From e1ac3614ff606ae03677f47459113f98a19af63c Mon Sep 17 00:00:00 2001 From: Paul Mackerras Date: Fri, 14 Aug 2009 15:39:10 +1000 Subject: perf_counter: Check task on counter read IPI In general, code in perf_counter.c that is called through an IPI checks, for per-task counters, that the counter's task is still the current task. This is to handle the race condition where the cpu switches from the task we want to another task in the interval between sending the IPI and the IPI arriving and being handled on the target CPU. For some reason, __perf_counter_read is missing this check, yet there is no reason why the race condition can't occur. This adds a check that the current task is the one we want. If it isn't, we just return. In that case the counter->count value should be up to date, since it will have been updated when the counter was scheduled out, which must have happened since the IPI was sent. I don't have an example of an actual failure due to this race, but it seems obvious that it could occur and we need to guard against it. Signed-off-by: Paul Mackerras Acked-by: Peter Zijlstra LKML-Reference: <19076.63614.277861.368125@drongo.ozlabs.ibm.com> Signed-off-by: Ingo Molnar --- kernel/perf_counter.c | 11 +++++++++++ 1 file changed, 11 insertions(+) (limited to 'kernel') diff --git a/kernel/perf_counter.c b/kernel/perf_counter.c index 534e20d14d6..b8fe7397b90 100644 --- a/kernel/perf_counter.c +++ b/kernel/perf_counter.c @@ -1503,10 +1503,21 @@ static void perf_counter_enable_on_exec(struct task_struct *task) */ static void __perf_counter_read(void *info) { + struct perf_cpu_context *cpuctx = &__get_cpu_var(perf_cpu_context); struct perf_counter *counter = info; struct perf_counter_context *ctx = counter->ctx; unsigned long flags; + /* + * If this is a task context, we need to check whether it is + * the current task context of this cpu. If not it has been + * scheduled out before the smp call arrived. In that case + * counter->count would have been updated to a recent sample + * when the counter was scheduled out. + */ + if (ctx->task && cpuctx->task_ctx != ctx) + return; + local_irq_save(flags); if (ctx->is_active) update_context_time(ctx); -- cgit v1.2.3 From f738eb1b63edf664da1b4ac76895d988749b2f07 Mon Sep 17 00:00:00 2001 From: Ingo Molnar Date: Tue, 18 Aug 2009 11:32:24 +0200 Subject: perf_counter: Fix the PARISC build PARISC does not build: /home/mingo/tip/kernel/perf_counter.c: In function 'perf_counter_index': /home/mingo/tip/kernel/perf_counter.c:2016: error: 'PERF_COUNTER_INDEX_OFFSET' undeclared (first use in this function) /home/mingo/tip/kernel/perf_counter.c:2016: error: (Each undeclared identifier is reported only once /home/mingo/tip/kernel/perf_counter.c:2016: error: for each function it appears in.) As PERF_COUNTER_INDEX_OFFSET is not defined. Now, we could define it in the architecture - but lets also provide a core default of 0 (which happens to be what all but one architecture uses at the moment). Architectures that need a different index offset should set this value in their asm/perf_counter.h files. Cc: Kyle McMartin Cc: Helge Deller Cc: linux-parisc@vger.kernel.org Cc: Peter Zijlstra Cc: Mike Galbraith Cc: Paul Mackerras Cc: Arnaldo Carvalho de Melo Cc: Frederic Weisbecker LKML-Reference: Signed-off-by: Ingo Molnar --- kernel/perf_counter.c | 4 ++++ 1 file changed, 4 insertions(+) (limited to 'kernel') diff --git a/kernel/perf_counter.c b/kernel/perf_counter.c index b8fe7397b90..36f65e2b8b5 100644 --- a/kernel/perf_counter.c +++ b/kernel/perf_counter.c @@ -2019,6 +2019,10 @@ int perf_counter_task_disable(void) return 0; } +#ifndef PERF_COUNTER_INDEX_OFFSET +# define PERF_COUNTER_INDEX_OFFSET 0 +#endif + static int perf_counter_index(struct perf_counter *counter) { if (counter->state != PERF_COUNTER_STATE_ACTIVE) -- cgit v1.2.3