[PATCH] pthread_detach: add a comment about a known segfault bug.

[Aaron Jacobs]

From: Aaron Jacobs <aaronjjacobs@gmail.com>

This bug has existed for nearly a decade, so doesn't seem like it's
likely to be fixed tomorrow. If this comment had been here yesterday it
would have saved me many hours of debugging.
---
 nptl/pthread_detach.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/nptl/pthread_detach.c b/nptl/pthread_detach.c
index 3bbc037bdc..1e26eaf09e 100644
--- a/nptl/pthread_detach.c
+++ b/nptl/pthread_detach.c
@@ -44,7 +44,9 @@ ___pthread_detach (pthread_t th)
     }
   else
     /* Check whether the thread terminated meanwhile.  In this case we
-       will just free the TCB.  */
+       will just free the TCB.  Note using pd after giving up ownership above is
+       incorrect, and this code may segfault. See
+       https://sourceware.org/bugzilla/show_bug.cgi?id=19951. */
     if ((pd->cancelhandling & EXITING_BITMASK) != 0)
       /* Note that the code in __free_tcb makes sure each thread
 	 control block is freed only once.  */
-- 
2.52.0.rc2.455.g230fcf2819-goog