Cygwin: open_shared: always bump next_address

The new loop in open_shared has a subtil performance problem.
Next_address is bumped only if mapping at this address
failed.  Every subsequent call to open_shared has a high probability
having to call MapViewOfFileEx twice, because next_address is still
set to the address of the last successful mapping.

Avoid this by bumping next_address every time.

While at it, fix a comment.

Conflict: Missing backport of 31302b267b86 ("Cygwin: drop fixed addresses for standard shared regions")
Fixes: dc0fe7742b8c ("Cygwin: open_shared: try harder allocating a shared region")
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

1 files changed, 8 insertions, 12 deletions

diff --git a/winsup/cygwin/mm/shared.cc b/winsup/cygwin/mm/shared.cc
index d23cc8e0e..2ea3a4336 100644
--- a/winsup/cygwin/mm/shared.cc
+++ b/winsup/cygwin/mm/shared.cc
@@ -184,25 +184,21 @@ open_shared (const WCHAR *name, int n, HANDLE& shared_h, DWORD size,
     {
       /* Locate shared regions in the area between SHARED_REGIONS_ADDRESS_LOW
 	 and SHARED_REGIONS_ADDRESS_HIGH, retrying until we have a slot.
-	 Don't use MapViewOfFile3 (loader deadlock during fork. */
+	 Don't use MapViewOfFile3 (STATUS_DLL_INIT_FAILED during fork). */
       bool loop = false;
 
-      addr = (void *) next_address;
       do
 	{
+	  addr = (void *) next_address;
 	  shared = MapViewOfFileEx (shared_h, FILE_MAP_READ | FILE_MAP_WRITE,
 				    0, 0, 0, addr);
-	  if (!shared)
+	  next_address += wincap.allocation_granularity ();
+	  if (next_address >= SHARED_REGIONS_ADDRESS_HIGH)
 	    {
-	      next_address += wincap.allocation_granularity ();
-	      if (next_address >= SHARED_REGIONS_ADDRESS_HIGH)
-		{
-		  if (loop)
-		    break;
-		  next_address = SHARED_REGIONS_ADDRESS_LOW;
-		  loop = true;
-		}
-	      addr = (void *) next_address;
+	      if (!shared && loop)
+		break;
+	      next_address = SHARED_REGIONS_ADDRESS_LOW;
+	      loop = true;
 	    }
 	}
       while (!shared);