perf: Restructure perf syscall point of no return

The exclusive_event_installable() stuff only works because its
exclusive with the grouping bits.

Rework the code such that there is a sane place to error out before we
go do things we cannot undo.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 32 insertions, 17 deletions

diff --git a/kernel/events/core.c b/kernel/events/core.c
index f548f69c4299..39679f749500 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -8297,13 +8297,30 @@ SYSCALL_DEFINE5(perf_event_open,
 
         if (move_group) {
                 gctx = group_leader->ctx;
+                mutex_lock_double(&gctx->mutex, &ctx->mutex);
+        } else {
+                mutex_lock(&ctx->mutex);
+        }
+
+        /*
+         * Must be under the same ctx::mutex as perf_install_in_context(),
+         * because we need to serialize with concurrent event creation.
+         */
+        if (!exclusive_event_installable(event, ctx)) {
+                /* exclusive and group stuff are assumed mutually exclusive */
+                WARN_ON_ONCE(move_group);
+
+                err = -EBUSY;
+                goto err_locked;
+        }
 
+        WARN_ON_ONCE(ctx->parent_ctx);
+
+        if (move_group) {
                 /*
                  * See perf_event_ctx_lock() for comments on the details
                  * of swizzling perf_event::ctx.
                  */
-                mutex_lock_double(&gctx->mutex, &ctx->mutex);
-
                 perf_remove_from_context(group_leader, false);
 
                 list_for_each_entry(sibling, &group_leader->sibling_list,
@@ -8311,13 +8328,7 @@ SYSCALL_DEFINE5(perf_event_open,
                         perf_remove_from_context(sibling, false);
                         put_ctx(gctx);
                 }
-        } else {
-                mutex_lock(&ctx->mutex);
-        }
-
-        WARN_ON_ONCE(ctx->parent_ctx);
 
-        if (move_group) {
                 /*
                  * Wait for everybody to stop referencing the events through
                  * the old lists, before installing it on new lists.
@@ -8349,22 +8360,20 @@ SYSCALL_DEFINE5(perf_event_open,
                 perf_event__state_init(group_leader);
                 perf_install_in_context(ctx, group_leader, group_leader->cpu);
                 get_ctx(ctx);
-        }
 
-        if (!exclusive_event_installable(event, ctx)) {
-                err = -EBUSY;
-                mutex_unlock(&ctx->mutex);
-                fput(event_file);
-                goto err_context;
+                /*
+                 * Now that all events are installed in @ctx, nothing
+                 * references @gctx anymore, so drop the last reference we have
+                 * on it.
+                 */
+                put_ctx(gctx);
         }
 
         perf_install_in_context(ctx, event, event->cpu);
         perf_unpin_context(ctx);
 
-        if (move_group) {
+        if (move_group)
                 mutex_unlock(&gctx->mutex);
-                put_ctx(gctx);
-        }
         mutex_unlock(&ctx->mutex);
 
         put_online_cpus();
@@ -8391,6 +8400,12 @@ SYSCALL_DEFINE5(perf_event_open,
         fd_install(event_fd, event_file);
         return event_fd;
 
+err_locked:
+        if (move_group)
+                mutex_unlock(&gctx->mutex);
+        mutex_unlock(&ctx->mutex);
+/* err_file: */
+        fput(event_file);
 err_context:
         perf_unpin_context(ctx);
         put_ctx(ctx);