lkml.org 
[lkml]   [2005]   [Nov]   [12]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
From
Subject[PATCH 2/9] uml: remove bogus WARN_ON, triggerable harmlessly on a page fault race
Date
From: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>

The below warning was added in place of pte_mkyoung(); if (is_write)
pte_mkdirty();

In fact, if the PTE is not marked young/dirty, our dirty/accessed bit emulation
would cause the TLB permission not to be changed, and so we'd loop, and given we
don't support preemption yet, we'd busy-hang here.

However, I've seen this warning trigger without crashes during a loop of
concurrent kernel builds, at random times (i.e. like a race condition), and I
realized that two concurrent faults on the same page, one on read and one on
write, can trigger it. The read fault gets serviced and the PTE gets marked
writable but clean (it's possible on a shared-writable mapping), while the
generic code sees the PTE was already installed and returns without action. In
this case, we'll see another fault and service it normally.

Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
---

arch/um/kernel/trap_kern.c | 9 +++++++++
1 files changed, 9 insertions(+), 0 deletions(-)

diff --git a/arch/um/kernel/trap_kern.c b/arch/um/kernel/trap_kern.c
index 95c8f87..0d4c10a 100644
--- a/arch/um/kernel/trap_kern.c
+++ b/arch/um/kernel/trap_kern.c
@@ -95,7 +95,16 @@ survive:
pte = pte_offset_kernel(pmd, address);
} while(!pte_present(*pte));
err = 0;
+ /* The below warning was added in place of
+ * pte_mkyoung(); if (is_write) pte_mkdirty();
+ * If it's triggered, we'd see normally a hang here (a clean pte is
+ * marked read-only to emulate the dirty bit).
+ * However, the generic code can mark a PTE writable but clean on a
+ * concurrent read fault, triggering this harmlessly. So comment it out.
+ */
+#if 0
WARN_ON(!pte_young(*pte) || (is_write && !pte_dirty(*pte)));
+#endif
flush_tlb_page(vma, address);
out:
up_read(&mm->mmap_sem);
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2005-11-12 19:06    [W:0.081 / U:0.072 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site