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Task local data #9315

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Pull request for series with
subject: Task local data
version: 6
url: https://patchwork.kernel.org/project/netdevbpf/list/?series=983442

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Upstream branch: 0768e98
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=983442
version: 6

@kernel-patches-daemon-bpf
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Upstream branch: 0768e98
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=983442
version: 6

@kernel-patches-daemon-bpf
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Upstream branch: 0ee30d9
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=983442
version: 6

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Upstream branch: 0ee30d9
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=983442
version: 6

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Upstream branch: beb1097
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=983442
version: 6

ameryhung added 3 commits July 18, 2025 17:21
@ameryhung
selftests/bpf: Introduce task local data
998a5bb 
Task local data defines an abstract storage type for storing task-
specific data (TLD). This patch provides user space and bpf
implementation as header-only libraries for accessing task local data.

Task local data is a bpf task local storage map with two UPTRs:
1) u_tld_metadata, shared by all tasks of the same process, consists of
the total count of TLDs and an array of metadata of TLDs. A metadata of
a TLD comprises the size and the name. The name is used to identify a
specific TLD in bpf 2) u_tld_data points to a task-specific memory region
for storing TLDs.

Below are the core task local data API:

                     User space                           BPF
Define TLD    TLD_DEFINE_KEY(), tld_create_key()           -
Get data           tld_get_data()                    tld_get_data()

A TLD is first defined by the user space with TLD_DEFINE_KEY() or
tld_create_key(). TLD_DEFINE_KEY() defines a TLD statically and allocates
just enough memory during initialization. tld_create_key() allows
creating TLDs on the fly, but has a fix memory budget, TLD_DYN_DATA_SIZE.
Internally, they all go through the metadata array to check if the TLD can
be added. The total TLD size needs to fit into a page (limited by UPTR),
and no two TLDs can have the same name. It also calculates the offset, the
next available space in u_tld_data, by summing sizes of TLDs. If the TLD
can be added, it increases the count using cmpxchg as there may be other
concurrent tld_create_key(). After a successful cmpxchg, the last
metadata slot now belongs to the calling thread and will be updated.
tld_create_key() returns the offset encapsulated as a opaque object key
to prevent user misuse.

Then, user space can pass the key to tld_get_data() to get a pointer
to the TLD. The pointer will remain valid for the lifetime of the
thread.

BPF programs can also locate the TLD by tld_get_data(), but with both
name and key. The first time tld_get_data() is called, the name will
be used to lookup the metadata. Then, the key will be saved to a
task_local_data map, tld_keys_map. Subsequent call to tld_get_data()
will use the key to quickly locate the data.

User space task local data library uses a light way approach to ensure
thread safety (i.e., atomic operation + compiler and memory barriers).
While a metadata is being updated, other threads may also try to read it.
To prevent them from seeing incomplete data, metadata::size is used to
signal the completion of the update, where 0 means the update is still
ongoing. Threads will wait until seeing a non-zero size to read a
metadata.

Signed-off-by: Amery Hung <[email protected]>
@ameryhung
selftests/bpf: Test basic task local data operations
894ee84 
Test basic operations of task local data with valid and invalid
tld_create_key().

For invalid calls, make sure they return the right error code and check
that the TLDs are not inserted by running tld_get_data("
value_not_exists") on the bpf side. The call should a null pointer.

For valid calls, first make sure the TLDs are created by calling
tld_get_data() on the bpf side. The call should return a valid pointer.

Finally, verify that the TLDs are indeed task-specific (i.e., their
addresses do not overlap) with multiple user threads. This done by
writing values unique to each thread, reading them from both user space
and bpf, and checking if the value read back matches the value written.

Signed-off-by: Amery Hung <[email protected]>
@ameryhung
selftests/bpf: Test concurrent task local data key creation
166f1e1 
Test thread-safety of tld_create_key(). Since tld_create_key() does
not rely on locks but memory barriers and atomic operations to protect
the shared metadata, the thread-safety of the function is non-trivial.
Make sure concurrent tld_key_create(), both valid and invalid, can not
race and corrupt metatada, which may leads to TLDs not being thread-
specific or duplicate TLDs with the same name.

Signed-off-by: Amery Hung <[email protected]>
@kernel-patches-daemon-bpf
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Upstream branch: 42be23e
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=983442
version: 6

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