qemu/0013-linux-user-fix-segfault-deadlock.patch

From ea757d5a85a7e218d34f95f21923dfa979d52fa2 Mon Sep 17 00:00:00 2001
From: Alexander Graf <agraf@suse.de>
Date: Fri, 13 Jan 2012 17:05:41 +0100
Subject: [PATCH] linux-user: fix segfault deadlock
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When entering the guest we take a lock to ensure that nobody else messes
with our TB chaining while we're doing it. If we get a segfault inside that
code, we manage to work on, but will not unlock the lock.

This patch forces unlocking of that lock in the segv handler. I'm not sure
this is the right approach though. Maybe we should rather make sure we don't
segfault in the code? I would greatly appreciate someone more intelligible
than me to look at this :).

Example code to trigger this is at: http://csgraf.de/tmp/conftest.c

Reported-by: Fabio Erculiani <lxnay@sabayon.org>
Signed-off-by: Alexander Graf <agraf@suse.de>
[AF: Drop spinlock_safe_unlock() and switch to tb_lock_reset() (bonzini)]
Signed-off-by: Andreas Färber <afaerber@suse.de>
---
 user-exec.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/user-exec.c b/user-exec.c
index d8d597b..f8b7752 100644
--- a/user-exec.c
+++ b/user-exec.c
@@ -94,6 +94,10 @@ static inline int handle_cpu_signal(uintptr_t pc, unsigned long address,
     printf("qemu: SIGSEGV pc=0x%08lx address=%08lx w=%d oldset=0x%08lx\n",
            pc, address, is_write, *(unsigned long *)old_set);
 #endif
+
+    /* Maybe we're still holding the TB fiddling lock? */
+    tb_lock_reset();
+
     /* XXX: locking issue */
     if (is_write && h2g_valid(address)
         && page_unprotect(h2g(address), pc, puc)) {