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Kernel: Implement flock downgrading
This commit makes it possible for a process to downgrade a file lock it holds from a write (exclusive) lock to a read (shared) lock. For this, the process must point to the exact range of the flock, and must be the owner of the lock.
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9b425b860c
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@ -298,7 +298,7 @@ static inline ErrorOr<void> normalize_flock(OpenFileDescription const& descripti
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return {};
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}
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bool Inode::can_apply_flock(flock const& new_lock) const
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bool Inode::can_apply_flock(flock const& new_lock, Optional<OpenFileDescription const&> description) const
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{
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VERIFY(new_lock.l_whence == SEEK_SET);
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@ -310,8 +310,19 @@ bool Inode::can_apply_flock(flock const& new_lock) const
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if (!range_overlap(lock.start, lock.len, new_lock.l_start, new_lock.l_len))
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continue;
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// There are two cases where we can attempt downgrade:
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//
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// 1) We're the owner of this lock. The downgrade will immediately
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// succeed.
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// 2) We're not the owner of this lock. Our downgrade attempt will
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// fail, and the thread will start blocking on an FlockBlocker.
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//
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// For the first case, we get the description from try_apply_flock
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// below. For the second case, the check below would always be
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// false, so there is no need to store the description in the
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// blocker in the first place.
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if (new_lock.l_type == F_RDLCK && lock.type == F_WRLCK)
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return false;
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return description.has_value() && lock.owner == &description.value() && lock.start == new_lock.l_start && lock.len == new_lock.l_len;
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if (new_lock.l_type == F_WRLCK)
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return false;
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@ -320,29 +331,38 @@ bool Inode::can_apply_flock(flock const& new_lock) const
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});
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}
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ErrorOr<bool> Inode::try_apply_flock(Process const& process, OpenFileDescription const& description, flock const& lock)
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ErrorOr<bool> Inode::try_apply_flock(Process const& process, OpenFileDescription const& description, flock const& new_lock)
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{
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return m_flocks.with([&](auto& flocks) -> ErrorOr<bool> {
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if (!can_apply_flock(lock))
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if (!can_apply_flock(new_lock, description))
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return false;
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if (lock.l_type == F_UNLCK) {
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bool any_locks_unlocked = false;
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for (size_t i = 0; i < flocks.size(); ++i) {
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if (&description == flocks[i].owner && flocks[i].start == lock.l_start && flocks[i].len == lock.l_len) {
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flocks.remove(i);
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any_locks_unlocked |= true;
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}
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bool did_manipulate_lock = false;
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for (size_t i = 0; i < flocks.size(); ++i) {
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auto const& lock = flocks[i];
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bool is_potential_downgrade = new_lock.l_type == F_RDLCK && lock.type == F_WRLCK;
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bool is_potential_unlock = new_lock.l_type == F_UNLCK;
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bool is_lock_owner = &description == lock.owner;
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bool lock_range_exactly_matches = lock.start == new_lock.l_start && lock.len == new_lock.l_len;
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bool can_manage_this_lock = is_lock_owner && lock_range_exactly_matches;
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if ((is_potential_downgrade || is_potential_unlock) && can_manage_this_lock) {
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flocks.remove(i);
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did_manipulate_lock = true;
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break;
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}
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if (any_locks_unlocked)
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m_flock_blocker_set.unblock_all_blockers_whose_conditions_are_met();
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// Judging by the Linux implementation, unlocking a non-existent lock also works.
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return true;
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}
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TRY(flocks.try_append(Flock { lock.l_start, lock.l_len, &description, process.pid().value(), lock.l_type }));
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if (new_lock.l_type != F_UNLCK)
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TRY(flocks.try_append(Flock { new_lock.l_start, new_lock.l_len, &description, process.pid().value(), new_lock.l_type }));
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if (did_manipulate_lock)
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m_flock_blocker_set.unblock_all_blockers_whose_conditions_are_met();
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// Judging by the Linux implementation, unlocking a non-existent lock
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// also works.
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return true;
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});
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}
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@ -104,7 +104,7 @@ public:
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ErrorOr<NonnullLockRefPtr<FIFO>> fifo();
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bool can_apply_flock(flock const&) const;
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bool can_apply_flock(flock const&, Optional<OpenFileDescription const&> = {}) const;
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ErrorOr<void> apply_flock(Process const&, OpenFileDescription const&, Userspace<flock const*>, ShouldBlock);
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ErrorOr<void> get_flock(OpenFileDescription const&, Userspace<flock*>) const;
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void remove_flocks_for_description(OpenFileDescription const&);
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