[PATCH 1/2] Revert "nptl: More useful padding in struct pthread"

[Florian Weimer]

This reverts commit 7c22dcda27743658b6b8ea479283b384ad56bd5a.

The padding is required by Chromium's MaybeUpdateGlibcTidCache
in sandbox/linux/services/namespace_sandbox.cc.
---
 nptl/descr.h | 56 +++++++++++++++++++++++++++++-----------------------
 1 file changed, 31 insertions(+), 25 deletions(-)

diff --git a/nptl/descr.h b/nptl/descr.h
index bcc537ed7d..d0d30929e2 100644
--- a/nptl/descr.h
+++ b/nptl/descr.h
@@ -129,32 +129,41 @@ struct priority_protection_data
 /* Thread descriptor data structure.  */
 struct pthread
 {
+  union
+  {
 #if !TLS_DTV_AT_TP
-  /* This overlaps the TCB as used for TLS without threads (see tls.h).  */
-  tcbhead_t header;
+    /* This overlaps the TCB as used for TLS without threads (see tls.h).  */
+    tcbhead_t header;
 #else
-  struct
-  {
-    /* multiple_threads is enabled either when the process has spawned at
-       least one thread or when a single-threaded process cancels itself.
-       This enables additional code to introduce locking before doing some
-       compare_and_exchange operations and also enable cancellation points.
-       The concepts of multiple threads and cancellation points ideally
-       should be separate, since it is not necessary for multiple threads to
-       have been created for cancellation points to be enabled, as is the
-       case is when single-threaded process cancels itself.
-
-       Since enabling multiple_threads enables additional code in
-       cancellation points and compare_and_exchange operations, there is a
-       potential for an unneeded performance hit when it is enabled in a
-       single-threaded, self-canceling process.  This is OK though, since a
-       single-threaded process will enable async cancellation only when it
-       looks to cancel itself and is hence going to end anyway.  */
-    int multiple_threads;
-    int gscope_flag;
-  } header;
+    struct
+    {
+      /* multiple_threads is enabled either when the process has spawned at
+	 least one thread or when a single-threaded process cancels itself.
+	 This enables additional code to introduce locking before doing some
+	 compare_and_exchange operations and also enable cancellation points.
+	 The concepts of multiple threads and cancellation points ideally
+	 should be separate, since it is not necessary for multiple threads to
+	 have been created for cancellation points to be enabled, as is the
+	 case is when single-threaded process cancels itself.
+
+	 Since enabling multiple_threads enables additional code in
+	 cancellation points and compare_and_exchange operations, there is a
+	 potential for an unneeded performance hit when it is enabled in a
+	 single-threaded, self-canceling process.  This is OK though, since a
+	 single-threaded process will enable async cancellation only when it
+	 looks to cancel itself and is hence going to end anyway.  */
+      int multiple_threads;
+      int gscope_flag;
+    } header;
 #endif
 
+    /* This extra padding has no special purpose, and this structure layout
+       is private and subject to change without affecting the official ABI.
+       We just have it here in case it might be convenient for some
+       implementation-specific instrumentation hack or suchlike.  */
+    void *__padding[24];
+  };
+
   /* This descriptor's link on the GL (dl_stack_used) or
      GL (dl_stack_user) list.  */
   list_t list;
@@ -398,9 +407,6 @@ struct pthread
   /* getrandom vDSO per-thread opaque state.  */
   void *getrandom_buf;
 
-  /* Can be used for backports preserving internal TCB layout.  */
-  void *padding[8];
-
   /* rseq area registered with the kernel.  Use a custom definition
      here to isolate from kernel struct rseq changes.  The
      implementation of sched_getcpu needs acccess to the cpu_id field;
-- 
2.47.1