[PATCH] Cygwin: gendef: fix AArch64 stabilize_sig_stack lock retry

[Aswin Kalies Ramkumar Mangayarkarasi]

Hi Everyone,
The AArch64 stabilize_sig_stack retry label sits below the code that
computes the stacklock address and loads the "locked" constant, so
neither is recomputed on a retry.  The signal-handler path branches
back to that label after reusing x9 and x12 for the incyg counter, so
a retry can operate on incyg instead of stacklock.  Move the label
above the setup, and clear the exclusive monitor before yielding, as
the ldaxr otherwise leaves one open across the yield.

Thanks and Regards,
Aswin Kalies

Inline patch:

---
 winsup/cygwin/scripts/gendef | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/winsup/cygwin/scripts/gendef b/winsup/cygwin/scripts/gendef
index 5f25c543e..b8684def7 100755
--- a/winsup/cygwin/scripts/gendef
+++ b/winsup/cygwin/scripts/gendef
@@ -495,11 +495,13 @@ stabilize_sig_stack:

      ldr   x10, [x18, #0x8]        // load TLS block base pointer into x10

-     // try to acquire the lock
+     // try to acquire the lock.  The address and the "locked" constant are
+     // recomputed on every iteration: the signal-handler path below branches
+     // back here after using x9/x12 for the incyg counter.
+1:
      mov   w9, #1                        // value to store (1 == locked)
      ldr   x11, =_cygtls.stacklock       // load the symbol offset
      add   x12, x10, x11                 // x12 = tls_base + &stacklock
-1:
      ldaxr w13, [x12]              // load old lock value
      cbnz  w13, 2f                       // if already locked, wait
      stxr  w14, w9, [x12]                // attempt to acquire the lock
@@ -507,6 +509,7 @@ stabilize_sig_stack:
      b     3f                      // lock acquired, continue

 2:
+     clrex                         // drop the exclusive monitor
      yield                         // yield to allow other threads to run
      b     1b                      // retry acquiring the lock

--
2.49.0.windows.1



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