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When enabling Context Analysis (CONTEXT_ANALYSIS := y) in arch/x86/kvm
code, Clang's Thread Safety Analysis failed to recognize that identical
per_cpu() accesses refer to the same lock:
| CC [M] arch/x86/kvm/vmx/posted_intr.o
| arch/x86/kvm/vmx/posted_intr.c:186:2: error: releasing raw_spinlock '__ptr + __per_cpu_offset[vcpu->cpu]' that was not held [-Werror,-Wthread-safety-analysis]
| 186 | raw_spin_unlock(&per_cpu(wakeup_vcpus_on_cpu_lock, vcpu->cpu));
| | ^
| ./include/linux/spinlock.h:276:32: note: expanded from macro 'raw_spin_unlock'
| 276 | #define raw_spin_unlock(lock) _raw_spin_unlock(lock)
| | ^
| arch/x86/kvm/vmx/posted_intr.c:207:1: error: raw_spinlock '__ptr + __per_cpu_offset[vcpu->cpu]' is still held at the end of function [-Werror,-Wthread-safety-analysis]
| 207 | }
| | ^
| arch/x86/kvm/vmx/posted_intr.c:182:2: note: raw_spinlock acquired here
| 182 | raw_spin_lock_nested(&per_cpu(wakeup_vcpus_on_cpu_lock, vcpu->cpu),
| | ^
| ./include/linux/spinlock.h:235:2: note: expanded from macro 'raw_spin_lock_nested'
| 235 | _raw_spin_lock(((void)(subclass), (lock)))
| | ^
| 2 errors generated.
This occurred because the default RELOC_HIDE() implementation (used by
the per-CPU macros) is a statement expression containing an intermediate
'unsigned long' variable (this version appears to predate Git history).
While the analysis strips away inner casts when resolving pointer
aliases, it stops when encountering intermediate non-pointer variables
(this is Thread Safety Analysis specific and irrelevant for codegen).
This prevents the analysis from concluding that the pointers passed to
e.g. raw_spin_lock() and raw_spin_unlock() were identical when per-CPU
accessors are used.
Simplify RELOC_HIDE() to a single expression. This preserves the intent
of obfuscating UB-introducing out-of-bounds pointer calculations from
the compiler via the 'unsigned long' cast, but allows the alias analysis
to successfully resolve the pointers.
Using a recent Clang version, I observe that generated code remains the
same for vmlinux; the intermediate variable was already being optimized
away (for any respectable modern compiler, not doing so would be an
optimizer bug). Note that GCC provides its own version of RELOC_HIDE(),
so this change only affects Clang builds.
Add a test case to lib/test_context-analysis.c to catch any regressions.
Reported-by: Bart Van Assche <bvanassche@acm.org>
Reported-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/all/e3946223-4543-4a76-a328-9c6865e95192@acm.org/
Link: https://patch.msgid.link/20260319135245.1420780-1-elver@google.com
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Remove __assume_ctx_lock() from lock initializers.
Implicitly asserting an active context during initialization caused
false-positive double-lock errors when acquiring a lock immediately after its
initialization. Moving forward, guarded member initialization must either:
1. Use guard(type_init)(&lock) or scoped_guard(type_init, ...).
2. Use context_unsafe() for simple initialization.
Reported-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/all/57062131-e79e-42c2-aa0b-8f931cb8cac2@acm.org/
Link: https://patch.msgid.link/20260119094029.1344361-7-elver@google.com
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Add scoped init guard definitions for common synchronization primitives
supported by context analysis.
The scoped init guards treat the context as active within initialization
scope of the underlying context lock, given initialization implies
exclusive access to the underlying object. This allows initialization of
guarded members without disabling context analysis, while documenting
initialization from subsequent usage.
The documentation is updated with the new recommendation. Where scoped
init guards are not provided or cannot be implemented (ww_mutex omitted
for lack of multi-arg guard initializers), the alternative is to just
disable context analysis where guarded members are initialized.
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/all/20251212095943.GM3911114@noisy.programming.kicks-ass.net/
Link: https://patch.msgid.link/20260119094029.1344361-3-elver@google.com
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Many patterns that involve data-racy accesses often deliberately ignore
normal synchronization rules to avoid taking a lock.
If we have a lock-guarded variable on which we do a lock-less data-racy
access, rather than having to write context_unsafe(data_race(..)),
simply make the data_race(..) macro imply context-unsafety. The
data_race() macro already denotes the intent that something subtly
unsafe is about to happen, so it should be clear enough as-is.
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20251219154418.3592607-27-elver@google.com
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Add support for Clang's context analysis for ww_mutex.
The programming model for ww_mutex is subtly more complex than other
locking primitives when using ww_acquire_ctx. Encoding the respective
pre-conditions for ww_mutex lock/unlock based on ww_acquire_ctx state
using Clang's context analysis makes incorrect use of the API harder.
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20251219154418.3592607-21-elver@google.com
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Add support for Clang's context analysis for local_lock_t and
local_trylock_t.
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20251219154418.3592607-20-elver@google.com
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Add support for Clang's context analysis for rw_semaphore.
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20251219154418.3592607-18-elver@google.com
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Add support for Clang's context analysis for SRCU.
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Paul E. McKenney <paulmck@kernel.org>
Link: https://patch.msgid.link/20251219154418.3592607-16-elver@google.com
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Improve the existing annotations to properly support Clang's context
analysis.
The old annotations distinguished between RCU, RCU_BH, and RCU_SCHED;
however, to more easily be able to express that "hold the RCU read lock"
without caring if the normal, _bh(), or _sched() variant was used we'd
have to remove the distinction of the latter variants: change the _bh()
and _sched() variants to also acquire "RCU".
When (and if) we introduce context locks to denote more generally that
"IRQ", "BH", "PREEMPT" contexts are disabled, it would make sense to
acquire these instead of RCU_BH and RCU_SCHED respectively.
The above change also simplified introducing __guarded_by support, where
only the "RCU" context lock needs to be held: introduce __rcu_guarded,
where Clang's context analysis warns if a pointer is dereferenced
without any of the RCU locks held, or updated without the appropriate
helpers.
The primitives rcu_assign_pointer() and friends are wrapped with
context_unsafe(), which enforces using them to update RCU-protected
pointers marked with __rcu_guarded.
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Paul E. McKenney <paulmck@kernel.org>
Link: https://patch.msgid.link/20251219154418.3592607-15-elver@google.com
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The annotations for bit_spinlock.h have simply been using "bitlock" as
the token. For Sparse, that was likely sufficient in most cases. But
Clang's context analysis is more precise, and we need to ensure we
can distinguish different bitlocks.
To do so, add a token context, and a macro __bitlock(bitnum, addr)
that is used to construct unique per-bitlock tokens.
Add the appropriate test.
<linux/list_bl.h> is implicitly included through other includes, and
requires 2 annotations to indicate that acquisition (without release)
and release (without prior acquisition) of its bitlock is intended.
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20251219154418.3592607-14-elver@google.com
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Add support for Clang's context analysis for seqlock_t.
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20251219154418.3592607-12-elver@google.com
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Add support for Clang's context analysis for mutex.
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20251219154418.3592607-11-elver@google.com
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Add support for Clang's context analysis for raw_spinlock_t,
spinlock_t, and rwlock. This wholesale conversion is required because
all three of them are interdependent.
To avoid warnings in constructors, the initialization functions mark a
lock as acquired when initialized before guarded variables.
The test verifies that common patterns do not generate false positives.
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20251219154418.3592607-9-elver@google.com
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Add a simple test stub where we will add common supported patterns that
should not generate false positives for each new supported context lock.
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20251219154418.3592607-4-elver@google.com
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