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[PATCH v2 08/21] nptl: aarch64: Fix Race conditions in pthread cancellation (BZ#12683)

This patch adds the aarch64 modifications required for the BZ#12683.
It basically adds the required ucontext_get_pc function.

The built cancelable syscall wrapper for aarch64 using GCC 7.2.1 and
default configuration flags shows an expected optimized version:

        .global __syscall_cancel_arch_start
.type __syscall_cancel_arch_start,@function
        ldr     w9, [x0]
        tbnz    x9, 2, .L7
        mov     x8, x1
        mov     x0, x2
        mov     x1, x3
        mov     x2, x4
        mov     x3, x5
        mov     x4, x6
        mov     x5, x7
        svc     0       // syscall nr
        .global __syscall_cancel_arch_end
.type __syscall_cancel_arch_end,@function

A similar code is obtained with GCC 5.3.1, so I see to need to provide
an arch-specific syscall_cancel.S for aarch64.

Checked on aarch64-linux-gnu.

	[BZ #12683]
	* sysdeps/unix/sysv/linux/aarch64/sysdep-cancel.h (PSEUDO): Redefine
	to call __syscall_cancel function for cancellable syscalls.
	(__pthread_get_ip): Add implementation.
	* sysdeps/unix/sysv/linux/aarch64/sysdep.h (SYSCALL_CANCEL_ERROR): Add
 ChangeLog                                        |  4 ++++
 sysdeps/unix/sysv/linux/aarch64/sigcontextinfo.h | 11 +++++++++++
 2 files changed, 15 insertions(+)

diff --git a/sysdeps/unix/sysv/linux/aarch64/sigcontextinfo.h b/sysdeps/unix/sysv/linux/aarch64/sigcontextinfo.h
index 04596ef..d9822dc 100644
--- a/sysdeps/unix/sysv/linux/aarch64/sigcontextinfo.h
+++ b/sysdeps/unix/sysv/linux/aarch64/sigcontextinfo.h
@@ -16,6 +16,9 @@
    License along with the GNU C Library; if not, see
    <>.  */
 #include <stdint.h>
 #include <sys/ucontext.h>
@@ -33,3 +36,11 @@
   (act)->sa_flags |= SA_SIGINFO; \
   (sigaction) (sig, act, oact); \
+static inline uintptr_t
+ucontext_get_pc (const ucontext_t *uc)
+  return uc->uc_mcontext.pc;

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