that is to be used by the new NMI + handler. In this case, the rcu_dereference_sched() would + be needed, because otherwise a CPU that received an NMI + just after the new handler was set might see the pointer + to the new NMI handler, but the old pre-initialized + version of the handler's data. + + This same sad story can happen on other CPUs when using + a compiler with aggressive pointer-value speculation + optimizations. + + More important, the rcu_dereference_sched() makes it + clear to someone reading the code that the pointer is + being protected by RCU-sched. |