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Kernel: Fix SharedBuffer reference counting on fork

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We need to not only add a record for a reference, but we need
to copy the reference count on fork as well, because the code
in the fork assumes that it has the same amount of references,
still.

Also, once all references are dropped when a process is disowned,
delete the shared buffer.

Fixes #4076
This commit is contained in:
Tom 2020-11-24 10:26:32 -07:00 • committed by Andreas Kling
parent 018eff802b
commit 68abd1cb29
3 changed files with 22 additions and 9 deletions
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18
Kernel/SharedBuffer.cpp
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@ -126,11 +126,14 @@ void SharedBuffer::share_all_shared_buffers(Process& from_process, Process& with
LOCKER(shared_buffers().lock());
for (auto& shbuf : shared_buffers().resource()) {
auto& shared_buffer = *shbuf.value;
if (shared_buffer.m_global)
continue;
// We need to clone all references (including for global shared buffers),
// and the reference counts as well.
for (auto& ref : shared_buffer.m_refs) {
if (ref.pid == from_process.pid()) {
shared_buffer.share_with(with_process.pid());
auto ref_count = ref.count;
shared_buffer.m_refs.append(Reference(with_process.pid(), ref_count));
// NOTE: ref may become invalid after we appended!
shared_buffer.m_total_refs += ref_count;
break;
}
}
@ -143,7 +146,9 @@ void SharedBuffer::deref_for_process(Process& process)
for (size_t i = 0; i < m_refs.size(); ++i) {
auto& ref = m_refs[i];
if (ref.pid == process.pid()) {
ASSERT(ref.count > 0);
ref.count--;
ASSERT(m_total_refs > 0);
m_total_refs--;
if (ref.count == 0) {
#ifdef SHARED_BUFFER_DEBUG
@ -164,7 +169,7 @@ void SharedBuffer::deref_for_process(Process& process)
ASSERT_NOT_REACHED();
}
void SharedBuffer::disown(ProcessID pid)
bool SharedBuffer::disown(ProcessID pid)
{
LOCKER(shared_buffers().lock());
for (size_t i = 0; i < m_refs.size(); ++i) {
@ -173,15 +178,18 @@ void SharedBuffer::disown(ProcessID pid)
#ifdef SHARED_BUFFER_DEBUG
dbg() << "Disowning shared buffer " << m_shbuf_id << " of size " << size() << " by PID " << pid.value();
#endif
ASSERT(m_total_refs >= ref.count);
m_total_refs -= ref.count;
m_refs.unstable_take(i);
#ifdef SHARED_BUFFER_DEBUG
dbg() << "Disowned shared buffer " << m_shbuf_id << " of size " << size() << " by PID " << pid.value();
#endif
destroy_if_unused();
return;
break;
}
}
return m_total_refs == 0;
}
void SharedBuffer::destroy_if_unused()