On 28/03/2019 02:54, Davide Italiano via lldb-commits wrote:
On Wed, Mar 27, 2019 at 6:49 PM Jim Ingham via lldb-commits
<lldb-commits@lists.llvm.org> wrote:
Author: jingham
Date: Wed Mar 27 18:51:33 2019
New Revision: 357141
URL: http://llvm.org/viewvc/llvm-project?rev=357141&view=rev
Log:
Copy the breakpoint site owner's collection so we can drop
the collection lock before we iterate over the owners calling ShouldStop.
BreakpointSite::ShouldStop can do a lot of work, and might by chance hit the
same breakpoint
site again on another thread. So instead of holding the site's owners lock
while iterating over them calling ShouldStop, I make a local copy of the list,
drop the lock
and then iterate over the copy calling BreakpointLocation::ShouldStop.
It's actually quite difficult to make this cause problems because usually all
the
action happens on the private state thread, and the lock is recursive.
I have a report where some code hit the ASAN error breakpoint, went to
compile the ASAN error gathering expression, in the course of compiling
that we went to fetch the ObjC runtime data, but the state of the program
was such that the ObjC runtime grubbing function triggered an ASAN error and
we were executing that function on another thread.
I couldn't figure out a way to reproduce that situation in a test. But this is
an
NFC change anyway, it just makes the locking strategy more narrowly focused.
<rdar://problem/49074093>
Modified:
lldb/trunk/include/lldb/Breakpoint/BreakpointLocationCollection.h
lldb/trunk/source/Breakpoint/BreakpointLocationCollection.cpp
lldb/trunk/source/Breakpoint/BreakpointSite.cpp
Modified: lldb/trunk/include/lldb/Breakpoint/BreakpointLocationCollection.h
URL:
http://llvm.org/viewvc/llvm-project/lldb/trunk/include/lldb/Breakpoint/BreakpointLocationCollection.h?rev=357141&r1=357140&r2=357141&view=diff
==============================================================================
--- lldb/trunk/include/lldb/Breakpoint/BreakpointLocationCollection.h (original)
+++ lldb/trunk/include/lldb/Breakpoint/BreakpointLocationCollection.h Wed Mar
27 18:51:33 2019
@@ -22,6 +22,8 @@ public:
BreakpointLocationCollection();
~BreakpointLocationCollection();
+
+ BreakpointLocationCollection &operator=(const BreakpointLocationCollection
&rhs);
//------------------------------------------------------------------
/// Add the breakpoint \a bp_loc_sp to the list.
Modified: lldb/trunk/source/Breakpoint/BreakpointLocationCollection.cpp
URL:
http://llvm.org/viewvc/llvm-project/lldb/trunk/source/Breakpoint/BreakpointLocationCollection.cpp?rev=357141&r1=357140&r2=357141&view=diff
==============================================================================
--- lldb/trunk/source/Breakpoint/BreakpointLocationCollection.cpp (original)
+++ lldb/trunk/source/Breakpoint/BreakpointLocationCollection.cpp Wed Mar 27
18:51:33 2019
@@ -178,3 +178,20 @@ void BreakpointLocationCollection::GetDe
(*pos)->GetDescription(s, level);
}
}
+
+BreakpointLocationCollection &BreakpointLocationCollection::operator=(
+ const BreakpointLocationCollection &rhs) {
+ // Same trick as in ModuleList to avoid lock inversion.
+ if (this != &rhs) {
+ if (uintptr_t(this) > uintptr_t(&rhs)) {
+ std::lock_guard<std::mutex> lhs_guard(m_collection_mutex);
+ std::lock_guard<std::mutex> rhs_guard(rhs.m_collection_mutex);
+ m_break_loc_collection = rhs.m_break_loc_collection;
+ } else {
+ std::lock_guard<std::mutex> lhs_guard(m_collection_mutex);
+ std::lock_guard<std::mutex> rhs_guard(rhs.m_collection_mutex);
+ m_break_loc_collection = rhs.m_break_loc_collection;
+ }
+ }
The two branches are identical, is this intentional?
It probably isn't. Also, a better way to avoid lock inversion would be
with std::lock <https://en.cppreference.com/w/cpp/thread/lock>.
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