[RFC PATCH for 4.21 01/16] rseq/selftests: Expose reference counter to coexist with glibc (v2)

[Mathieu Desnoyers]

In order to integrate rseq into user-space applications, expose a
reference counter TLS as a __rseq_refcount weak symbol so many rseq
users can be linked into the same application (e.g. librseq and glibc).
The reference count ensures that rseq syscall
registration/unregistration happens only for the most early/late user
for each thread, thus ensuring that rseq is registered across the
lifetime of all rseq users for a given thread.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoy...@efficios.com>
CC: Shuah Khan <sh...@kernel.org>
CC: Carlos O'Donell <car...@redhat.com>
CC: Florian Weimer <fwei...@redhat.com>
CC: Joseph Myers <jos...@codesourcery.com>
CC: Szabolcs Nagy <szabolcs.n...@arm.com>
CC: Thomas Gleixner <t...@linutronix.de>
CC: Ben Maurer <bmau...@fb.com>
CC: Peter Zijlstra <pet...@infradead.org>
CC: "Paul E. McKenney" <paul...@linux.vnet.ibm.com>
CC: Boqun Feng <boqun.f...@gmail.com>
CC: Will Deacon <will.dea...@arm.com>
CC: Dave Watson <davejwat...@fb.com>
CC: Paul Turner <p...@google.com>
CC: linux-...@vger.kernel.org
---
Changes since v1:
- Error out on refcount overflow/underflow.
- Expose __rseq_refcount weak symbol.
---
 tools/testing/selftests/rseq/rseq.c | 23 ++++++++++++++++-------
 tools/testing/selftests/rseq/rseq.h |  1 +
 2 files changed, 17 insertions(+), 7 deletions(-)

diff --git a/tools/testing/selftests/rseq/rseq.c 
b/tools/testing/selftests/rseq/rseq.c
index 4847e97ed049..835e3917d220 100644
--- a/tools/testing/selftests/rseq/rseq.c
+++ b/tools/testing/selftests/rseq/rseq.c
@@ -25,18 +25,19 @@
 #include <syscall.h>
 #include <assert.h>
 #include <signal.h>
+#include <limits.h>
 
 #include "rseq.h"
 
 #define ARRAY_SIZE(arr)        (sizeof(arr) / sizeof((arr)[0]))
 
-__attribute__((tls_model("initial-exec"))) __thread
+__attribute__((weak)) __thread
 volatile struct rseq __rseq_abi = {
        .cpu_id = RSEQ_CPU_ID_UNINITIALIZED,
 };
 
-static __attribute__((tls_model("initial-exec"))) __thread
-volatile int refcount;
+__attribute__((weak)) __thread
+volatile uint32_t __rseq_refcount;
 
 static void signal_off_save(sigset_t *oldset)
 {
@@ -70,7 +71,11 @@ int rseq_register_current_thread(void)
        sigset_t oldset;
 
        signal_off_save(&oldset);
-       if (refcount++)
+       if (__rseq_refcount == UINT_MAX) {
+               ret = -1;
+               goto end;
+       }
+       if (__rseq_refcount++)
                goto end;
        rc = sys_rseq(&__rseq_abi, sizeof(struct rseq), 0, RSEQ_SIG);
        if (!rc) {
@@ -78,9 +83,9 @@ int rseq_register_current_thread(void)
                goto end;
        }
        if (errno != EBUSY)
-               __rseq_abi.cpu_id = -2;
+               __rseq_abi.cpu_id = RSEQ_CPU_ID_REGISTRATION_FAILED;
        ret = -1;
-       refcount--;
+       __rseq_refcount--;
 end:
        signal_restore(oldset);
        return ret;
@@ -92,7 +97,11 @@ int rseq_unregister_current_thread(void)
        sigset_t oldset;
 
        signal_off_save(&oldset);
-       if (--refcount)
+       if (!__rseq_refcount) {
+               ret = -1;
+               goto end;
+       }
+       if (--__rseq_refcount)
                goto end;
        rc = sys_rseq(&__rseq_abi, sizeof(struct rseq),
                      RSEQ_FLAG_UNREGISTER, RSEQ_SIG);
diff --git a/tools/testing/selftests/rseq/rseq.h 
b/tools/testing/selftests/rseq/rseq.h
index c72eb70f9b52..47f815c19cef 100644
--- a/tools/testing/selftests/rseq/rseq.h
+++ b/tools/testing/selftests/rseq/rseq.h
@@ -45,6 +45,7 @@
 #endif
 
 extern __thread volatile struct rseq __rseq_abi;
+extern __thread volatile uint32_t __rseq_refcount;
 
 #define rseq_likely(x)         __builtin_expect(!!(x), 1)
 #define rseq_unlikely(x)       __builtin_expect(!!(x), 0)
-- 
2.11.0