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Kernel: Resolve deadlock in MasterPTY due to mutex in spinlock scope

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MasterPTY::read called DoubleBuffer::read which takes a mutex (which
may block) while holding m_slave's spinlock. If it did block, and was
later rescheduled on a different physical CPU, we would deadlock on
re-locking m_slave inside the unblock callback. (Since our recursive
spinlock implementation is processor based and not process based)
This commit is contained in:
Idan Horowitz 2023-12-16 00:12:37 +02:00 committed by Andreas Kling
parent ab06a76920
commit 662143e0a9
Notes: sideshowbarker 2024-07-16 22:51:10 +09:00 
Author: https://github.com/IdanHo
Commit: https://github.com/SerenityOS/serenity/commit/662143e0a9
Pull-request: https://github.com/SerenityOS/serenity/pull/22310
1 changed files with 10 additions and 5 deletions
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15
Kernel/Devices/TTY/MasterPTY.cpp
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@ -55,11 +55,16 @@ MasterPTY::~MasterPTY()
ErrorOr<size_t> MasterPTY::read(OpenFileDescription&, u64, UserOrKernelBuffer& buffer, size_t size)
{
return m_slave.with([this, &buffer, size](auto& slave) -> ErrorOr<size_t> {
if (!slave && m_buffer->is_empty())
return 0;
return m_buffer->read(buffer, size);
});
// Note that has_slave() takes and then releases the m_slave spinlock.
// Not holding the spinlock while calling m_buffer->read is legal, because slave starts non-null,
// and can only change its state to null once (and never back to non-null) in notify_slave_closed.
// So if the check happens, and it returns non-null, and then it turns null concurrently,
// and we call m_buffer->read, the behaviour from the perspective of the read caller is
// the same as if the slave turned null after we called m_buffer->read. On the other hand,
// if the check happens and returns null, then it can't possibly change to non-null after.
if (!has_slave() && m_buffer->is_empty())
return 0;
return m_buffer->read(buffer, size);
}
ErrorOr<size_t> MasterPTY::write(OpenFileDescription&, u64, UserOrKernelBuffer const& buffer, size_t size)