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authorOleg Nesterov <oleg@redhat.com>2011-01-19 19:22:07 +0100
committerIngo Molnar <mingo@elte.hu>2011-01-19 20:04:27 +0100
commitdbe08d82ce3967ccdf459f7951d02589cf967300 (patch)
tree2f8fc95749e47ff0de4b369f298898e4db5ad391 /kernel
parentc56eb8fb6dccb83d9fe62fd4dc00c834de9bc470 (diff)
perf: Fix find_get_context() vs perf_event_exit_task() race
find_get_context() must not install the new perf_event_context if the task has already passed perf_event_exit_task(). If nothing else, this means the memory leak. Initially ctx->refcount == 2, it is supposed that perf_event_exit_task_context() should participate and do the necessary put_ctx(). find_lively_task_by_vpid() checks PF_EXITING but this buys nothing, by the time we call find_get_context() this task can be already dead. To the point, cmpxchg() can succeed when the task has already done the last schedule(). Change find_get_context() to populate task->perf_event_ctxp[] under task->perf_event_mutex, this way we can trust PF_EXITING because perf_event_exit_task() takes the same mutex. Also, change perf_event_exit_task_context() to use rcu_dereference(). Probably this is not strictly needed, but with or without this change find_get_context() can race with setup_new_exec()->perf_event_exit_task(), rcu_dereference() looks better. Signed-off-by: Oleg Nesterov <oleg@redhat.com> Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Alan Stern <stern@rowland.harvard.edu> Cc: Arnaldo Carvalho de Melo <acme@redhat.com> Cc: Frederic Weisbecker <fweisbec@gmail.com> Cc: Paul Mackerras <paulus@samba.org> Cc: Prasad <prasad@linux.vnet.ibm.com> Cc: Roland McGrath <roland@redhat.com> LKML-Reference: <20110119182207.GB12183@redhat.com> Signed-off-by: Ingo Molnar <mingo@elte.hu>
Diffstat (limited to 'kernel')
-rw-r--r--kernel/perf_event.c34
1 files changed, 20 insertions, 14 deletions
diff --git a/kernel/perf_event.c b/kernel/perf_event.c
index 84522c796987..4ec55ef5810c 100644
--- a/kernel/perf_event.c
+++ b/kernel/perf_event.c
@@ -2201,13 +2201,6 @@ find_lively_task_by_vpid(pid_t vpid)
if (!task)
return ERR_PTR(-ESRCH);
- /*
- * Can't attach events to a dying task.
- */
- err = -ESRCH;
- if (task->flags & PF_EXITING)
- goto errout;
-
/* Reuse ptrace permission checks for now. */
err = -EACCES;
if (!ptrace_may_access(task, PTRACE_MODE_READ))
@@ -2268,14 +2261,27 @@ retry:
get_ctx(ctx);
- if (cmpxchg(&task->perf_event_ctxp[ctxn], NULL, ctx)) {
- /*
- * We raced with some other task; use
- * the context they set.
- */
+ err = 0;
+ mutex_lock(&task->perf_event_mutex);
+ /*
+ * If it has already passed perf_event_exit_task().
+ * we must see PF_EXITING, it takes this mutex too.
+ */
+ if (task->flags & PF_EXITING)
+ err = -ESRCH;
+ else if (task->perf_event_ctxp[ctxn])
+ err = -EAGAIN;
+ else
+ rcu_assign_pointer(task->perf_event_ctxp[ctxn], ctx);
+ mutex_unlock(&task->perf_event_mutex);
+
+ if (unlikely(err)) {
put_task_struct(task);
kfree(ctx);
- goto retry;
+
+ if (err == -EAGAIN)
+ goto retry;
+ goto errout;
}
}
@@ -6127,7 +6133,7 @@ static void perf_event_exit_task_context(struct task_struct *child, int ctxn)
* scheduled, so we are now safe from rescheduling changing
* our context.
*/
- child_ctx = child->perf_event_ctxp[ctxn];
+ child_ctx = rcu_dereference(child->perf_event_ctxp[ctxn]);
task_ctx_sched_out(child_ctx, EVENT_ALL);
/*